Sciencemadness Discussion Board - Anyone Up for a DIY Particle Accelerator?

Anyone Up for a DIY Particle Accelerator?

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Sauron - 21-2-2007 at 06:38

Not since Bill Murray and Ivan Reitman got together with Dan Ackroyd has science been this mad.

In connection with the mild and unassuming Analytical Machinery thread I went googling for Type 220A 150 magnetron magnets which had been referred to in a quote by @Leu from an old article. I found the complete old article, from April 1959 The Amateur Scientist, and am now downloading the entire website (quite large) into a pdf.

Anyway immediately following that article there was a sort of Letters to the Editor section discussing a January 1959 on building your own particle accelerator at home. I dug up that article, and sure enough it describes how to construct an electron (or proton, your choice) accelerator of >300-350 KeV or more than twice that built in the early 1930s by the inventors of particle accelerators.

It so happens I was just reading about the use of high energy electrons to drive a reaction between carbon tetrachloride and NO gas to give a 50% yield of trichloronitrosomethane, an intensely blue liquid. Clearly, we are talking about a free radical process here, the generation of trichloromethyl radicals by irradiation with charged particles.

Highly interesting.

The same radical could be generated by UV or by a radical intiator like benzoyl peroxide or AIBN but I think doing so with an electron beam is well, cool. The device uses a van de Graf generator (with plans from yet another article to build your own) and a 2 x 36" Pyrex tube evacuated to 0.01 micron Hg for accelerator tube, an Al foil window for beam with Al plate support, etc. I will supply this article here shortly.

No it isn't quite the backpack Ghostbuster weapon but it is still neater than hell.

Attachment includes above article plsu safety discussion from later issue plus VDG Generator construction article referenced. I just updated this file with new material from a 1955 article on electrostatic generators.

[Edited on 22-2-2007 by Sauron]

[Edited on 22-2-2007 by Sauron]

Attachment: Home Scientists Particle Accelerator 2.pdf (1.1MB)
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Sauron - 21-2-2007 at 06:50

PS The British physicists who built a very similar accelerator in 1932 of 150 KeV were J.D.Cockcroft and E.T.S.Walton. They also employed an electrostatic generator as power source.

Gentlemen and ladies, this is not a toy. Watson and Cockcroft transmuted Al to an unstable Be isotope that decayed to He and released energy of 17 MeV.

Of course by today's standards 150 or 350 KeV is small, but you and I are not likely to get our hands on FermiLab are we? Or even the sorts of equipment that Lawrence had at Berkeley just a few years after Watson and Cockcroft started the ball rolling.

[Edited on 21-2-2007 by Sauron]

[Edited on 21-2-2007 by Sauron]

YT2095 - 21-2-2007 at 07:13

currently (no pun intended) my CW Voltage multiplier only pulls 36Kv, but the capacitor bank will more than easily contain this charge, albeit a little to small to do anything of significance.

I Do have plans in the pipeline for a Bigger Better device though, and have most of the parts I need also, I just need a few good summer days in the back yard to construct the thing, and couple of strong and willing people to help move it into location and a few prayer for the correct weather to make it work

be sure, I WILL be Posting Pics, probably sometime in the Autumn this year

Sauron - 21-2-2007 at 07:35

The van der Graf generator they propose is supposed to be capable of 20 uA @ 500,000V which sounds like 10 VA to me.

This is connected to the anode and cathode, the cathode is a nichrome filament. The optimum temperature for the filament is determined empirically; too hot and it will release too many electrons. A simple transformer regulates the filament.

They propose a diffusion pump to pump down the tube during operation. I am wondering why not just evacuate it once, seal it and be done? It's just a vacuum tube albeit a big one. How often would the filament need service?

They propose an Hg diffusion pump for the high vacuum stage, I'd rather go with an oil diffusion pump to avoid having 3 lbs of Hg around the lab, but hey, this was 1959.

I was going to invest in an Ace photochemical (UV) reactor but with the lamps and power supply and special glass it is costly and UV is hazardous. I am already blind in one eye, and can't see out of the other. A particle beam is not without hazards but, I think these are more easily manageable. Radical initiators are not without hazard (peroxides, energetics like AIBN). It's a matter of TANSTAAFL.

YT2095 - 21-2-2007 at 08:57

that`s not the sort with the Alpha emitter and the lithium target is it?
that`s the sort I was planning several years ago, the only problem being is that in my highly evacuated apparatus that I make, I use Lithium as the Getter.

Sauron - 21-2-2007 at 09:35

No He nuclei, no Li target, and 6Li is RIGHT OUT.

No, this is just a circa 1932 Cockcroft & Watson electrostatic powered, heated filament source, not very tightly collimated particle beam. Under 400 KeV, which greatly simplifies construction. Good for chemistry, not so hot for physics. But not to be taken lightly. In particular I would not recommend doing what Lawrence did when he put his head in the beam.

You can accelerate electrons, or hydrogen nuclii (protons) but that's it. See the attached article above., along with safety discussion and article on constructing the electrostatic generator.

roamingnome - 21-2-2007 at 10:00

In a very recent Chemical & Engineering News (C&EN) a young female scientist is holding a Frisbee size neutral particle accelerator that was looking neat, in the next model they hope to collide them….

But for real experimentation I haven’t even made a working rail gun yet… and that stump still needs a quarter stick in it.. darn it…

Sauron - 21-2-2007 at 10:32

Did you read the article?

It's too early for April Fool's.

This is neither a joke nor a toy, and definitely not a weapon.

But it could be a practicable tool for a chemist. And not costly to build.

Read the article.

tnhrbtnhb - 21-2-2007 at 11:31

Quote:

Originally posted by Sauron
Did you read the article?

It's too early for April Fool's.

This is neither a joke nor a toy, and definitely not a weapon.

But it could be a practicable tool for a chemist. And not costly to build.

Read the article.

Personally, I think you might be better off with a cascade, not a van de graff generator. See www.powerlabs.org. You can get cascades out of the back of monitors and TVs. Also wouldn't building a cyclotron be cooler? Maybe you could use the magnets in a magnetron?
A Neutral particle accelerator?? How does that work? What was the article and where can I see it?

Sauron - 21-2-2007 at 11:51

Lotf things might be cooler but beyond the reach of anything less than a government or a multinational corporation.

I am not a nuclear physicist, and nuclear physics is not my goal. Nor was it the goal of the fellow in Buffalo NY who built this one in '59 for $100 or so. That was a piker's budget, even Cockcroft and Watson at Rutherford's lab at Cambridge spent $200 in 1932 dollars (or I suppose the equivalent in sterling) on theirs.

By the way apparently the proper spelling is Van de Graaff. I know I have bounced all over with variations myself...

By the way the reference I saw to a chemical reaction using high energy electrons was in PATR 2700 and they were citing a 1960 proceedings of a conference in Warsaw, which they called "Most interesting." I agree.

Ozone - 21-2-2007 at 16:55

All sorts of fun things can be done with high density electron flux...Like making X-rays and the like (100-150kV does this nicely, especially with a good target, like W).

Another interesting thing that can be done is to create Lichtenberg discharges, which I like very much. In this case, a piece of lucite it bombarded with electrons. The electrons lodge into the lattice structure and create electrostatic instability. A perturbation of the "charged" material, say, a swift rap with a grounded nail, produces an explosion as rediculous voltage is discharged through the plastic. The result--dendritic discharge channels.

I think these things are very cool!

Cheers,

O3

Theodore Gray_Lichtenberg discharge.jpg - 73kB

Theodore Gray_Lichtenberg discharge.jpg - 73kB

12AX7 - 21-2-2007 at 16:56

Beloit's physics department has a 500keV accelerator. Sort of. It's been in parts for a while without instructions, so on and off over the last 5 years they've been putting it back together.

It consists of a 15HP VdG, belt charged with a rectified NST (fed by variac) and charge comb. The (linear) tube appears to have concentric conical electrodes for focus. Thermionic cathode in the end. I recall hearing something about 1mA, so it's got pretty good capacity.

I don't see what good accelerators are, chemically. It's bad enough to run 50Ah through an electrochemical cell at 5V, but at 500,000!? (Granted incident electrons will scatter and cause multiple ionizations, but what's so special about that which lower energy bombardment or a plain mercury discharge can't do?)

I think accelerators are illegal (i.e. need license), at least those capable of ionizing radiation. Obviously there's a danger of soft x-ray radiation (bremstrahlung (sp)) above a few kV, turning into hard radiation in the hundreds of kV. Some thickness of metal will help that, starting with some sheet lead in the hundreds of k.

Tim

Sauron - 21-2-2007 at 20:00

Dr.Lee, who built this one in Buffalo, tested it for radiation for a year before publishing that article. As long as no heavy-element targets are irradiated this does not seem to be an issue. Typically, Lee was interested in irradiating organic substrates and so am I. Crowcroft and Watson irradiated Al and Li and made He in both cases. And some energy. For safety purposes I would like to know more about just what sort of energy they released and just where in the periodic table does light vs heavy demark? Obviously a W target, water cooled, is the classic target for X-ray generation. To be shunned, for my purposes. But W is very much on the heavy side, what about silicon as my liquids will be in Pyrex or quartz. Or do I have to build this vertically and irradiate through a flask mouth? Inconvenient for reacts involving a gas interaction with a liquid or another gas.

Yes this could be done with UV, but I find this more interesting.

The NRC's writ does not run where I am. The Thai Office of Atomic Energy for Peace is a lot easier to deal with, and as I am not going to be making isotopes or X-rays or endangering the public I do not think there will be a problem. I have a friend at OAEP, a gamma spectroscopist. He's a professor of physics at a Thai university. I will get his opinions and support.

Sauron - 21-2-2007 at 20:30

@O3, that is interesting, I take it the photo was a time exposure to catch the dendritic discharge which was a very rapid event. Lucite (acrylic) is the only material with this proprty?

This instrument wouldn't cost much to build - I am thinking the 1959 $100 is likely $1000 now. It wouldn't be all that big and heavy although it would be delicate. It would not consume much power (just the VdG motors and the filament heater. Not much downside.

By comparison a UV reactor with power supply and safety cabinet by the time it was shipped here from Vineland NJ would cost me $5000 on a 1 L scale (500 ml pot charges) and consume more electricity than this midget LINAC. Electricity isn't the issue obviously, I am just thinking out loud. A 5L US reactor would need a different lamp and power supply, and would not fit in safety cabinet so I'd have to make one here - I'd probably do that anyway with smaller one just because it's stupid to ship a cabinet from NJ.

I expect reactions of the FR type to be faster this way than with UV. Sounds like a job for a (chemical) loop reactor.

Sauron - 22-2-2007 at 00:53

Those Lichtenberg figures are fractal in nature, and while you can manually trigger them (after irradiation) with that grounded nail, they will also happen spontaneously if you keep irradiating to the point where dielectric breakdown occurs. Acrylic is a good dielectric (insulator) and it takes millions of volts to break it down like this.

