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Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843
Below is a link to an interesting paper about a marx generator designed for high reliability use in an industrial application. The spark gaps supposedly have a profile designed to wear away evenly...a so-called "Borda profile"...but I can't seem to find any analytic expression for this kind of curve. Does anyone have any info on this I wonder?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
There is a section on calculating the geometry of Borda profile and other Uniform Field Electrodes in Optimization in Electric Field Domain, by T. Takuma and T. Kawamoto in Applied Electromagnetics and Computational Technology IOS Press 1997, ISBN 905199320X
The Borda profile basically has a flat central section moving on to rounded edges.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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Thanks for the info, Harry. Unfortunately the book is about a hundred bucks or so :(.
Anyway, here's a simulated gap between two circular disks (with the edges merely rounded to a radius of half the thickness of the disk). The field seems quite uniform over a large area but there is significant (about 50%) field enhancement at the edges. So it looks like I'll have to experiment with it a little bit or maybe try and derive it or something.
Registered Member #1321
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You're probably right that it wouldn't matter all that much...except that it would offend my perfectionist sensibilities.
Anyway, I'm still doing some research to see if a Marx generator is likely to be able to do what I want, or if a pulse transformer (either "conventional" or transmission line type) or maybe even a stacked Blumlein or spiral vector-inversion type generator would be better.
Basically I'd like to have a pulse generator capable of a few hundred kv (the more the better) into 50 ohms, with a pulse width of 5 to 10 ns. max.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I have seen some very interesting configurations used in plasma pinch experiments where a beefy but compact Marx is outputed - via a peaking capacitor - into the primary of a pulse transformer with a relatively low turns ratio - one to five, or so.
The idea is to quintuple the voltage, but keep the secondary output impedance low, and the pulse edges crisp.
I have thought of trying this myself with a large bundle of ferrite rods as the transformer core, but haven't got round to it yet.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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What kind of voltages were they getting out of the transformer I wonder?
BTW I have several interesting papers on the design of high voltage air-core pulse transformers. IIRC several megavolts have been achieved and even 10 MV has been proposed as possible (in one particular paper at least). If you want to see them maybe I could upload them here.
One thing that interests me is the tapered line transformer. I'm working on simulating a coaxial one right now with a 3D EM solver. My ultimate interest is to maybe build a physically small pulsed electron accelerator using a tapered line loaded with a short diode gap. I may never try to do it, but I've been studying pulsed power for a while and I think it may be possible. Even if I never get around to going that far it's still very interesting subject matter.
I forgot to mention a site that has some info on a compact fast Marx generator they refer to as a "wave erection Marx". You probably are already familiar with it, but in case you aren't:
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What's a "Borda profile"?
