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Sauron
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Mercury or oil?
If mercury you might seriously want to consider NOT using it.
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jtkelectroman
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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.
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Sauron
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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!
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jpsmith123
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A Pulsed HV Gadget
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]
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12AX7
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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
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jpsmith123
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| Quote: | Originally posted by 12AX7
It's not particularly easy to generate megavolt pulses,
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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
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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.
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-- they probably use an encapsulated spark discharge (tube or otherwise) to generate the pulse in the first place!
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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.
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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...
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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.
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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)
This file has been downloaded 811 times
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Sauron
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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
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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
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| Quote: | Originally posted by jpsmith123
| Quote: | Originally posted by 12AX7
It's not particularly easy to generate megavolt pulses,
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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.
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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.
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
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Sauron
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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.
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jpsmith123
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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.
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Sauron
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Sure, thanks, let me take a look.
Nothing sacrosanct about thermionic rmisssion.
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jtkelectroman
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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! |
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jpsmith123
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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
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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.
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Bander
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pyroelectric acceleration and ferroelectric electron guns
[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]
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jpsmith123
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@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.
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Bander
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| 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]
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Sauron
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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.
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jpsmith123
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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.
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jpsmith123
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A scaled up version of the AET device...
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
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jtkelectroman
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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.
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jtkelectroman
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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.
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gregxy
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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.
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jtkelectroman
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| 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.
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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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