Does anyone sell these Lichtenberg figures commercially? They are so neat looking, like frozen lightning, I bet they would sell.

The oil diffusion pump is an easy $1000 to $1500 by itself, capable of 10x-6 torr (10x-5 required) plus a power supply to heat a nichrome element and a cold trap. This is an auxiliary pump and still needs a high performance two stage primary pump to get down to where the diffusion pump will start to operate. The good news is that the oil diffusion pump will work, Hg diffusion pumps wll get down to 10x-7 but obviously are unjustified, cost more, and are hazardous (8 lbs Hg in glass.)

This, not the tube and not the VdG's will be the major expense.

Here's another Lichtenberg figure, courtesy Wilkepedia.

BTW Lichtenberg is also the fellow who discovered the principle behind xerography.

[Edited on 22-2-2007 by Sauron]

lichtenberg2.jpg.jpg - 99kB

not_important - 22-2-2007 at 02:51

look here

http://205.243.100.155/frames/interesting.html

Sauron - 22-2-2007 at 05:00

Oh well. I was right...just too late.

I do like the the photo of the actual dielectric breakdown.

Loud bang, bright flash.

[Edited on 22-2-2007 by Sauron]

12Inch_Discharge2x.jpg - 9kB

Sauron - 22-2-2007 at 05:17

The anode end of this LINAC tube is sealed with only a 0,001" thick Al foil supported by 1/4" thick Al disc that has been drilled with a pattern of closely spaced 3/16" holes in staggered rows, so that about 50% of the disc in beam path is absent.

Is that foil really supposed to not rupture with the tube evacuated to 0.01 micron (0.00001 torr)? I guess the key is the small diameter of the holes in the support plate.

I am going to investigate the vacuum holding capacity of the unions for process pipe, because if they will hold a high vacuum (0.01 micron) then this will allow for interchangeable cathodes and anodes which will greatly simplify maintanence and make the tube more versatile. I was unimpressed by the construction details of the cathode end of the Lee design. I'm sure it worked, but it seemed a wee but quick and dirty.

If not process pipe then flat ground flanged joints maybe with an O-ring groove in one face. Those are stock glassblower items from Ace and ChemGlass etc.

[Edited on 22-2-2007 by Sauron]

not_important - 22-2-2007 at 08:54

What's the force (pressure differential) at 100 mm vac? At 10, and at 1 mm? Then how much additional force is applied in going from 1 mm to 0.01 micron?

Sauron - 22-2-2007 at 09:10

I know a socratic question when I see one but I don't want to do the math. I'm too old for pedagogy.

But thanks for asking.

I have appended the compilation of Amateur Scientist articles dealing with the LINAC and VDG generator power supply for it, with an additional article from 1955 referenced in the 1957 article.

I have replaced the compilation in the first post of this thread, and those of you who haven't yet downloaded it can get the latest one there.

For those of you who already have the main file here's the latest addition.

[Edited on 22-2-2007 by Sauron]

[Edited on 22-2-2007 by Sauron]

Attachment: http___vacuum.ramapo.edu_physics_physics-doc_amsci_AmSci01_1955_04_1955-04-fs.pdf (397kB)
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Sauron - 22-2-2007 at 12:18

At first I assumed quite incorrectly that this 2 ft long LINAC tube is mounted horizontally, however, Franklin R. Lee built this one vertically. He has the collector if the VdG generator touching the collector of the anode end of the LINAC and he has a microammeter (of sorts) built into the collector dome. This leaves only a small space for a sample to be interposed for irradiation.

This would still allow me to plumb a loop through that space. I was figuring on a CSLR setup anyway (continuously stirred loop reactor) as most of the free-radical reactions I am interested in using this for involve a gas and a liquid. Chlorine and acetic acid, chlorine and methyl formate, CCl4 and NO, etc. A peristaltic pump with a PTFE head and tubing would be chemically inert and thin enough. (The thickness issue is only critical in beam path). I have a good number of Microflex LE peristaltic pumps on hand, no PTFE drive head but I do have a PTFE diaphragm head that will work.

What tubing material to put in beam path? Lee gives some rough numbers for what a 250KeV electron beam will penetrate, this machine is about half again as powerful. For example he says 0.25 mm Al (about 0.01 in) vs 3 mm of PE or several meters of air. And he uses a 0.001 in Al foil to close the tube window. He describes the relationship between beam penetration and density of target and also target molecular weight.

I think a thin glass or quartz tube in beam path may be best. The beam effect on plastics may be harder to predict. Lee discusses PE crosslinking briefly; a good effect.

Obviously metal tubing is out, the lighter metals lack inertness and the inert ones like tantalum are heavy enough to produce hard X-rays when struck by an electron beam.

I guess I will be reading up on electron beam effects on materials.

Sauron - 22-2-2007 at 14:51

The design theory of a VdG is interesting.

The theoretical potential of the machine is determined by the smallest radius of curvature of the spherical collector. Multiply that by 70,000 and you have the theoretical voltage - if the collector were a perfect sphere.

However, as the collector generally has a hole in its bottom for the belt system to enter, the best real world performance is on the order of 84-85% of theory. That assumes a few things about proper machine design were obeyed.

The current is determined by the surface area of the belt rubbing the fixed upper bearing per second. The rule of thumb is: 100 sq in per second = 1 microampere. The partical limit to belt speed is 100 ft per second so a 1" wide belt gives 12 microamps, a 2" wide belt 24 microamps

I am evaluating a commercial VdG that is 10" diameter collector, 30" high and they claim 400,000 volts. This is a $600 unit.

Well, from Dr.Lee's numbers we can quickly see the theoretical maximum is 350,000 V and the likely real potential is 300,000. The seller does not enlighten about how many microamps. Anyway the seller's claims are looking a trifle high. Of course it is not necessary to reply on rules of thumb as an electrometer will tell the tale.

It is certainly true that anyone feeling deprived by the odd 100,000 extra V or two can replace the collector with a larger one without a tgreat deal of effort. As long as you don't exceed half a million volts your LINAC tube design problems remain simple. You wrap 4 turns of copper wire asoubd the tube every 2 inches or so and twist the ends into a corona point. Above 500,000 V this is done with fixed resistors. Anyway 300-400 K is enough for me.

The belts are rubber and the thinner the better. So you can see why rpms can't be pushed beyond a certain limit and why spare belts are a Really Good Idea. Like, there you are doing your E.O.Lawrence impression and the experiment halts because you have to change what is after all just a big rubber band. Really! It's like having your Bently stop because the hamsters in the engine need a break.

Fear not, latex sheet is easy to come by, one does not have to pay ripoff spare parts prices from the OEM.

Sauron - 23-2-2007 at 02:04

Upon reflection, I don't need flanged ground joints to achieve modularity, rge ebds of this 2 to 3 ft Pyrex tube (not process pipe) 2" diameter will just have 45/50 standard taper joints and these can be sealed with high vacuum grease and clamped quite conventionally. The screw-together clamps probably better than Keck plastic clamps. A great variety of adapters to choose from for the cathose thermionic source (filament and leads) and the vacuum takeoff. Probably Ace threads and their fittings.

Same at the other end, allowing for change of the anode, window etc.

I found a second large commercial VdG, this one has a 12" collector rather than 10" and is about 10% cheaper ($550 vs $605).

I found a smaller VDG locally for $300 but it seems like maybe 150,000 volts (6 or 8 jnch collector) so not very attractive. Still it is unclear whether or not I can get 220V AC versions from the US so I may snag this one just to get the motor. I also might take it, plu a larger collector on it, increase the height of the insulator and length of the belt, and be happy. That sort of depends on whether the belt diameter is adequate (>1.5") which is what determins the surface area of friction and therefore the current.

I found an article with construction details of a 500 KeV 200 uA VDG circa 1938 with a 24 x 12" flattened toroidal collector and obviously a very wide belt. The only advantage of the toroidal collector compared to a 12" sphere was extra capacitance (fatter sparks) not extra voltage but that 10X increase in amperage is impressive. To do that with a sphere you'd have a diameter for insulator of 20 inches so a sphere of 1 to 1.5 meters and a voltage of 1.5 to 2 MeV less yhat pesky 15%. Still a real beast, it would need to be 5-6 meters high.

Sauron - 23-2-2007 at 05:42

Many more commercial VDGs have turned up, the top end of anything affordable being 400-500KV. Which is fine. Resonance Research makes museum grade machines up to 3 Megavolts but I doubt any of us could handle the cost. Or have anyplace to put it.

One commercial machine claims to be 750KV with a 17" collector, while their 14" collectors are 400K. I think the 17" models are interesting but won't exceed 500K (what does an extra 1.5 inches of radius get you at 70,000 per inch?)

A word of caution to those who like me are inclined to throw $$$ at problems.

The VDG required to build an electron accelerating LINAC needs to be positively charged at the collector. Duh! Well, most sellers do not even tell you whether their VDGs are built to be + at collector or - at collector. And it seems that many of them are minus, because such machines charge faster and give longer sparks. One company makes them both ways and specifies this in description. Their units are 400KV, 14" SS collectors and c.$600, and they offer both 110V and 220V models. These folks get full marks from me.

They even put the manual on the web, something I wish all sellers would do.

Here it is as attachment.

[Edited on 23-2-2007 by Sauron]

Attachment: 10085[1].pdf (762kB)
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Sauron - 23-2-2007 at 06:16

say, isn't that Hillary hiding behind the VdG?

[Edited on 23-2-2007 by Sauron]

10085.jpg - 92kB

Sauron - 23-2-2007 at 22:35

I just found out that this neat company in Buffalo was in fact started by Frank Lee, the guy who designed this VDG-powered LINAC in the first place (well, with a lot of help from Messers Cockcroft & Watson) and after he retired his daughter Nancy Bell has kept it going. So it is fitting and proper that I buy my VDG from same folks. See their website

Http://www.sciencefirst.com

Sauron - 24-2-2007 at 08:32

Robert Van de Graaff after WWII made a fortune in X-ray machines and was heavily involved in the establishment of Brookhaven National Laboratory, which was the Eastern Establishment's rival to the Berkeley physica mafia (Lawrence, Teller, Oppenheimer, et al).

The larger more powerful electrostatic generators for particle accelerators and x-ray machines (very closely related) used compressed Freon as insulator - the entire Van de Graaf apparatus was pressurized. Freon did same job as air but at 1/3 the pressure. These machines were in the 1.5 to 3 megavolt class with belts typically 12-16 inch wide.

Incidentally, internal excitation (friction type) VDGs are the simplest but not necessarily the best. External excitation machines still use a belt for charge transport but have an external high voltage DC power supply that is connected to a lower comb, this charges the belt which continuously transports the charge to the collector terminal, then returns to pick up more charge. The DC supply might be 5000-25,000 V or more, at an amperage 10-20X that of the VDG output. So for example a 2" belt for a benchtop machine in the 500,000V class would be about 20 uA so the DC supply ought to be 200 uA. At 5000 V this would be a 1 Watt supply; at 25,000 V a 5 W supply.

