Pinch plasmas

[Chris Bradley]

Sure. I see that. It's a nice demonstrator effort, but I'm just wondering if you're able to put more current discharge through the coil. Or are you running that with a constant DC current? I'm just thinking that if you are not already doing so, can you trigger an HV discharge through the discharge tube, and a high current/low DC discharge through the coil simultaneously?

[Mark Rowley]

"I'm just thinking that if you are not already doing so, can you trigger an HV discharge through the discharge tube, and a high current/low DC discharge through the coil simultaneously?"

As presented, that is exactly how the circuit is arranged.

"......but I'm just wondering if you're able to put more current discharge through the coil."

I would really like to do this, but am cautious as I dont want my coil to explode and damage the tube. I cannibalized a small percentage of my quarter-shrinker capacitor bank for this experiment so I have the capability for several larger tests. This would necessitate the construction of an improved coil design both in deformation resistance and flux placement.

Any ideas for a more rigid coil?

Hi Frank, you guessed it...The Final Countdown

Mark Rowley

[Doug Coulter]

Way back in the day (early '70s) I was a grad student running a pinch experiment for a professor. In this case we had no interest in fusion, but the idea has stuck with me since, because the results were so good for what we WERE trying to do. Our apparatus was quite simple, but very effective at getting pinches down to the 1u size region. As I have no drawing right now, I'll attempt to describe this, and soon will be posting on a new test I plan to make, now that I have all the pieces other than hours in the days to get it done quickly.

Imagine a piece of large coax, with a spark gap in the center conductor a little back from one end (which is shorted). That's about it, except we did this bit with a 3" OD brass tubing as the outer conductor, and a 1/4" tungsten rod as the inner conductor, using a piece of pyrex glass as the insulator inside the big brass pipe (length about 5"). The other center conductor at the "shorted" end was a piece of about 1" diameter copper with a hole drilled through the middle for a small trigger spark gap electrode, also copper. We used a large capacitor (call it 8" by 20" by 30" tall) at around 15kv discharged into this via 10 pieces of RG-8 in parallel. I found out the hard way that you need at least 10 or they explode from magnetic repulsion at the peak currents we had -- in the 100kA region. We were using perhaps (my memory...) 15kV and about 100 uF Maxwell capacitor for this.
It was one designed for very low series L and R, center bolt and case ground. We built a special plate and ring connector for the top of this to keep inductance low and provide for all the coax pieces to connect.

Procedure was to get the best vacuum possible (our setup was a diff pump, no cold trap), then charge the cap and fire the trigger gap via a small coupling cap and an auto spark coil. There was a 1/4" hole in the side of the tube(s) with a Be window to let the Xrays out, and indeed there were plenty of X-rays far higher in energy than the kV we were using. After taking spectra with this setup for my advisor using a salt (NaCl) crystal and the film about 4 ft away, which took about 20 shots to expose the film, we looked at the thing as a point source for Xray microscopy, and indeed it was very good for that -- the pinch was calculated to be down to about 1 micron in size. Just put your sample close, and the film far, and you have a nice camera with plenty of high resolution magnification.

I of course plan to try this with other materials that might do fusion in these conditions, we saw Xrays up to the 100kV region from this. That's one heck of a compression ratio considering we had perhaps several cc's of plasma to squish down to that size in the pinch. I've managed to acquire the vac system (very nice new Pfeiffer turbo etc etc) and the capacitors -- Maxwell 120uF @ 10kV, rated at 100kA per shot at 10^5 shot life, and I have 3 of them (eg two spare if anyone is interested).

Warning to anyone trying this -- the magnetic field in the coax leadin at the 100kA level is very serious and makes the coax want to explode. I spent one day picking untold thousands of pieces of braid out of my hide when we found that 7-8 pieces in parallel weren't enough. I was young then, wouldn't want to repeat that experience and only have my eyes because I was looking the other way at the time..

My new setup will use 12 pieces of RG213 about 3 ft long apiece, and I am taking serious measures in the connections and blast shielding and to equalize the currents in the coaxes...This lashup could also duplicate the focus fusion experiments with the right stuff at the vacuum end, which I also plan to do. But I think this does a better job, I don't hear him claiming under 1u concentration sizes...

Also you should know that despite "heroic" measures in shielding there is going to be enough EMP when you do this to fry any PC or other electronics within perhaps 30 ft. We even fried tube type stuff (including a Tektronix 555 scope) and had to build a 6'x6' Faraday cage for the operator and test gear to make measurements. There was no more than one inch of exposed non-coax conductor in this lashup, everything else was totally shielded...

In the pictures, see one of my caps with the coax connector I machined for this. There will be heat shrink on the conductors, and a blast shield cap on top of this, as the coax tends to blow right were it has to be stripped and separated for the connection. 100kA will be hard to get with only 12 pieces of coax, but possible, with a sadly slow risetime due to the impedance mismatch.

[Andrew Haynes]

Mark, hows the z pinch going. Is 7kv 11uf enough for fusion.