High Voltage Generation

Good Evening everyone,

I need to generate about 1.5 million volts for an electrostatics experiment (I need positrons, if you're curious...)

I currently have a 200 KeV Van De Graaff generator at my disposal. I have two options, wondering if y'all would have any input:

1.) This one's the simpler of the two: I buy another 7 Van De Graaff generators, and put them in series, or I get bigger spheres and get say 4 extra, or some variant of that.

2.) I use the generator to charge a Leyden Jar. I disconnect the generator once it's fully charged. Then, in order to increase voltage, I increase distance between the plates, lowering capacity, increasing voltage. This will almost certainly be done mechanically, with a lot of rubber insulation. See the attached diagram for this one if it'd help!

Thoughts? I'm leaning option 2 because of how expensive option 1 will be, though 2 has its distinctive sketchiness...

Thanks for your time, other ideas are welcome!

I expect that you will have serious corona problems,

charge can ionise air once the field gradient is sufficient, for rounded objects use 3 kV per mm of radius.

So 1.5 MV = 1500/3 = 500mm

therefore, to avoid breakout at 1.5 MV, the minimum diameter of spheres or cylinders is 1 metre.


One common 'trick' is to use +0.5.V and -0.5.V = +/- 750 kV in this case.

I remember a tall brick building at Harwell, built to house a tandem VDG to generate 1 - 2 MeV particles by this method, it was over 100' tall.

You would probably find a microwave LINAC or synchrotron more practical.
(I helped to commission a 55 MeV microwave LINAC at Harwell in the '70's)

The classic h.v. generator is https://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_gener...
though I suspect that 1.5 MV will require a lot of input power just due to leakage/corona.

As above, a Marx generator for +/- 750 kV may be doable at hobby scale https://en.wikipedia.org/wiki/Marx_generator

P.S. due to the very high dV/dt generated by a Marx generator, EXTREMELY long arcs through the air can be formed
https://markobakula.wordpress.com/about/100kv-marx-generator... , http://www.capturedlightning.com/frames/longarc.htm
(you thought chemistry was a good mix of fun & danger ?)

IF you do generate 1.5 Mv (or +/- 750 kV) the machine will have to be quite large to withstand such voltages,
so the mean free paths will be measured in metres - quite a hard vacuum
(fingerprints on the inside of the machine can add many hours to pump-down time, that was using roughing-, turbo- and ion-pumps)


[Edited on 2-3-2017 by Sulaiman]

Invictos: Your option 2 is a form of influence machine. The changing of capacitance can be done with rotating discs. One form of influence machine is a self starting Wimshurst machine that would not require your Van De Graaff generator. The one below has 7 feet discs allegedly the largest in the world. Yours would be huge as it might need 50ft discs unless you put it in a pressure tank of .sulfur hexafluoride but the discs would still have to be say 3ft thick.

Start the crowd funding now and call Guinness LOL

Check out Antonio Carlos M De Queiroz (a mad science type of guy) site http://www.coe.ufrj.br/~acmq/bigwin.html with links to lots of HV machines and info.

From http://www.scitechantiques.com/wimshurst1/



If you don't mind the high voltage being alternating you could do it with a big Tesla coil like the one below.



From https://www.quora.com/What-does-a-Tesla-coil-do

[Edited on 2-3-2017 by wg48]

I remember Greg Leyh sharing the development of the Electrum on pupman.com
where there is enough information to reproduce an Electrum,
a fantastic project, not for the faint-hearted.

.........................................................................................
for option 2, the voltage will rise proportional to distance (ideally)
e.g. capacitor plates 5mm apart charged to 15 kV would only need to be increased to 500 mm separation for 1.5 MV
- why have I never tried this !

insulation will be 'interesting'
and 'stray/parasitic' capacitance reduction will be challenging.