Friction based machines are ertainly safer.

Hey you electronics genuises: would it be feasuble to take the output of a Tesla coil and rectify it (diode?) and use that as the DC supply for an externally excited VDG? I will have to look at the output specs of a typical small, commercially available Tesla coil.

12AX7 - 24-2-2007 at 09:52

You have all the voltage you want, but you need a diode to rectify it, without also disturbing the resonance too much. A tube rectifier would be perfect, but the best you'll find off the shelf is maybe 50kV, 1mA or so (3A3C, etc.).

Still enough for kicking electrons around and x-ray nuking someone's testicles, but nowhere near 500keV.

Tim

Sauron - 24-2-2007 at 11:55

50 KV and 1 mA would be perfectly fine, the Tesla coil I am looking at puts out 5 KV, the frigging manual does not say how much current, but it will be measurable, therefore known.

Anyway another solution is at hand. Several VDG sites recommend for this purpose using a negative ion generator (examples, $11 from Electronics Goldmine, 12V DC input and 120 VAC input).

The voltage of the external excitation does not determine the maximum potential of the VDG, it's an additive machine. It keeping fetching more and more and more till the max potential is achieved, and that is determined by the radius of the collector terminal (sphere at top). As mentioned above, the rule of thumb is 70,000V per inch of radius so for example a 7" radius, 490,000 V. The deviation from a perfect sphere because of the insulating column and its requisite hole in the collector cuts that back by 16% so roughly 400,000 V.

The external excitation source replaces the friction of belt and top roller as the source of the electricity being collected. The external power supply's appropriate output polarity (it is DC) is connected to a comb (brush) at the lower end of the belt which almost but not quite touches. This "sprays" charge onto the belt.

The speed of rotation and the surface area of the belt determine the (small) current output maximum of the VDG. For a 2" belt at 100 foot/seconds that is theoretically 24 microamperes. That is typical for a large benchtop machine.

There's a modification I am just studying now which apparently allows that current capacity to be doubled.

So far we still have a very inherently safe system. Where you can get into dangerous waters is if you add some serious capacitance between the VDG terminal and the anode (sphere of same size in contact with the collector) of the LINAC. Seperate those spheres and interpose one or more Leyden jars and you have a LOT of DC in there, high voltages and dangerously high currents. NOT recommended! One mistake and no second chances.

Incidentally, I found a page about a quite recent (1990s) invention, a very simple elegant electrostatic generator by a university professor in spain.

Four rollers all in same plane in a frame. All in contact with the next. #1 is metal, #2 is insulating (teflon), $3 is insulating (nylon) and #4 is metal. Turn one (by hand or by motor) and all turn because they are in contact. The two insulators are at opposite ends of the electrostatic scale and so electrons are lost by one to the other. Opposite charges accumulate on the conductive metal end rollers #1 and #4. Typical output for 2" diameter rollers of say 12" length 50,000V at a few microamps.

Simple, cheap, elegant. String 10 together in series, you have half a million volts Anyone want the URL, the US patent number, and/or the EU patent number? There are still worlds to conquer.

tnhrbtnhb - 24-2-2007 at 13:34

Why do you keep posting like this? It's not hard to get 50 Kv at 1mA DC. Like I said, just use a cascade and a flyback transformer. If you are not picky about noise, use a marx generator.

What you are trying to do here is not that hard - just go for it.

Personally I think getting the beam out of the accelerator will be the hardest part.

Sauron - 24-2-2007 at 17:23

I'm not trying to get 50K DC @ 1 mA. I only need about 150-250 uA and as little as 10-20K DC maybe less, so a cascade is not ideal for me. But thanks for the suggestion.

Frank Lee's machine had more than enough penetration to make it through his Al foil 1/1000-inch window and that was at 350 KV. So why would this one have a penetration problem at somewhere between 500 and 750 KV?

[Edited on 25-2-2007 by Sauron]

not_important - 24-2-2007 at 22:37

Sounds similar to USP 4990813 developed in the late 1980s. I'e just a terse description, so I don't know how close it is and I'm too lazy to go do a retrieval of the patent.

Sauron - 24-2-2007 at 22:54

Here's the web page of the Spanish one;

In fact thisis US 4994813.

Well done, @n_i !

Now, couldn't the two metallic rollers very easily be replaced with combs thus reducing size and weight? The business end of the generator is the pair of dissimilar insulators as far as I can see.

(Perhaps that would not have been patentable.)

Maybe the metal rollers are more electricallyefficient but not from standpoint of size and weight. By eliminating the two conductive rollers you could double the diameter of the remaining two rollers this increasing the output of the generator for a given height and rpm; only downside would be doubling thickness.

Perhaps I should read the patent before speculating.

I am not sure (per the remarks of the fan of the cascade and flyback xformer) that the Lorentz is necessarily better than some other solutions or even some other electrostatic generator designs but it IS damned simple isn't it? Its main downside is that it simply does not have the look of some classic Hammer Horror lab equipment like the Wimhurst and the Van de Graaff and the Jacob's Ladder -- things you'd expect to see Peter Cushing using (never mind that there's a serious anachronism problem.)

[Edited on 26-2-2007 by Sauron]

Attachment: lorentz.pdf (401kB)
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jpsmith123 - 25-2-2007 at 19:00

I bought some Van De Graaff parts from Frank Lee about 20 yrs ago. I think it was a kit, minus a motor and chassis.

The collector was an oblate spheroid of about 14" diameter, IIRC.

It worked quite well and was a lot of fun to play with.

I never did get around to building the accelerator tube, although I did think about it quite a bit, and I had a few designs on paper.

Nowadays, if I were going to build something and extract a beam into the atmosphere, I think I would opt for some kind of pulsed accelerator, possibly using either an air-core pulse transformer or a transmission line transformer (I guess technically that could be considered an air-core transformer too).

Attached is Van De Graaff's patent.

Attachment: VDG_Patent.pdf (1002kB)
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Sauron - 26-2-2007 at 02:43

Thanks. I've got the abstract that Van de Graaff presented at the 1931 Schenectdahy meeting of the APS describing his first machine and I have requested the 1933 paper he published in Physical Review with a complete description. As you can surmise the 1931 presentation obviously inspired Cockcroft and Watson's experiment the following year but indeed it was Rutherford's 1927 call for high voltage technology for particle accelerators that at least partially was Van De Graaff's own inspiration, or so he says himself. As Cockcroft and Watson were at Rutherfor'd laboratory this is a really sterling example of scientific cross-fertilization.

The machine you bought, @jpsmith, is still sold but not in kit form. They rate it as 400,000V and that's correct for the collector diameter. The motor is 3000 rpm, and that ought to produce 20 uA with the 2" belt.

Van de Graaff's original machine had larger copper collectors, solid pyrex rods and the silk belts ran externally. The generators operated in pairs with opposite polarity so the original machine was rated at 1.5 million volts. Excitation was external, a 10,000VDC power supply connected to a comb at the bottom of the belt run spraying ions onto the belt. I am not sure of the output current of the supply, probably 1 mA or less.

I gather that the first article, and very likely the patent, which I have not yet read, cover induction as excitation, in addition to ion spraying, and also the option of internal (rolling friction) excitation which is the norm on small didactice machines like yours and the ones Frank Lee's company (ScienceFirst) still sells. This has the virtue of great cimplicity and low cost but at the expense of reliability. High humidity can be a real problem, and dust is a serious problem, and all electrostatic devices are dust magnets.

But I guess for younger amatuers a HV DC supply is a wee bit hazardous.

Van de Graaff studied under Marie Curie at the Sorbonne and got his PhD in the UK, but he was an American. He was a National Research Fellow at Princeton when he invented the generator and an associate professor at MIT when he published the first of many article about it in PR.

Here's a ohoto of him with (I think) the original, bipolar VDG generator. I believe those collectors are 24" spheres.

[Edited on 26-2-2007 by Sauron]

Fig3.jpg - 38kB

Sauron - 26-2-2007 at 03:23

As big as those units are compared to the usual tabletop didactice machines we see today, they are nothing alongside the monster MIT dual VDG built at Round Hill and presently at the Thomson Theatre of Electricity, Boston Museum of Science. That one is the largest extant VdG surviving. The collector is 6 feet diameter and the column 23 feet high, maximum potential of the dual machine was in excess of 5 megavolts.

A giant bipolar set built in France in the 30s and exhibited was planned to be used for isotope production but due to wartime exigencies was taken apart and later scrapped. C'est dommage.

[Edited on 26-2-2007 by Sauron]

Fig5.jpg - 27kB

Sauron - 26-2-2007 at 18:24

I just got off the phone with my physics professor buddy who is now a very senior official in the Thai Office of Atomic Energy for Peace, within the Ministry of Science & Technology. His specialty is gamma spectroscopy.

Anyway he tells me that as long as I am not bombarding heavy-element targets (to produce X-rays) or making isotopes that the VDGs and small LINAC are not under the authority of his office or covered by national law.

Green light! as I have no interest in either X-rays or making isotopes. The tube in Frank Lee's design is not designed for either purpose as it is not tightly collimated and the (liquid/gas) target is not in vacuo.

This is good news.

Sauron - 28-2-2007 at 07:16

With some help from the angels of References I obtained the original Cockcroft and Walton papers from Proc.Roy.Soc.London (1932).

I'm afraid that Carl Stong's account of their experiment was not particularly accurate.

Contrary to his description in The Amateur Scientist, the British team at Rutherford's lab did not employ an electrostatic generator to power their accelerator tube. They did decide that they wanted steady current rather than AC or Tesla coil or rippling current. They used conventional electronic components, albeit large and custom ones, to make a voltage multiplier that could delliver up to 800,000 VDC @ 10 microamperes.

And they did demonstrate that transmutation of elements could occur with particles charged to only 150 KV but that was the threshold of their experimental potential not the maximim. The practical maximum was about 710 KV.

They bombarded metallic and nonmetallic targets in air, after determining the (quite short) penetration of protons and H2+ ions in air (a few cm). Lithium, beryllium and aluminum metals along with fluorine (as CaF2) and boron all had high nuclear reactivity with the proton beam and emitted alpha particles (He nuclei). Other elements were less reactive, up to and including uranium. Reactivity was monitored by observing a ZnS screen through an optical microscope and counting scintillations. These became more numerous as voltage was stepped up above 150 KV.

Now, they could have used an electrostatic generator to good effect, but the fact is they did not. Maybe they did so later; certainly in 1932 they felt that their voltage multiplier circuitry had reached its limits. VDG generators can and do operate in multiple megavolts at a steady state.

No question but that Van de Graaff's work was partially inspired by Lord Rutherford's 1927 call for steady high voltage sources for nuclear research. The inventor says so himself in his early papers in Physical Review. And he lauds Cockcroft and Walton's experiemnts as "brilliant" which they were. But, contrary to Stong, electrostatics played no part in those experiments.

