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Sauron
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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
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Sauron
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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.
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12AX7
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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
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Sauron
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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.
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tnhrbtnhb
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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.
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Sauron
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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]
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not_important
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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.
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Sauron
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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)
This file has been downloaded 1600 times
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jpsmith123
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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
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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]
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Sauron
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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]
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Sauron
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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.
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Sauron
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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.
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Sauron
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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]
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Maya
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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
\"Prefiero ser yo extranjero en otras patrias, a serlo en la mia\"
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Sauron
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@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.
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Sauron
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@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.
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Sauron
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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.
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Maya
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| Quote: |
But you know all that. I am preaching to the choir.
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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..............
\"Prefiero ser yo extranjero en otras patrias, a serlo en la mia\"
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Maya
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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]
\"Prefiero ser yo extranjero en otras patrias, a serlo en la mia\"
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jpsmith123
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Pulsed Accelerators
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)
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Sauron
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@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.
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Sauron
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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.
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unionised
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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.
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Sauron
International Hazard
Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline
Mood: metastable
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So who's a size queen?
No moths on my balls.
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