Reply To: NIMBY FUD

[asymmetric_implosion]

The beams exist; I’m not saying they don’t. People have measured the beams using all sorts of techniques from nuclear activation to magnetic spectrometers. The beams have been an accepted part of plasma focus operation since the 1960’s. My comment regards extracting energy from them. I’m saying that the beams are composed of both ions and electrons. When the beam is composed of both ions and electrons, the net current is lower. If 100 kA of ions are moving toward the generator but 10 kA of electrons are moving with them it is a small issue. If 50 kA of electrons are along for the ride, you’ve lost half your generator current. Now you have a pretty serious problem. I don’t know the answer on magnitude of the electrons because it is strongly related to the experimental conditions. I’m simply saying it is observed and a potential problem to address. I recall a post earlier about the ion beam missing the generator region or the current seeming to be low. This could explain it. People have observed an off axis maximum in the ion beam emission. It is accepted that the ion beams have some angular profile from parallel to the pinch (0 deg) to perpendicular to the pinch (90 deg). The contribution of ions is suggested to peak +/- 5-10 deg off 0 deg for some gases using electric probes. Nuclear activation suggests the ions peak on axis (0 deg). The two measurements are reconciled when electrons are added to the picture. Electrons cannot activate the materials used in activation experiments but they can reduce the observed current on an electric probe. The reduction was ~20% so it is probably getting serious.

There is a world of literature out there that is not derived from LPP or their experiments. I know this message board is not familiar with it but relying solely on one group’s writing is not a good idea. It is easy for someone to lead you down a path when you rely on one source. From my perspective, LPP is doing two things different than the rest of the folks in the PF community; small electrode geometry and as they call it chewing the sheath. The small electrodes are interesting because it challenges some conventional wisdom about plasma focus devices. I think this is an interesting path which will benefit the Z-pinch community in general. Chewing the sheath is interesting but I don’t know if it is the reason for the results observed by LPP. We have observed filament breakdown leading to pinches as good as uniform breakdown. My opinion is filaments exist with or without the tungsten ring and there are other ways to create them that don’t rely on tungsten pins with poor current contact. Another PF company a few years back showed a ring with a triangular cross section produced very reproducible pinches operating at modest repetition rates (~1 Hz) for hundreds of shots. Rep rate operation changes matters as well as plasma is left behind between shots. We observe improvements in uniformity with increasing repetition rate. If chewing the sheath is essential, it will be hard to do at 200 Hz when a bunch of electrons are hanging out ready to breakdown uniformly.