Sauron - 1-3-2007 at 00:36

I have been assiduously studying VDG design and now know a little of how these electrostatic generators work, how to make them more reliable, how to double the charge (in microamps) they can produce, and how to set up an external excitation system if desired, using either a small electronic high voltage DC supply like a negative ion generator, or a different electrostatic generator like a Toeppler or Bonetti machine (which resemble the old Wimhurst machine of Frankenstein films, but put out 2-4X more current.)

So I have now turned my attention to the accelerator tube design. The use of a metal foil "window" on a honeycomb support was pretty standard for cathode ray tubes where it was desired to have the beam exit the tube into the air.

Lenard, Ann.d.Physik. 51, 225-267 (1894)

But aluminum foil wasnot the best choice for the material. At least as of 1926 Coolidge (of GE fame) taught that nickel foil was superior to aluminum and enumerated the reasons.

Alfa Aeser has nickel foil; household Al foil is clearly just an expedient.

Coolidge describes nickel foil 0.0127 mm thick for window. Stong and Lee specify Al foil not more than .001 inch thick (.025 inch) or about double what Coolidge advised. Maybe this is within acceptable limits.

Further study is indicated. There are additional issues concerning homogeneity of the Pyrex tube (air bubble can provide spots for breakdown by perforation); difficulties with the high vacuum system, etc.

Coolidge resolved a lot of these problems with his "cascade" multisegmented tube array. Shorter segments of cathode ray tubed flanged together, seperated only by the foil windows, had several advantages. They could be evacuated to high vacuum and sealed rather than having to be evacuated continuously as was the usual practice. The foil windows (except for the external one) required t\no support, since there was vacuum on both sides. The internal foil windows functioned as the anode for one tube and cathode for the next. Most importantly the overall voltage of the array is divided among the individual tubes as example, a four segment tube having first tube of 250,000 V, second, third and forth of 350,000 V each, total 1.3 MV which would be a tricky tube to build and operate as a single unit.

W.D.Coolidge, J.Frank.Instit. 202, p 693 (1026)

The details of the Coolidge cascade however are quite a bit more complex than the single tube described by Stong and Lee, and are not so amatuer-friendly.

So as I said, I must read on before plunging in.

[Edited on 1-3-2007 by Sauron]

Maya - 1-3-2007 at 08:35

How are you going to get a VDG to work in high humidity, Thailand?

won't happen. you are way better off with a CW multiplier, only thing is the diodes are critical. you need high speed switching HV ones. the common ones won't switch at higher than maybe 400 Hz and you need over 10,000 Hz

then get some caps and you're done after mastering your corona minimizing techniques ( which is an absolute BITCH at anything over 40,000 Volts! )

Personally , I have a stack that'll easily get to 60,000V but made from the slow switching kind.

parts to make a high speed multiplier up to 200,000V, and a pair of spellman dual polarity +/- 40Kv = total 80Kv modules!

The hardest thing to get rid of is the corona, humidity and dirt kill you

Sauron - 1-3-2007 at 11:41

@Maya

First of all Thailand's humidity isn't that high. Secondly, there are a few tricks to getting a VDG towork in humidity at least to 90% or so.

The belts run inside of the insulator (tubular column) and it is not a big deal to add inlets and outlets for a recirculating warm air system with an inline dessicant tower (indicating Drierite). Same way we keep our dry box (glove box) dry even when it is pourind down raining outside. What's the big deal?

The old trick was to use an electric hair dryer to blow hot air through the column before using.

Frankly, it is more of a problem to keep the sphere and the belt free of dust and oil (even oil from the hands and fingers can bugger up a VDG) especially given that static is a dust magnet.

Another thing is that a lot of commercial VDG manufacturers are not too thoughtful about selection of materials for the rollers. For self excited systems some work better than others. Obviously, it's hard to make specific comments because it depends on polarity and other factors but just changing a roller out to a PTFE roller can greatly improve performance and reliability of a self excited VDG and make it work first time every time even in high humidity.

There's an academic physicist down in Brasil whose passion is old electrostatic generators, he builds replicas of antique machines and has a huge well documented, referenced, and brilliantly illustrated website on the subject. VDGs are almost too modern for him, his main focus are Wimshurst machines and their variants. But he did built a pair of smaller VDGs patterned after the ones I have pictured above, and over the years he overcame the problems he initially had with humidity.

IMO part of the problem was that he went with an external belt and solid insulator and therefore repeated an early Van de Graaff mistake. The entry and exit slits in the sphere were too proximate to the belt. Going to a hollow (tube) insulator and placing the belt inside it helped. And that is why you hardly ever encounter any other sort of VDG today.

I am only mucking around at or below 500 KV per machine. If I wanted or needed to cross into megavolts, then I would have to build a talk around the VDG, fitted with pressure tight electrical connections in and out, pump it full of freon @100-150 spi (which is not a whole lot) and no longer be limited to the 70,000 V dielectric strength of air. And obviously humidity would not be any sort of issue.

BTW ozone from the corona discharge slowly deteriorates rubber belts. However, lots of other materials work fine as belts. The trick is to make sure it is not hygroscopic.

Many DIY builders use PVC insulators and PVC rollers and that is not a really good choice, for that reason.

Sauron - 1-3-2007 at 12:00

@Maya

I see you did discuss dirt (dust)

For corona management on the generator (VDG) they went to equipotential rings between the terminal and ground to achieve electrical equilibrium. On units up to 500 KV they are not mandatory. On the accelerator tube up to 500 KV you wind 4 turns of copper wire around the tube every 2" and twist the ends into corona points. Above 500 KV you are better off going to a (Coolidge) cascade series of smaller tubes strung end to end. These tubes (unlike the one in the SA article) have complex shielding and equipotential distribution, and no individual tube bears the entire potential.

Both Van de Graaff and his associates at MIT and HVEC, and Coolidge at GE were deeply involved in X-ray machines, power supplies and tubes and licked all these problems. Basically an X-ray tube is just a cathode ray tube with a heavy metal target (like W) so the technologies for X-ray ("artificial gamma") and electron beams, proton beams, and various ion beams are really the same.

But you know all that.

I am preaching to the choir.

Sauron - 1-3-2007 at 13:02

http://www.coe.ufrj.br/~acmq/myvdg.html

That's the great Brasilian site, specifically the page on the suthor's matched pair of VDGs, built himself.

In particular see the section on "Improving the Machines"

With those changes the machines went from being erratic, unpredictable performers in self excited (rolling friction) mode to being highly reliable.

He also did a lot of work with external excitation of the belt, which is inherently more reliable than rolling friction, but is rarely encountered on the inexpensive single polarity machines usually sold for educational or "let's make some pretty sparks" purposes. Formerly they were more common; a kenotron rectifier supplies 10-15KV at maybe 100-200 microamps to the belt by "spraying" from a comb.

Today experimenters usually acquire a surplus "negative ion generator" and modify it to do the same thing. The ones I have looked at are typically 7.5 to 15 KV and less than a mA and are available for mains power input or 12VDC input. They sell for as little as $11-$13. The charge that the belt can carry is determined by the surface area of the belt (mostly) but you want the supply to always be 10-20X the capacity.

The Brasilian physicist went one further and excited his VDGs with another one of his electrostatic generators, one that puts out a lot more current than most, the Toeppler or Bonetti machines. I rather like this outright refusal to incorporate any electronics at all! and it does work. He built those disk machines himself, too.

The machines (VDGs) built by the author of the website linked above are c.180KV each, or about 1/3 the potential of mine (now on order). My intention is to build a tube to handle 500 KV first, and then use a pair of VDGs of opposite polarity and build a tube for 1 megavolt at 50-100 microamps. For my purposes (chemical in nature) I don't need to exceed that and if the second phase is too difficult the first phase will suffice.

Maya - 1-3-2007 at 17:20

Quote:

But you know all that. I am preaching to the choir.

yeah,

I can see your hellbent for > 500Kv which only VDG's deliver

my suggestion was only for meager under 300Kv

constant high current. But, I can see that you've not handled
corona issues B4. otherwise you woulda' acknowledged them as quite a drain and burden as a function of increasing voltage upwards of 50,000V

well , at least small sphere vdg's you can't kill yourself, you need a huge sphere to build up capacitance

think joules..............

Maya - 1-3-2007 at 17:27

yeah , I forgot

Big spheres are emormously expensive

much more than solid state devices

Maximum V is obviously a function of sphere size

??what exactly are you hoping to transmut?

[Edited on 2-3-2007 by Maya]

Pulsed Accelerators

jpsmith123 - 1-3-2007 at 18:44

Pulsed accelerators may offer some advantages if continuous operation is not a necessity. And there are some interesting animals out there.

The high voltage source can be a pulse transformer, a tesla type resonance transformer, a marx generator, a stacked blumlein, a spiral generator, other "adder" topologies, or various combinations thereof.

Under pulsed conditions, insulators can hold off much higher voltages than in the steady state.

And pulse transformers can be very compact. Sandia Labs has got 3 MV transformers in sizes of only a few feet in the largest dimension.

I have many papers (pdf) covering design of high vpltage pulse transformers that could be used to power an accelerator. A user-friendly spice simulator like CircuitMaker can be easily used to explore the effect of source and load impedance, capacitive loading, coil inductance and coupling coefficient, pulse width, etc.

Just for reference, I'm attaching a paper describing a rather novel pulse accelerator that uses what I guess you would call a helical resonator.

Attachment: Helical Resonator Accelerator 2.pdf (98kB)
This file has been downloaded 1133 times

Sauron - 1-3-2007 at 20:15

@Maya

I am hoping to transmute boredom into amusement. My ambitions for the electron beam are purely free radical reactions, naught else. Oh, maybe I might stick some acrylic in there and make a lichtenberg figure, those are nifty.

Yeah, it is a Good Thing, from my viewpoint, that it is really kind of hard to kill yourself with 50 microamps even at these voltahes.

Sauron - 1-3-2007 at 20:43

The spheres on mine are 17" and the belts are 2". The ones above in the pic with the inventor are 24" and the belts 2.2".

Theory says 70,000 V per inch of radius and the rule of thub is you really get 84-85% of that because of the losses associated with the opening at bottom of sphere for column. There are a few tricks about fabrication, the edges of the opening must be curved inwards with a generous radius because sharp edges are Bad from a corona point of view.

Mine are SS, most cheap ones are Al, Van de Graaff used Cu for those 24 inchers and Al for that monsyer at Round Hill (which is still working, at Museum of Science, Boston.) There's a guy in Calif. who built his own 30" VDG (just a few years ago.)

I am buying mine from Frank Lee's old company in Buffalo now called Science First. They mostly make models with negative collectors but will build positive-collector units on request.

And they do them in 220V which is important for me. So I like this company.

unionised - 3-3-2007 at 06:22

That's a nice little V de G they have in the boston museum. If you want a big one have a look on Google earth.
The foot of it is at 53 20 34 North 2 38 17.5 West
The end of the shadow is at 53 20 36N 2 38 20W
There's a tandem 30 MV V de G accelerator in that tower- unfortunately, due to government cutbacks, it is currently mothballed.
There's a picture of the lab here.
http://www.cclrc.ac.uk/Activities/Gallery/DL0508202_500.jpg
The V de G is in the tower at the back.

