Wow. Could this be caused by expansion of a hot cathode? If that’s the case, some things to think about would be increasing the tolerance space between cathode and insulator, or increasing the tensile strength of the insulator (rather than flexural strength, as I had been assuming).
Or maybe this might be caused by arcing? In which case, dielectric strength might indeed be a consideration.
I’ve seen that before. :( I thought it was the disk part shattering but it is the cylinder. A plasma is forming behind the insulator. The rapid heating of the gas to plasma increases the pressure and shocks the insulator. I’ve cracked many insulators at the 0.25 MA level because of this. You need to increase the wall thickness of the alumina. It increases the inductance but you gain in lifetime by a great deal. It should take minor changes to the anode to make this work. The rest of the hardware can remain as is.
Also, the disk diameter seems to be overkill unless you’ve had arcs in the air before.
Dow corning 1540-20p is a pourable silicone that can be formed in a mold any shape. Mix it with alumina or other insulator dust and you have a tailor made insulator with a lot of flex, insulating properties thermal capabilities, its off the shelf and inexpensive. 1 mold can make as many as you need and can easily be modified. Samples are easily obtained and would most likely cover the first few trials.
Dow corning 1540-20p is a pourable silicone that can be formed in a mold any shape. Mix it with alumina or other insulator dust and you have a tailor made insulator with a lot of flex, insulating properties thermal capabilities, its off the shelf and inexpensive. 1 mold can make as many as you need and can easily be modified. Samples are easily obtained and would most likely cover the first few trials.
Silicone is like a polymer. It has many of the same problems of the polymer including UV sensitivity. Can alumina be mixed with it and will the silicone wet to the alumina? Wetting is key to building a strong compound. There are alumina and binders that can be molded into any shape but they are prone to cracking in the presence of intense plasma discharges like the plasma focus breakdown.
Maurice Ward died in May of last year. If he managed to file a patent application before his death, it should be published before the end of this year.
However, if the article is correct, this would not be a case of an inventor taking his secrets to his grave, because the formula has been written down, and close family members know it.
A plasma is forming behind the insulator. The rapid heating of the gas to plasma increases the pressure and shocks the insulator.
Can the gas behind the insulator be displaced by more insulation as either a solid or packed in powder? Can some type of pressure relief be formed in the insulator or the anode to accomodate the shock wave?
Dow corning 1540-20p is a pourable silicone that can be formed in a mold any shape. Mix it with alumina or other insulator dust and you have a tailor made insulator with a lot of flex, insulating properties thermal capabilities, its off the shelf and inexpensive. 1 mold can make as many as you need and can easily be modified. Samples are easily obtained and would most likely cover the first few trials.
Silicone is like a polymer. It has many of the same problems of the polymer including UV sensitivity. Can alumina be mixed with it and will the silicone wet to the alumina? Wetting is key to building a strong compound. There are alumina and binders that can be molded into any shape but they are prone to cracking in the presence of intense plasma discharges like the plasma focus breakdown.
Another cheap material to experiment with is sodium or potassium silicate. It has good thermal and dielectric strength but it is brittle. That can be partially handled with different additives. It can be used as a binder for alumina or other insulating material and it can handle ultraviolet rays.
A plasma is forming behind the insulator. The rapid heating of the gas to plasma increases the pressure and shocks the insulator.
Can the gas behind the insulator be displaced by more insulation as either a solid or packed in powder? Can some type of pressure relief be formed in the insulator or the anode to accomodate the shock wave?
I have not tried either solution. In my case, the problem is the thermal expansion of the anode. You need to leave a little room for the anode to expand. LPP likely has tight tolerances for other reasons. My concern with powders is that gas can get into the powder. The plasma forms and explodes the powder. After some number of shots the powder is all over the vacuum chamber. Good, bad, other…I don’t know. The another insulator would still have to be plasma facing so it would likely need to be alumina or some other ceramic material. I don’t know of a flexible ceramic.
To be fair, the insulator problem is a very hard problem and with limited resource it hasn’t been fully explored. Packed powder or another solution might work. In the case of limited funds, I typically suggest the tried and the true…add more of what you need. It comes at a cost, but I think that cost can be handled more easily than venturing into the world of materials development. I might not have pointed it out, but many machines at the >1 MA level have insulator lifetimes of hundreds of shots at most with tens of shots being more common. There are ideas on how to increase the insulator lifetime but there are tradeoffs. It depends largely on your pulse power requirements and goals. I know for my work, a thicker insulator was an acceptable solution. It cost me 5-10 kA at peak current but I can easily compensate with a slightly larger charge voltage. I’m sure these discussions are going on at LPP.
This is what we’ve decide to do, near-term and medium-term. Since we have gone at times for hundreds of shots without breaking insulators, we don’t think we need a thicker one right now, with 1 MA or so currents. It seems to be a question now of avoiding any lateral stresses that accumulate when we are centering the electrodes and insulator. So first, we are separating the centering of the electrodes from final assembly, so that when we assemble, everything will already be vertical. Second, we’ve enlarged a cushioning gasket to eliminate a small ring of the insulator that was unsupported. Third, we are eliminating a second, ‘centering’ o-ring, so there will be no lateral pressure conveyed from the anode to the insulator. I estimate that together these steps should take any stresses on the insulator down by at least a factor of 5. And, the plastic shims we were using to measure the gap between the insulator and the electrodes could have left behind residue that explosively vaporized when the plasma hit it. So we are changing the probes to eliminate this problem.
In the medium term, stresses will increases as we go to higher currents. So, thanks in part to the ideas from the forum, and searching the literature, we’ve got three “finalists” to look at for a new insulator material—silicon nitride, zirconia, and cubic boron nitride. We’re seeing what suppliers can come up with and quotes. Anybody use any of these?
Focus Fusion Society 
