Update on my "Mini-F" Dense Plasma Focus device

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SteveHansen
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Update on my "Mini-F" Dense Plasma Focus device

Post by SteveHansen » Tue Jan 10, 2012 3:48 pm

The Mini-F is a 3 kJ device fed by a single 10 kV, 59 uF capacitor with 35 nH internal inductance. The initial uses for it will be to study x-ray production and ion beam characteristics. A big part will be to characterize and optimize the electrode geometries in consideration of the total short circuit inductance and other parameters (energy, pressure, gas type, etc.). Key diagnostic devices will include Rogowski coils, x-ray diodes and eventually a Thompson spectrometer.

I had some concerns with the interface between the capacitor and the electrodes as most of the DPF devices have more complex spark gap and interface arrangements. I ran my design past Prof. Sing Lee (probably the world expert on DPF) and he gave it a general thumbs up. My thanks to him.

At the moment my device is scattered around the basement as a result of our move. I will begin to move it into the lab/office/ham shack over the course of the month and start to reassemble it.

At some point I will probably use it with deuterium to produce neutrons but that is not a near term goal.

For those who might be interested in DPF, the site to go to is Dr. Lee's Institute for Plasma Focus Studies at http://www.plasmafocus.net/. A tremendous amount of data is accessible from here including design rules, the numerical DPF model (an Excel worksheet with the model code inserted), links to other programs and hundreds of downloadable papers.

One interesting thing about the DPF - the parameters are very scalable. Working DPFs from under 1 joule to a megajoule fit the model.

One device that is similar to mine is described in this 2007 paper:
http://www.icenes2007.org/icenes_procee ... 0FOCUS.pdf

Steve

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Doug Coulter
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Re: Update on my "Mini-F" Dense Plasma Focus device

Post by Doug Coulter » Tue Jan 10, 2012 6:59 pm

Go Steve!

I happened to collect some real nifty caps for this sort of thing awhile back - Maxwell's 120uf/10kv times 3 (35 nh each) and another one 1 uf / 30kv.

I haven't yet had the guts to charge one of those big guys up and arc it. Back in the day, I had to run an experiment for an advisor that used one in a plasma pinch, and was wired with 8 pieces RG8 in parallel. They weren't strong enough to not explode at the peak current - and I got to pick braid bits out of my hide for a long time after that. Next time I use twice as many!

I think you match various parameters of the electrodes to the cap you've got, then adjust gas pressure to match up with all that or something similar?

I'd be real interested in your results, and also how you keep the EMI out of whatever you use to measure them.

I'd been researching inverse pinch type spark gaps to be the main switch here - last longer and shut themselves off after a given pulse duration (easier on the capacitors). Might get you a little more leeway with cap parameters.
Why guess when you can know? Measure!

SteveHansen
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Re: Update on my "Mini-F" Dense Plasma Focus device

Post by SteveHansen » Tue Jan 10, 2012 7:36 pm

Let's see 120 uF and 10 kV - looks like 6 kJ for each cap. Healthy.

As to design, you basically want the run down of the plasma to match the rise time of the cap. That sets the length of the electrodes to get the maximum pinch. At the end of the run down the current, for a few kJ, should be in the 200 kA range, give or take.

When I was a kid I put a piece of fine wire between two plates of glass and discharged a small cap (maybe 200 -300 joules) through the wire. The interesting thing was that no real pieces of glass remained - it was just powder.

For the George Schmermund types who like really small things, here's an interesting paper on a tiny 0.1 J plasma focus out of Chile: http://www.plasmafocus.net/IPFS/associa ... 202010.PDF

Steve

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Re: Update on my "Mini-F" Dense Plasma Focus device

Post by Richard Hester » Wed Jan 11, 2012 4:30 am

I have a 2uF 60kV cap - DPF may be about the only valid reason to keep it around, other than the difficulty of getting it out of my basement. It was crowned with a complicated nest of spark gaps when I got it, which I still have somewhere. I figure the lab that used it in NM (it came from there) used it in a plasma experiment of some sort or some godawful gas laser.

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Re: Update on my "Mini-F" Dense Plasma Focus device

Post by Richard Hester » Wed Jan 11, 2012 4:31 am

Another interesting app for those big hv caps may be a capillary discharge X-ray laser...

SteveHansen
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Re: Update on my "Mini-F" Dense Plasma Focus device

Post by SteveHansen » Wed Jan 11, 2012 1:07 pm

Not of so much use for a DPF but 2 uF at 60 kV would make an excellent vacuum spark x-ray source.

Steve

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Ed Meserve
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Re: Update on my "Mini-F" Dense Plasma Focus device

Post by Ed Meserve » Fri Jan 13, 2012 9:02 pm

Steve,
Neat! How close are you to firing your "Mini-F"?

Ed
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Re: Update on my "Mini-F" Dense Plasma Focus device

Post by SteveHansen » Fri Jan 13, 2012 10:18 pm

That's the tough question Ed. Should be sometime in March.

Steve

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Re: Update on my "Mini-F" Dense Plasma Focus device

Post by SteveHansen » Mon Jan 16, 2012 9:20 pm

I put up a project page for my Mini-F on my website. I have also added similar pages for two other projects that are active - a small vacuum spark soft x-ray source and my pseudospark electron beam source. Two more projects will be added soon.

Go to http://www.belljar.net/articles.htm and scroll about halfway down to "Ongoing Projects." The links are there.

Steve

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