Sauron - 3-3-2007 at 06:55

So who's a size queen?

No moths on my balls.

unionised - 4-3-2007 at 06:01

"No moths on my balls. "Glad to hear it.

Anyway, none of the "big boys' toys" really addresses the topic; they are hardly DIY.
As has been mentioned, the tricky bit is getting enough energy into the "projectile". A V de G will give you practically unlimited voltage if you have the balls for it (if you will forgive the expression) but you could always look at cyclotron type accelerators.

Sauron - 4-3-2007 at 06:56

If I were after nuclear interactions I'd have little choice. But I'm just interested in chemical reactions (like free radical production) and so 500 KV to 1 MV and 50-100 microamps is plenty. Cockcroft and Walton made do with 150 KV to 700 KV and they were doing transmutations (albeit really miniscule ones that they could only detect by scintillation counting.)

In reading about what the Big Boys are doing I am somewhat disheartened at the utter insignificance of such a setup as I have in mind, even in Thailand they had a major accelerator at Chiang Mai at the Fast Neuiron lab. Serious electron beams are merely technology for welding, evaporation, etc. Even more serious electron beams are input for free electron lasers. Or injectors for synchrotrons. So what I am doing is like a caveman rubbing two sticks together standing next to a guy with a plasma arc welder. No Can Compete!

------------

Actually I ought to eat those words. In the 1940s a group of high school kids in El Cerrito CA (a suburb of Berkeley) decided to build a small cyclotron. They got technical advice from the Rad Lab at UC Berkeley (Lawrence's lab), they got financial support from the school and the school district, it took them 2 years and they DID it. A 1000 lb magnet, 800 lbs of magnet wire, a working 6" cyclotron putting out a 7 microamp proton beam. I am in awe. They worked with high voltages and currents of very lethal proportions on equipment they built themselves. All this was detailed in a 1953 The Amateur Scientist article (before C.T.Stong's tenure) which I will post here.

Stong actually revisited the subject of VDG-powered DIY particle accelerators that he and Frank Lee wrote about in the same SA column in 1957. Stong wrote another column in 1971 about a different homemade accelerator, this one for protons and deuterons, but using a Frank Lee (Morris and Lee, now Science First) 500 KV VDG. The accelerator tube is quite different from the one described 14 years earlier, and naturally the source is different as well. I will also post this article here.

The Bell Jar is a quarterly publication devoted to vacuum science published by Steve Hansen in New Hampshire. Many of the articles from this fine periodical are available online, and anyone interested in medium to high vacuum will benefit from checking this out. Frank Lee has some recollections published there, and there's an article about vacuum and The Amateur Scientist column under Stong.

Of particular interest to me is an article on how to achieve hard vacuum without diffusion pump, using a two stage mechanical oil sealed pump and a special zeolite trap to remove water vapor and back-diffusing oil vapor. Diffusion pumps are costly and require a roughing pump anyway so this technique will be a good one for me in my accelerator tube work.

While the homemade tubes described by Stong relied on vacuum wax for sealing and assembly, I prefer to put my faith in kovar or copper graded seals that are professionally done by glassblowers who know how to produce glass to metal and metal to metal seals and joints that are vacuum tight to 10x-5 torr.

[Edited on 7-3-2007 by Sauron]

Attachment: http___vacuum.ramapo.edu_physics_physics-doc_amsci_AmSci01_1953_09_1953-09-fs.zip (374kB)
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cyclotron

jtkelectroman - 15-3-2007 at 14:07

For starters I would say Hi this the first time I have replied to this board but I have visited it many times before.
Now I saw this thread and I thought I would take the time to say that since august of 2003 I have been building my own Cyclotron with some help from the staff over at St.Cloud state but keep in mind that I am the sole funder of this project and it was my Idea to start it.
So far I have mannaged to build the cyclotron cavity as can be seen in the picture below. I have also nearly completed the UHV system but I still need a mechanical vacuum pump and foreline filters.
The gas handeling system is complete minus pressure guages.
I have also aquired a 19"W by 56" tall rack mount type cart and I have begun installing the cyclotrons controll systems. I have allready installed an ion beam current meter, a kepco DC power supply for the ion source, and an Rf frequency generator. I also was offered a 1500 watt Vicor MegaPack power supply from Timension Inc.

I also have several Ionization guages but no controllers for them.
I also aquired a small compact Varian Ion implantation magnet which I plan on refurbishing so that it will create a magnetic field somewhere arround 1.2T.

Now as you may have guessed this equipment is very expensive even the second hand stuff and So I still need to find a way to come up with the funds for the following equipment yet:
Cryotrap
High power RF Amplifier with 1Kv Ac p-p or higher
mechanical pump
Rf frequency counter
Large spool of magnet wire to make new 8" diameter coils/yolkes for the ion implantation magnet.
Two 7inch diameter pole faces for magnet
used ocilloscope 20Mhz or higher

Please visit my website and give me your input and if anyone has any old equipment lying around that they don't need that they wouldn't mind donating for the cyclotron project please let me know.

[img]http://www.angelfire.com/weird2/chemistrymn/cyclotron_photo_album/index.album/cyclotron-cavity?i=1[/img]

Cyclotron Photo

jtkelectroman - 15-3-2007 at 14:10

Here hopefully the immage shows up. If it does here is a picture of the cyclotron cavity.

My website

jtkelectroman - 15-3-2007 at 14:12

Heres a link to my webpage.

http://www.angelfire.com/weird2/chemistrymn/cyclotron_photo_...

Maya - 15-3-2007 at 16:47

<<Cryotrap
1 High power RF Amplifier with 1Kv Ac p-p or higher
2 mechanical pump
3 Rf frequency counter
4 Large spool of magnet wire to make new 8" diameter coils/yolkes for the ion implantation magnet.
5 Two 7inch diameter pole faces for magnet
6 used ocilloscope 20Mhz or higher>>

# 1, 5 IDK about
but
# 2, 4, 6 I've picked up for under $100 ea at a famous action
# 3 I bet U could as well

# 4 actually I've picked up all made as from an x ray transformer secondary at least
10 inches across

Oh, I forgot the cryotrap,
I picked up one savant for $10, didn't work.
I picked up another for $49 which got down to -90 degrees

[Edited on 15-3-2007 by Maya]

Sauron - 15-3-2007 at 20:12

Very ambitious!

I salute you.

HOWEVER your attachment or its host placed a popup on my PC which is likely to be malware. At least, the popup occured just after I opened your link. It was a come on for some screensaver (ostensibly).

Members be warned.

Here is an illustration of a Coolidge cascade accelerator tube from a 1935 Physical Review article by Tuve et al, referenced below.

[Edited on 17-3-2007 by Sauron]

[Edited on 17-3-2007 by Sauron]

[Edited on 17-3-2007 by Sauron]

Tuveetal.jpg - 31kB

not_important - 15-3-2007 at 21:57

www.angelfire.com

is warning enough. Don't go there using IE

Sauron - 15-3-2007 at 22:52

Here is the second C.L.Stong article in his Amateur Scientist column in Scientific American concerning DIY particle accelerators based on Van de Graaff generator high voltage electrostatic supplies. This one was in 1971; the first was in 1957. Based on remarks made in the Bell Jar by Frank Lee who supplied the VDGs for both projects Stong was the instigator of the first project so he could write about it.

While Lee's 1957 linear accelerator had a simple, single chamber partial tube with a thermionic electron source at the cathode and an uncollimated electron beam, the tube in the 1971 article was a collimated, Coolidge-cascade type of multiple sections built around copper corona rings which also served to support the copper electrostatic shields. Compare the Stong illustration with this detail from a 1935 paper in Physical Review by Tuve et al detailing a cascade accelerator tube employed with a two meter VDG at the Carnegie Institute in Washington DC. The similarity is striking. The rounded spun ends of the shields are to avoid cold-cathode effect. The unshielded gaps between the tubular shields are for collimation of the beam (in this case, protons or deuterons) by the electrostatic charge of opposite polarity along the experior of the vacuum tube. In the case of this 1.25" OD tube described by Stong the proton beam emerges with a diameter of about 1 mm and is made visible, for purposes of adjusting the ion source, by use of a quartz target which fluoresces.

I also have Coolidge's first paper describing his accelerator tube and also the cascade version. Unlike the tube Stong describes, both Coolidge's cascade sections and the Tuve tube at Carnegie have bulb sections at the unshielded regions. Stong's amateur designer omitted these for simplicity and economy. His sections were fastened to the corona/shield assemblies by vacuum wax, a fragile method to say the least. I intend to have my sections professionally blown and made with graded seals at each end that can be brazed or welded to the lathe turned copper pieces. Kovar and copper graded seals are quite standard items in scientific glassblowing, the Kovar alloy being preferable because its thermal expansion coefficient is same as Pyrex. (Coolidge in 1926 was using the Invar alloy for same reason.) The contractor will deliver a vacuum tight assembly to the specified 10x-5 torr. The resulting tube will be a lot less temporary than the one described by Stong. Most likely this will be about a 1 m long tube of between 50 and 75 mm OD at the cylindrical sections and will be designed to be mounted between two 17" VDG spheres, one positively charged and the other negative, with a total potential (unloaded) of 1 MV and an actual potential across the tube (which loads them) of 500 KV to 750 KV. I am hoping to be able to achieve an electron beam energy of 100 microamps by employing high voltage DC charge spraying onto the VDG belts and using charge doublers to make the system more efficient.

[

[Edited on 17-3-2007 by Sauron]

Attachment: http___vacuum.ramapo.edu_physics_physics-doc_amsci_AmSci01_1971_08_1971-08-fs.pdf (263kB)
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jtkelectroman - 16-3-2007 at 07:29

Quote:

Originally posted by Sauron
Very ambitious!

I salute you.

HOWEVER your attachment or its host placed a popup on my PC which is likely to be malware. At least, the popup occured just after I opened your link. It was a come on for some screensaver (ostensibly).

Members be warned.
I seem to get popup adds from them too. I think it is because I am still using they're free webpage member ship I haven't upgraded the account to an add free page yet. I plan on doing it once I get more stuff put on the page

[Edited on 16-3-2007 by Sauron]

jtkelectroman - 16-3-2007 at 07:32

Yes you have to b quite ambitious to do something of this magnetude. I'll tell you one thing it was a pain in the arse trying braze weld 316 stainless steel to copper metal.

Sauron - 16-3-2007 at 07:55

What is the diameter of the cyclotron (distance between the poles of the main magnet) and how much does your magnet weigh?

Are you familiar with the cyclotron that a group of high school boys in El Cerrito CA built in the mid 1940s? This was written up in Scientific American in early 1950s, by which time one of the boys had graduated, went on to Berkeley and graduated from there and was hired by the Rad Lab there as a researcher.

