Reply To: turn heat into electricity

[asymmetric_implosion]

Lerner wrote: Question to assymetric implosion: Did you ever calculate the stresses and impulses invovled with your insulator? We are running at 1 MA and intending to go above 2 MA, so we need to know in advance what tolerance we need to achieve.

Simple calculations were done but I wasn’t the one that did them. Our cathode (anode pointing skyward in our case) does not move the hardware at 0.25 MA in an appreciable way as determined by experimental testing. The calculations supported this conclusion.

I would point out that imperfections in the alumina can cause problems. I’ve encountered “bad” alumina that has voids or smaller scale imperfections in the past. I would like a second opinion from one more knowledgeable but I speculate that local defects increase local stress making it easier to break. Once a fracture starts the insulator is broken in a shot or so. This was my hypothesis with quartz. I could fire 1 or 2 shots before it breaks. My best suggestion is to increase the alumina thickness and buy the highest grade alumina which should have minimum defects. I use 1/4 inch alumina without any problems at 0.25 MA. My guess is other 1-2 MA machines use much thicker insulators to avoid the shock problem.

I mention three other options but all have problems and cost money. The first is to tie the anode and cathode together mechanically using nylon or some other insulating ties to limit the motion. The problem is how to tie the two together without flashing over. The second is to make the plasma focus a tri-plate instead of a bi-plate. If you carry the cathode current in two plates that surround the anode (cathode-anode cathode sandwich) you should be able to balance the force on both sides of the anode and it should reduce the impulse. The same problem as the first; how do you get the current to flow through the anode without a short. Techniques like these are used on >2MA Z-pinches which have large impulses. Tri-plates are the norm for large machines to reduce the impulse of the current pulse and reduce inductance. The problem with tri-plates is they tend to flash on large voltage spikes (like the pinch) splitting your current. The third possibility is to change the insulator design from a hat or reduce the radius of disk. I found that reducing the disk radius improved insulator survivability during assembly. I typically make the disk diameter just 10-30% larger than the outer diameter of the cylinder of a hat insulator. It is just enough space to put a thin o-ring in place on both the anode and cathode side to maintain the vacuum seal. I wish I could say more but I am straying into proprietary design information.