[Edited on 2-3-2017 by Sulaiman]

Quote: Originally posted by Sulaiman

I remember Greg Leyh sharing the development of the Electrum on pupman.com where there is enough information to reproduce an Electrum, a fantastic project, not for the faint-hearted. [Edited on 2-3-2017 by Sulaiman]

You're all amazing, wow is all I can say!

The biggest thing I've pulled from this is that these things don't quite scale

The biggest, most out there end goal is to accelerate electrons in a vacuum tube by using this voltage differential, use the Bremsstrahlung radiation in order to convert the energy in the electrons to photons, and then use the photons in pair production. Pair production for positron/electron pairs occurs at 1.1 MeV, so I was shooting for the 1.5 MeV range to account for the energy lost in Bremsstrahlung. Given the scaling difficulty, I may need to aim closer to 1.1 MeV apparently! If I shoot enough electrons down the pipe, some of them should make it at just the right angle to get that energy, right?

Note, I crunched the numbers at 100 KeV for now, and found the power given off by Bremsstrahlung to be 3.31e-20 Watts, while the energy of the particle itself is 1.59e-14 Joules, so it does seem remotely possible. However, 1.1 MeV is really cutting it!

As for some of these ideas...

This is an idea...I'd bet if I used some industrial level transistors, it could be done, right? That would increase the voltage an insane amount too, because you're switching between inverse addition to regular...I don't even think you could get a transistor that big, I'd probably be easier off hooking up servo motors to metal rods to use as switches!

Hey I did a thing! But wow those are some numbers The disk approach is so majestic I love it. Going off of that, one could, in theory, use some sort of conductive fluid for the dialectric, and drain and fill the fluid in order to change capacitance, right? It'd be clunky, but it's a thought!

Quote: Originally posted by Sulaiman

The problem with putting one voltage source on top of another is insulation ... how do you supply power to something that is at e.g. 100 kV ? (Cockroft-Walton / Villard / Greinacher) voltage multipliers and Marx generators are fairly easy to configure for +ve and/or - ve voltages A VDG can be configured to produce +ve or -ve voltage if a h.v. supply is used to spray charge onto the belt. Tesla coils can also be bipolar, but not useful here I think. P.S. the idea of charging leyden jars (capacitors) in parallel then stacking them up in series . is incredibly hazardous. . is a manually switched Mark Generator

I UNDERSTAND MARX GENERATORS NOW

I was looking over the circuit drawings but it never really clicked on why it worked...I see...


Thanks for your time, I've got plenty to look through and consider at this point!

Quote: Originally posted by Invictos

I UNDERSTAND MARX GENERATORS NOW

The more that I think of generating 1.5 MV the less practical I consider it;

constant potential difference (d.c.) causes air to ionise at about 3 kV/mm,
1.5 MV is therefore equivalent to a minimum gap in air of 500mm
or a curved conductor of 500 mm min. radius.

Due to inevitable surface contamination, monoatomic layers of water etc.
Electricity will arc along surfaces at much much lower v/m.


with high dV/dt the breakdown of air is much more complicated,
related to streamer growth at the tip of the arc.
e.g. you may have seen Steve Ward's tesla guns https://www.youtube.com/watch?v=R2jlgTA0QtI
< 100 kV in bursts produced those arcs if I remember correctly !

When Marx generators are used, if the risetime is just right ...
http://teslamania.delete.org/frames/longarc.jpg

In short, I doubt that the simple approach of accelerating charged particles in a linear electric field is a viable method of producing 1.5 MeV electrons.
(with the possible exception of bipolar VDG, BIG, EXPENSIVE, but doable)


I think that in an amateur setting, the most likely to succed is a cyclotron https://en.wikipedia.org/wiki/Cyclotron


A LINAC using a microwave oven magnetron may be another viable option ...
https://www.youtube.com/watch?v=jSgnWfbEx1A
http://hpschapters.org/florida/13PPT.pdf
http://users.ox.ac.uk/~atdgroup/referencematerial/The%20novi...