I am in midst of a large project that consumes most of my resources, and a number of smaller projects that get funded with leftovers, the VDG accelerator is one of those. The big one is a peptide synthesis and a serious HPLC lab to suport it. So I am not about to mount a Herculean effort like yours. But you have my respect, and you probably deserve the Hubris Award for the decade.

If you have not seen this I will post it.

Their cyclotron took two years to do and worked. If I recall, they achieved a beam current of 14 microamps. I think these teenagers had incredible chutzpah - but they carried it off with panache. You apparently have the same daring-do, It's astonishing.

[Edited on 17-3-2007 by Sauron]

Attachment: http___vacuum.ramapo.edu_physics_physics-doc_amsci_AmSci01_1953_09_1953-09-fs.pdf (434kB)
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jtkelectroman - 16-3-2007 at 18:37

The distance between the two pole faces will be about 2.5-3 inches when the magnet is finished being refurbished. The magnet that I have right now is extremelly small in fact it is only 42 pounds with two pole faces that are about 2.5-3inches in diameter.
What I plan on doing is removing the two pole faces and replacing them with two larger pole faces that go from 2.5 inches to 7.5 inches in diameter. This should give me enough room to add on two large approximately 8in diameter water cooled coils. If all goes well this set up should allow me to design a small compact electromagnet that has an intense magnetic field in the center and the magneti field should decrease at the edges of the pole faces. I plan on adding some shim coils to the unit too for magnetic field corrections.

"I am in midst of a large project that consumes most of my resources, and a number of smaller projects that get funded with leftovers, the VDG accelerator is one of those. The big one is a peptide synthesis and a serious HPLC lab to suport it. So I am not about to mount a Herculean effort like yours. But you have my respect, and you probably deserve the Hubris Award for the decade."

Peptide synthesis. Sounds cool. Hey I've been setting up a DNA analysis lab at my house . I've got all sorts of electrophoresis equipment, transfection reagents, dyes and so on.

BTW Sauron thanks for posting the articles on the van de graaf acellerators I personally wouldn't mind building them once I finish the cyclotron in fact I would like to use a van de graaf generator for building a tandem particle acellerator that is flushed with SF6. Also I found that the articles you posted had interesting information on ionization guage circuits and thermocouple circuits. I think I will try to use this info for making my own guage controllers this should save me alot of money.

Yes I am familiar with the high school students that built the cyclotron, what an interesting article.

Quote:

Originally posted by Sauron
What is the diameter of the cyclotron (distance between the poles of the main magnet) and how much does your magnet weigh?

Are you familiar with the cyclotron that a group of high school boys in El Cerrito CA built in the mid 1940s? This was written up in Scientific American in early 1950s, by which time one of the boys had graduated, went on to Berkeley and graduated from there and was hired by the Rad Lab there as a researcher.

I am in midst of a large project that consumes most of my resources, and a number of smaller projects that get funded with leftovers, the VDG accelerator is one of those. The big one is a peptide synthesis and a serious HPLC lab to suport it. So I am not about to mount a Herculean effort like yours. But you have my respect, and you probably deserve the Hubris Award for the decade.

If you have not seen this I will post it.

Their cyclotron took two years to do and worked. If I recall, they achieved a beam current of 14 microamps. I think these teenagers had incredible chutzpah - but they carried it off with panache. You apparently have the same daring-do, It's astonishing.

[Edited on 17-3-2007 by Sauron]

Sauron - 16-3-2007 at 20:24

Here's enlarged image of the cascade LINAC tube in the SA article. Note strong similarity to the Carnegie (Tuve et al, PR, 1935) tube detailed above. The general layout of the Carnegie 1 meter and 2 meter VDG based accelerators was also similar to what Stong described in his 1971 article and quite different from Robert Van de Graaff's own early designs.

My own intention is to try to follow Van de Graaff's design for the general machine design but use the cascade tube, with improvements, based on the later work.

[Edited on 17-3-2007 by Sauron]

1971-08-02.jpg - 51kB

Sauron - 16-3-2007 at 20:37

(I beg the forum's indulgence for this double post and apologise to Lord Vulture for the transgression.)

Hmmm. 42 lbs? The El Cerrito High School 6" cyclotron of mid 1940s had a 1000 lb mild steel magnet (they couldn't get soft Norwegian iron at the time) and was too large to machine in-house, they talked a technical school in Oakland into machining it. Granted that with special alloys you might get some improvement in the strength of field but, 42 lbs seems likely to be insufficient for the instrument's needs.

Recently we were looking at some NMRs on the market using surplus magnetron magnets and these were also hundreds of lbs. up to 1000 lbs mass of magnet. So my question is, what is the basis of your design with such a small main magnet?

Regarding vacuum gauges of all types, I have a lot of information regarding circuit diagrams, DIY, commercial models of thermocouple gauges, Pirani gauges etc. much better than what was in those three SA articles. Happy to pass it along to you.

[Edited on 17-3-2007 by Sauron]

1971-08-01.jpg - 48kB

12AX7 - 17-3-2007 at 04:45

I wonder what the saturation, coercivity and permeability of mild steel (circa 1020, annealed), cast iron (ferritic, gray or ductile) and cast silicon steel (not necessarily as available, but high saturation (~1.2T) suitable for 99% of all transformers in use today).

Tim

jpsmith123 - 17-3-2007 at 06:50

I got a Varian #801 TC gauge on ebay, rather cheaply. Then I bought an enclosure for it at "Duniway Stock Room"...they call it a "TGC Box", for $57.00.

Duniway actually sells all the parts. The two main items are the TC tube, and the meter. The power supply (a slightly adjustable current source), you could easily build yourself, if you can't get a Varian #801 somewhere.

http://www.duniway.com/images/pdf/pg/p-85-controllers-new.pd...

As far as the tube, when I was toying with the idea of building one, I contacted a few places, e.g., "Larson Electronic Glass", but the prices were higher than I could go, so I decided that I would try to solder the glass to the copper or brass electrodes using indium.

Lastly, for admitting small amounts of H2 or D2 into the tube, I got some thin-walled 1/8" dia. Pd-Ag alloy tubes with one closed end. I was going to use the tubes as a combination electrolytic cathode/vacuum interface; IOW, I was hoping the Pd-Ag tubes would act as a controlled leak, with the amount of gas admitted proportional to the electrolysis current.

Sauron - 17-3-2007 at 13:40

My own setup will strictly be for electron beam rather than proton beam, as I'm not interested in anything other than chemical reactions promoted by such a beam. I posted the illos of the proton accelerator just for the tube details.

Along with Dunway, the Bell Jar lists a number of other commercial suppliers of vacuum gauges, parts for same, etc. on their website and also has several free articles about gauge building and use. It is well worth buying their first five year compilation and getting on the list for the next one. They are supposed to pubish it this year as well as resuming publication of the journal. Anyone interested in medium to high vacuum should contact Steve Hansen.

jtkelectroman - 17-3-2007 at 13:52

When I origionally designed the machine the design specs called for a much larger magnet which would have been about a couple thousand pounds but as I started buying the equipment to build this machine I became strapped for cash wrather quickly keep in mind the fact that I am a colledge student that has no financial aid available. That being said I determined that it would be too expensive to purchase the metal to make the magnetic core that large also I determined that having a pre-assembled surplus magnet of that size would also be too expensive to ship to my house via freight . So I began redesigning the magnet. THe Idea is to try and create the most compact core as possible while still obtaining a high magnetic field. The current core only ways about 42 1bs but keep in mind that this thing could easily weigh 300+ lbs by the time I'm finished. I'm going to be re placing the pole faces with much larger ones made from 7.5 in diameter cold rolled low carbon steel. I also plan on using much thinner copper wire to make the coils and give them efficient water cooling. Now I should point out that I haven't actually finished designing the magnet in fact I would say it's an experimental one. But I believe it's possible to do this. I've done enough research and talked to enough physics proffs at SCSU and they seem to agree. I've seen several extremelly small high powered electromagnets that are very similar to what I'm trying to create. Some of them were actually made with high impedence coils believe it or not..

Quote:

Originally posted by Sauron
(I beg the forum's indulgence for this double post and apologise to Lord Vulture for the transgression.)

Hmmm. 42 lbs? The El Cerrito High School 6" cyclotron of mid 1940s had a 1000 lb mild steel magnet (they couldn't get soft Norwegian iron at the time) and was too large to machine in-house, they talked a technical school in Oakland into machining it. Granted that with special alloys you might get some improvement in the strength of field but, 42 lbs seems likely to be insufficient for the instrument's needs.

Recently we were looking at some NMRs on the market using surplus magnetron magnets and these were also hundreds of lbs. up to 1000 lbs mass of magnet. So my question is, what is the basis of your design with such a small main magnet?

Regarding vacuum gauges of all types, I have a lot of information regarding circuit diagrams, DIY, commercial models of thermocouple gauges, Pirani gauges etc. much better than what was in those three SA articles. Happy to pass it along to you.

[Edited on 17-3-2007 by Sauron]

jtkelectroman - 17-3-2007 at 13:54

Hey Sauron if you could post those diagrams that would be great.

Sauron - 17-3-2007 at 18:45

Try these

http://www.belljar.net/tcgauge.htm

http://www.ee.ualberta.ca/~schmaus/vacf/thermis.html

And attachment below on how you might be able to get to high vacuum with a good two stage mechanical pump alone (no oil or Hg diffusion pumps) by using a zeolite trap. Maybe just maybe you can skip the cryo trap as well. Depends on what ult.vacuum you require.

[Edited on 18-3-2007 by Sauron]

Attachment: 82trap[1].pdf (33kB)
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jpsmith123 - 18-3-2007 at 05:29

I don't know where Steve Hansen got his specs from, but the Varian 531 tube is normally operated with about 165 mA of heater current.

Also, the 531 tube is usually loaded with about 10 ohms, not 55 ohms.

http://www.sisweb.com/vacuum/sis/thermcpl.htm

http://www.varianinc.com/cgi-bin/nav?products/vacuum/measure...

As far as using Zeolite to get a higher vacuum (higher than what you would get from a mechanical pump alone), yes, I can say from experience it does work. (I read a few microns on one side of my MDC foreline trap, and the TC gauge is buried on the other side, my ion gauge was not working so I couldn't tell exactly how low it gets).

Sauron - 18-3-2007 at 09:18

I don't know Hansen personally, but the Bell Jar is the most respected amateur high vacuum publication there is. I am not an electronics type and so cannot speak to the particulars but I'd drop Steve a note and ask him for clarification before dismissing those diagrams and details out of hand. You might be surprised.

The Zeolite 13X trap article was by its developers and was originally published in a peer reviewed journal. As mentioned the use of this technique is based upon the experimental observation that at the normal limits of a two stage mechanical oil sealed pump, the remaining vapors are water outgassing from metal surfaces and oil backdiffusing from pump. The idea of using Zeolite 13X to trap both of these was and is highly succesful and is now commercially available and widely used. So for many applications diffusion pumps and other auxiliary high vacuum pumps are no longer needed. As I'm sure you know, they are expensive, the Hg types are hazardous and the oil types require a really good roughing pump. So from my point of view eliminating them, as long as the ultimate vacuum remains in the required range and pumping speed is still adequate, is very very desirable. The glass diffusion pumps from Ace and Kontes etc. run $1500-$4000 with controllers for heater elements and without Hg or oil. Ouch! And you still need the roughing pump, a cryo trap, gauges, valves etc. Dropping the diffusion pump(s) and optionally the cryo trap saves a lot of money. (Still works better with a cryo trap than without.)

jtkelectroman - 18-3-2007 at 10:21

well I'm happy to say that I had a diffusion pump donated to me from the university that I go to.

Quote:

Originally posted by Sauron
I don't know Hansen personally, but the Bell Jar is the most respected amateur high vacuum publication there is. I am not an electronics type and so cannot speak to the particulars but I'd drop Steve a note and ask him for clarification before dismissing those diagrams and details out of hand. You might be surprised.

The Zeolite 13X trap article was by its developers and was originally published in a peer reviewed journal. As mentioned the use of this technique is based upon the experimental observation that at the normal limits of a two stage mechanical oil sealed pump, the remaining vapors are water outgassing from metal surfaces and oil backdiffusing from pump. The idea of using Zeolite 13X to trap both of these was and is highly succesful and is now commercially available and widely used. So for many applications diffusion pumps and other auxiliary high vacuum pumps are no longer needed. As I'm sure you know, they are expensive, the Hg types are hazardous and the oil types require a really good roughing pump. So from my point of view eliminating them, as long as the ultimate vacuum remains in the required range and pumping speed is still adequate, is very very desirable. The glass diffusion pumps from Ace and Kontes etc. run $1500-$4000 with controllers for heater elements and without Hg or oil. Ouch! And you still need the roughing pump, a cryo trap, gauges, valves etc. Dropping the diffusion pump(s) and optionally the cryo trap saves a lot of money. (Still works better with a cryo trap than without.)

jpsmith123 - 18-3-2007 at 10:29

I don't mean to disparage "The Bell Jar"...I've known about it ever since Hansen started it, and in fact I've corresponded with him about a few things over the years.

I may send him an email just out of curiosity, but I mainly mentioned the discrepancy for your benefit. (In fact I think the pin numbers may be wrong too, although I'm not sure - I'll have to look at one of mine).

BTW, I've built two diffusion pumped HV systems, and in my experience, although they can be a pain, they generally work quite well. You can usually find a half decent one, used, for no more than a few hundred dollars. As far as backing pump requirements go, well, with a half-decent pump oil like DC704, the backing requirements are generally quite modest; even a cheap single stage air-conditioner service pump should work, IMO.

I should say I agree with you about the Zeolite. If you can use it, that would be the way to go.

[Edited on by jpsmith123]

Sauron - 18-3-2007 at 10:29

Mercury or oil?

If mercury you might seriously want to consider NOT using it.

jtkelectroman - 18-3-2007 at 10:44

It's an older hot oil diffusion pump that was made by the bendix corporation and it runs off of 208v 1ph AC the only problem that I have with it is that I can't find anything to mate up to it's 4in flange so I have decided to use some C-clamps if those don't work I'll have to machine a fitting.

Sauron - 18-3-2007 at 10:54

OK, oil diffusion is fine. They require lower pressure from roughing pump thahn Hg diffusion pumps to operate properly, is about their only drawback.

As to your flange problem, I think you had better figure on C-clamps not making the grade. Study high vacuum flange design, an arcane art, before drawing the mating part to be machined. I think you will need really good surface finishes (the flange face on the pump might need resurfacing) and you may want to provide for some sort of sealing system to avoid leaks. High vacuum is totally unforgiving of the slightest leaks. In fact you will absolutely need a good leak detector because you will need to use it a lot!

A Pulsed HV Gadget

jpsmith123 - 18-3-2007 at 11:07

This looks like a nifty little x-ray source. And they claim it's possible to extract the electron beam out into the atmosphere through a thin foil, if you should want the beam.

http://www.aetjapan.com/english/hardware/pdf/MiMi-X.pdf

Note the extremely small size of the unit - apparently only a few mm in diameter. This is one main reason why I'm interested in pulsed HV.

At nanosecond (and even tens of nanoseconds) time scales, you can have voltage gradients approaching 10^9 V/m, without breakdown.

If that thing operated at a continuous 60 kv, it would probably have to be at least 10 to 20 times larger, IMO.

Just think... little or no field grading problems, simple field emission cathode, small size and rugged, mostly metal construction, ability to use things such as tapered line transformers, etc.

It should be possible to scale something like that up to megavolt levels.

[Edited on by jpsmith123]

12AX7 - 18-3-2007 at 12:01

It's not particularly easy to generate megavolt pulses, and there's no getting away from insulation, breakdown occurs in the nanosecond regime -- they probably use an encapsulated spark discharge (tube or otherwise) to generate the pulse in the first place!

And note the cable is bigger than the tube...

The tube looks like it will work based on their diagram, but axial oriented carbon nanotubes aren't particularly easy to get a hold of. Maybe in a few decades when they have carbon nanofiber yarn at the hardware store, but until then, you're stuck with thermionic emission and its difficulties.

Incidentially, I have some HV diodes that look about like that. Cylindrical anode though.

Tim

jpsmith123 - 18-3-2007 at 14:12

Quote:

Originally posted by 12AX7
It's not particularly easy to generate megavolt pulses,

It may not be "particularly" easy, but it's no more ambitious than many other amateur projects I see going on, IMHO.
Marx generators, transmission line transformers, and air-core transformers are three possible approaches.

Take 25 several-meter lengths of RG8 cable. Connect them in parallel at one end, i.e., the "input", and in series at the other end, the "output". You've just built a 1 megavolt transmission line pulse transformer. (See the attached patent for example).

Also, you could even build a tapered line transformer into the device itself, and multiply the applied pulse voltage that way as well.

Quote:

and there's no getting away from insulation, breakdown occurs in the nanosecond regime

What I'm saying is that, generally speaking, the breakdown strength of insulating materials (and vacuum breakdown too) is a function of pulse length. For example you wouldn't normally stress transformer oil higher than, say, 60 kv/inch or so, for a continuously applied voltage, but in a short pulsed situation you can go to perhaps 600 kv/inch without fear of breakdown; so generally speaking, a system that operates in a pulsed mode can be much smaller and lighter than a CW system.

Quote:

-- they probably use an encapsulated spark discharge (tube or otherwise) to generate the pulse in the first place!

Well they may use a spark gap, triggered spark gap, "pseudospark" switch, thyratron, etc., to drive a blumlein or something. I don't know as they didn't go into detail on their power supply system.

Quote:

And note the cable is bigger than the tube...

The tube looks like it will work based on their diagram, but axial oriented carbon nanotubes aren't particularly easy to get a hold of.

Actually, you can buy axially oriented arrays of SWNT (for FE cathode service) online:

http://www.xintek.com/products/materials/index.htm

But you actually don't need anything so extravagant. Just mixing up some arc-produced MWNTs with some epoxy will apparently work.

http://physics.berkeley.edu/research/zettl/projects/emission...

Quote:

Maybe in a few decades when they have carbon nanofiber yarn at the hardware store, but until then, you're stuck with thermionic emission and its difficulties.

Not at all. You can do it right now. In fact, you don't even need nanotubes. Many pulsed HV electron diode projects have used nothing but a piece of aluminum rod for a FE cathode. If your voltage is high enough, you can almost use *anything* for a FE cathode...it's a matter of engineering: adjusting the cathode area, shaping the field, and adjusting the anode-cathode gap to get the approximate diode impedance you need.

Attachment: 5651045.pdf (553kB)
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Sauron - 18-3-2007 at 14:35

Partial-tube electron guns that allow beams to exit the tube through an appropriate supported thin foil have been around since the 1920s or before. W.D.Coolidge of General Electric, the father of the modern X-ray tube, turned his hand to electron beam tubes like that in 1926 both single tube and the famous Coolidge cascade. The latter allowed previous voltage limits to be exceeded and also made it practical to evacuate the tube sections permanently rather than having the tube(s) tethered to a pump system. I attach Coolidge's original publication in J.Frank.Inst. that year. The two cascade tubes described above are simplified forms of the Coolidge cascade.

Note that while Stong's articles both describe Al foil windows Coolidge eschewed Al for Ni foil, and Mo support rather than Al. An electron beam of 500 KV potential will travel 7-8 meters through the atmosphere. But rather thin shielding of a light element like Al will stop it, and the resulting soft x-rays are also easy to shield.

[Edited on 19-3-2007 by Sauron]

Attachment: JFranklin[1].pdf (1.2MB)
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jpsmith123 - 18-3-2007 at 14:49

I would think that something like beryllium foil would make an excellent beam exit window.

But if you *really* want to impress the neighbors, then you've simply got to go with a plasma window:

http://www.acceleron-enbeam.com/news/Acc-AIP-PlasmaArc-20050...

Of course, for serious high power work, maybe even some kind of "beam weapon", see the attached patent.

Attachment: 4931700.pdf (492kB)
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12AX7 - 18-3-2007 at 15:09

Quote:

Originally posted by jpsmith123

Quote:

Originally posted by 12AX7
It's not particularly easy to generate megavolt pulses,

It may not be "particularly" easy, but it's no more ambitious than many other amateur projects I see going on, IMHO.

Meh, vacuum experiments are pretty easy once you have the hardware.

Quote:

Take 25 several-meter lengths of RG8 cable. Connect them in parallel at one end, i.e., the "input", and in series at the other end, the "output". You've just built a 1 megavolt transmission line pulse transformer.

That would work for this application where sheer voltage is needed. I shudder at the appearance of the tail of that pulse though!

Quote:

Also, you could even build a tapered line transformer into the device itself, and multiply the applied pulse voltage that way as well.

There has been talk of tapered lines here before, but has anyone actually built one?

Suppose I should some day. Take a heavy garbage bag perhaps, cut it into strips and lay out narrow triangles of tin foil on either side. Charge to 20V with bench supply and resistor, short one end with a fast MOSFET and see what comes out the other side (and observe impedance matching, energy transfer, etc. at the end).

It seems to me, ideally you want the edges to taper narrower, but also the dielectric thickness to increase, *smoothly* so as to avoid reflection.

Quote:

Actually, you can buy axially oriented arrays of SWNT (for FE cathode service) online:

http://www.xintek.com/products/materials/index.htm

Well I'll be, that's interesting. Shame on me for not checking!

Quote:

Many pulsed HV electron diode projects have used nothing but a piece of aluminum rod for a FE cathode.

Yeah, but you have to quantify it (and probably under certain conditions, like temperature) first. If you're using the same sealed tube, that's fine. It would be very annoying if you have to quantify it every time you adjust the device inside your bell jar though.

Tim

Sauron - 18-3-2007 at 15:31

I'll be happy to get a few score microamps, I'm not after a beam weapon just something to tickle a few organic reactions. Maybe make myself a Lichtenberg figure or two.