P.S. it seems to me that a 1.5 MeV accelerator would become your only hobby for the forseeable future if you go that way.


Is there a university near you ?
Academics (desperate to publish a paper), and facility operators (I suggest 1 beer per MeV.minute) can be incredibly helpful !
Once an Academic has introduced you to the operator/technician, access is easy (in my case it was for S.E.M. time)

[Edited on 5-3-2017 by Sulaiman]

[Edited on 5-3-2017 by Sulaiman]

Quote: Originally posted by WGTR

How much beam current do you need (at 1.5MV)? There are a number of ways to make low power, fairly compact modules at these high voltages, but the materials are not cheap, and the whole high voltage section would need to be encapsulated in a dielectric.

Whatever current we could reasonably get! We're trying to get pair production to occur, and I don't believe current is a factor in that (correct me if I'm wrong)

As for the target, we were planning on using tungsten.

Also, as this thread is taking a bit of a...pessimistic.. turn (darn Sulaiman, you and your knowledge...pft...), I've got a question about plan B that I'm going to put in another thread in a minute here...

Again, y'all are perfect

even if a potting gell is the ultimate goal,
I would start with oil first,
unless you are confident that no future repairs or modifications may be required

Quote: Originally posted by WGTR

Is this a personal curiosity, or a funded project? It's not a trick question; it would help you the most to have a solution tailored to your needs and resources. Much of my electronics work has been in general power electronics design, up to about 50kV, but others in our facility work specifically with high voltage. The power side sounds doable on a benchtop to me, but without understanding all the details of your application it's impossible to say for sure. It would require some rather cleverly enforced isolation techniques. The beam current question comes back to me not understanding the application very well. Are we talking about pA, or several amps? Does the current need to be continuous, or pulsed? If pulsed, then with what pulse length? If current requirements are in the pA or nA range, then power supply design becomes much simpler (and safer for you to use). Dow Corning is my go-to place for dielectric potting gels, but generally their stuff is going to run several $100 per gallon. If it's what the Dr. orders, though...

Quote: Originally posted by jpsmith123

I'm wondering, are you just looking to make big sparks, or do you need an electron beam? [Edited on 12-3-2017 by jpsmith123]

We just need a high voltage in order to get a short burst of an electron beam. (I think that is a valid answer to that question )

Quote: Originally posted by Invictos

H The dielectric breakdown of something macroscopic like glass or plastic is probably stronger than the breakdown of a store bought capacitor, right? [Edited on 3-12-2017 by Invictos]

Quote: Originally posted by unionised

Quote: Originally posted by Invictos   H The dielectric breakdown of something macroscopic like glass or plastic is probably stronger than the breakdown of a store bought capacitor, right? [Edited on 3-12-2017 by Invictos] If it was, guess what they would make store bought caps from...

Quote: Originally posted by Invictos

Quote: Originally posted by jpsmith123   I'm wondering, are you just looking to make big sparks, or do you need an electron beam? [Edited on 12-3-2017 by jpsmith123] We just need a high voltage in order to get a short burst of an electron beam. (I think that is a valid answer to that question )

Quote: Originally posted by jpsmith123

High voltage will make sparks, but that's not the same thing as an electron beam. You might have to do something like what's described in this paper: http://www.sciencemadness.org/talk/files.php?pid=76343&a... But you'd have to scale it up to get the voltage that you want, which would probably involve replacing the spiral generator with an appropriately designed and driven air core transformer, and increasing the size of the device to prevent HV breakdown across insulator surfaces. Quote: Originally posted by Invictos   Quote: Originally posted by jpsmith123   I'm wondering, are you just looking to make big sparks, or do you need an electron beam? [Edited on 12-3-2017 by jpsmith123] We just need a high voltage in order to get a short burst of an electron beam. (I think that is a valid answer to that question )

I'll look into that! And we have electron emitters and a tube, we're just using the voltage difference to actually accelerate them