My target is 100 microamps but with a pair of VDGs with 2" belts that's a tall order even with good external excitation and charge doublers. We will see.

I have several article on hand about generating X-rays with off the shelf vacuum tubes and HV supplies for amateur radiography. While it is not of any particular interest to me, I can post these for you guys if this floats your boat. One is from ClL.Stong and the other is more contemporary from the Bell Jar.

jpsmith123 - 18-3-2007 at 18:18

Sauron, you really should consider using a field emission cathode.

For practical purposes, your electron source will need to be inside your collector electrode, and you'll accelerate the electrons down through the tube and out into the atmosphere through your grounded foil.

If you're using thermionic emission, this imposes the hardship of having to generate filament power locally inside the collector using a battery or a generator. It also makes the tube more complicated in that you'll have two electrical feedthroughs to deal with, instead of robust one.

The FE cathode could just be merely a small blob of MWNTs and epoxy on the end of a screw or plunger of some sort sealed by a simple o-ring. I have the two papers by Zettl et al. on simple FE cathode material. If you want them I'll upload them.

Sauron - 18-3-2007 at 20:42

Sure, thanks, let me take a look.

Nothing sacrosanct about thermionic rmisssion.

jtkelectroman - 19-3-2007 at 08:38

Thanks for the tips I'll keep that in mind and perhaps keep searching for an adapter a little while longer. As far as the leak detector goes I'm sure SCSU has one lying around somewhere because theu do a small amount UHV stuff.

Quote:

Originally posted by Sauron
OK, oil diffusion is fine. They require lower pressure from roughing pump thahn Hg diffusion pumps to operate properly, is about their only drawback.

As to your flange problem, I think you had better figure on C-clamps not making the grade. Study high vacuum flange design, an arcane art, before drawing the mating part to be machined. I think you will need really good surface finishes (the flange face on the pump might need resurfacing) and you may want to provide for some sort of sealing system to avoid leaks. High vacuum is totally unforgiving of the slightest leaks. In fact you will absolutely need a good leak detector because you will need to use it a lot!

jpsmith123 - 19-3-2007 at 12:45

Sauron here are two papers to look at regarding a FE cathode made from nanotubes.

At one time there was a guy selling arc-produced MWNTs on ebay, but it seems he is no longer doing so.

If you can find a supplier for small quantities of these, please let me know as I'd like to buy a gram or two myself.

(BTW if you have an idea of the geometry of your tube, post it here and I will look into the electron optical situation, i.e., I will see how a beam propagates through it).

Attachment: Zettl.zip (237kB)
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Sauron - 20-3-2007 at 04:58

Looks like an Unobtainium procurement, been there before.

I think I can lick the problem of the leads for powering the thermionic cathode by running them along the tube wall (inside) and bringing them out through the wall exterior to the collector.

The emission from cathode must be regulated to match the current across the tube, this is done by trial and error varying the heating of the filament with a small transformer and measuring the beam current with microammeter at the exterior of anode.

I have no clue as to how to regulate output from a FE cathode even if I can get the MTNTs. As this would not seem likely to be wireless I'd be back to same problem of interaction between those wires and the HV electrostatic propagating along the exterior of the tube (or the collector.)

Anyway thanks for the articles, I will read with interst.

pyroelectric acceleration and ferroelectric electron guns

Bander - 20-3-2007 at 21:44

[context pressure ~=10^-3 Pa]

Has anyone considered using pyroelectric crystal mediated acceleration of charged particles (spontaneous polarization of voltage oriented through the c-axis crystal faces (when heating the , +z surface has a negative potential and the -z surface has a positive potential, reverse for cooling) as the result of shifting ions in the crystal shape/lattice combined with funky dilute gas charge masking behavior, ref: main, http://arxiv.org/abs/physics/0404113, bipolar setup for higher energies, more detailed, even more detailed)? Particle (electrons or ions depending on the crystal face and heat/cool cycle) energies up in the 150keV+ range are very doable (or 200kev+ with dilute light gases and a bipolar setup). Plus with cylindrical crystals the beams are self focusing. A common choice for this is Lithium Tantalate, and making thin films and stacking this material is possible, but there are lots of tricks involved (and making crystal boules is a magnitude harder):

Quote:

http://dx.doi.org/10.1016/j.jallcom.2005.01.063
http://riverwayparkpartnership.com/old/vacuum/Ferroelectric%...
In this paper, a new sol–gel method was developed to prepare the LiTaO3 powder and thin film. In this method, hydrogen peroxide aqueous solution (H2O2) reacts with tantalum and lithium ethoxides ethanol solution. The property of transformation into pure LiTaO3 was demonstrated through annealing the precursor powder dried from the solution. Nano-sized lithium tantalate particles with dimension of 40–50 nm were obtained at a sintering temperature of 700 °C. Such aqueous solution is suitable for fabrication of lithium tantalate thin film. LiTaO3 thin films on Si and Pt/Si were fabricated through heat treatment of the spin coated wet films. The obtained thin film is characterized through measuring its ferroelectric and dielectric properties.

Using Silicon(111) c-cut wafer instead of the traditional and expensive sapphire wafers to orient and control nucleation so that you actually have a defined c-axis and a net pyroelectric effect is noted in above and covered more in depth in: http://riverwayparkpartnership.com/old/vacuum/c-axis-texture...

...and while all that is feasible, it isn't easy.

An alternate option might be large trigylcine sulphate (TGS) monocrystals. Growing TGS of of few cm in size is well within the range of amateur methods (ref:http://physics.technion.ac.il/~jammia/advlab/advlab.htm). l-arginine phosphate monohydrate looks good too (a bit more robust but still water soluble), but I haven't explicitly seen it referenced for this role.

As for electron guns, a wide range of ferroelectrics, when hit with a couple microfarads at ~2kv or so throw off a lot of monoenergic electrons.
http://riverwayparkpartnership.com/old/vacuum/Ferroelectric%...
http://riverwayparkpartnership.com/old/vacuum/FERROELECTRIC%...
[/context]

For the vacuum required: Obtaining pressures in the 10–5 Pa range with oil-sealed rotary vacuum pumps, http://riverwayparkpartnership.com/old/vacuum/High%20Vacuum%...).

Anyway, pyroelectric acceleration is basically just getting the voltage for a linear accelerator out of a cousin of the piezoelectric effect--the complexities of crystal growth balance out with the ease of operation, emphasis on chemistry, and lack of high voltage feedthroughs. I've collected all the accelerator and vacuum information that is too much to spam and/or attach to this post here:
http://riverwayparkpartnership.com/old/vacuum/

Sorry about the nested quotes.

[Edited on by Bander]

jpsmith123 - 24-3-2007 at 06:51

@Sauron: I found two potential sources of supply for small quantities of arc-produced MWNTs:

http://www.n-tec.no/ and www.nanocs.com

I think getting the current in the range you want will involve some trial and error. Depending on your situation, I suppose it may be easier to just use a thermionic emitter, but if you use a nanotube FE cathode, it could be notable "first" for an amateur.

@Bander: I've looked into ferroelectric cathodes a little bit in the past. I think they show some promise for certain applications, but probably not for Sauron for his application.

BTW, that's an interesting abstract you have there re: air dissolved in vacuum pump oil imposing a significant limit on the ultimate pressure obtainable from oil sealed mechanical pumps. I always thought that adsorbed water vapor was the main culprit. I may try to get the full paper.

Bander - 25-3-2007 at 03:13

Quote:

Originally posted by jpsmith123BTW, that's an interesting abstract you have there re: air dissolved in vacuum pump oil imposing a significant limit on the ultimate pressure obtainable from oil sealed mechanical pumps. I always thought that adsorbed water vapor was the main culprit. I may try to get the full paper.

Adsorbed water vapor just limits the time required to pump down to the ultimate pressure. Leaving the a mechanical pump on overnight should remove most of it.

[Edited on by Bander]

Sauron - 25-3-2007 at 07:09

Outgassing the pump oil prior to installation would help.

Anyway did you see the article from the Bell Jar about zeolite trapping of water vapor and backdiffusion from pump oil?

According to the authors this is a succesful technique for achieving high vacuum in the range I need without auxiliary pumps. Very nice numbers with or without a cryo trap.

jpsmith123 - 25-3-2007 at 10:11

I was always under the impression that a fraction of the water initially physisorbed in the system would end up contaminating the pump oil...in part because this would seem to explain my own observation regarding the pressure differential across my MDC foreline trap, which had a charge of Linde 13X.

A scaled up version of the AET device...

jpsmith123 - 25-3-2007 at 11:16

12AX7, I found the following interesting gadget, which seems to be almost a scaled up (x83) version of the AET electron source.

It uses a tapered line to multiply the air-core transformer voltage by a factor of four, giving an output energy up to 5 MeV.

Note that the high impedance end of the tapered line is only 6mm in diameter (which gives rise to a very high electric field), yet there is apparently no appreciable field emission, since the pulse is so short.

http://www.sunysb.edu/icfa2001/Papers/tu4-6.pdf

jtkelectroman - 31-5-2007 at 12:06

Well Here is a picture of one of the two magnetic yokes that I am building for the cyclotron's magnet.I should point out that I built in a water cooling line.

Coil.jpg - 51kB

jtkelectroman - 31-5-2007 at 12:11

Here is a picture of the faraday cup that houses the target inside of the cyclotron. The faraday cup is located in the upper right hand side of the picture it is that rectangular object with the wire mesh screen which acts as an rf shield for the target.

Target.jpg - 82kB

gregxy - 5-6-2007 at 16:34

Did you know that the first cyclotron was made inside of a wine bottle? I don't know if it actually worked on not but it was what Lawrence tried first. There is an interesting cyclotron museum at the 88" cyclotron in Berkeley that has
it on display.

We used to bonbard integrated circuits with high energy ions from the cyclotron to simulate bit-flips that occur from cosmic rays in satellites.

jtkelectroman - 7-6-2007 at 13:12

Quote:

Originally posted by gregxy
Did you know that the first cyclotron was made inside of a wine bottle? I don't know if it actually worked on not but it was what Lawrence tried first. There is an interesting cyclotron museum at the 88" cyclotron in Berkeley that has
it on display.

We used to bonbard integrated circuits with high energy ions from the cyclotron to simulate bit-flips that occur from cosmic rays in satellites.

I didn't know that a wine bottle was used.Thanks for sharing that tidbit of info. I think I read somthing before about how radiation can screw up computers and the like. So I'd be interested in getting some feedback on the my coils the cyclotron cavity. Please everyone give me your input.
By the way I just picked up some arc welding cables and I plan on using them for making the connections between my 1.5Kw supply and the coils on the magnet. So I won't have to worry about the cables overheating. But the coils? I don't know about them I figure they should be fine due to the fact that I wrapped 20Ft of 1/4in copper tubing arround them so and I will be forcing high pressure water through the lines so hopfully the magnet will be okay.

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