[asymmetric_implosion]

Here is one site with some of their papers put up for free. In the interest of disclosure, I am not a part of this group but I do know their work reasonably well.

http://www.intimal.edu.my/school/fas/UFLF/

I’m sad to say that experience is expensive. I pay $30 or more per article. I guess the question one must ask is the level of devotion to learning the subject. Just a warning, these are peer-reviewed publications. The authors expect some minimum understanding of plasmas and little is provided to help you in the journal articles other than references to other articles and more money spent. If you are seeking a basic appreciation of the subject, popular science type literature on the plasma focus exists and some of it is linked on this site. I look forward to the day when the pay walls fall down so everyone can see all the different interpretations of available data and decide from themselves what is the best interpretation.

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But you did use the tungsten pins? If so, that is very different than most PF devices I’ve run across in literature. Does that not qualify as chewing the sheath? I am curious if tests have been done without the tungsten pins and compared to tests with them.

I agree that an experiment is important to evaluate neutralization of the beam. The point was to say that a particle accelerator is not a good test bed for testing the generator without a gas box to serve as an electron source.

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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.

[asymmetric_implosion]

Maybe, but MHD generators tend to favor quasi-steady plasma flows and the plasmoid is far from quasi-steady unless you are firing at very high repetition rate. Can the plasmoid survive a modest transit in the dense fuel gas? If I’m understanding the long term goal of LPP, they wish to operate the focus at ~100 Torr. Even an alpha particle at 3 MeV has a range limited to 15-20 cm (using an analytical alpha particle slowing down model from Mayo, Intro to NE). The plasmoid is unlikely to travel that far so the MHD generator is right on top of the pinch which is probably bad for its lifetime. High energy pinches produces copious amounts of UV, X-rays and charged particles that can chew up materials.

[asymmetric_implosion]

Joe,

That is a typical view of the plasma focus. It is also demonstrated to be incorrect. In the last couple years, plenty of measurements support that ions and electrons are traveling in both directions. Work by Roshan et al at Nanyang Technical University showed that ions are indeed traveling toward the anode when they should be moving away. It is about local potentials. The conventional view of a pinch creating fast ions is that magnetic field rapidly evacuates a small region of the pinch due to instabilities. With the magnetic field changing in time, you generate an electric field which accelerates electrons and ions. These ions and electrons move in opposite directions as one might expect so you get two opposite moving particle beams. The problem is the electric field has a radial profile with some electrons and ions accelerated in both directions. None of this requires Gigagauss fields. For the tech savvy, Malcom Haines wrote a review article on the subject of pinches which contains the full argument; just pony up $90 for it. Worth the read in my opinion as it gives the history of pinches back to the 1900’s. Anyway, even if you create pure beams of ions and electrons in opposite directions, you have the send these beam through a dense gas. As the ions fly by they pull electrons from neutral atoms. It is possible that some electrons chase the ions. More likely, the gas in the direction of the generator is partly ionized due to UV so the electrons are already hanging out. They hitch-hike with the ions leading to a reduced current. I know a number of measurements disagree on the impact of these electrons but it is another loss term to consider.

[asymmetric_implosion]

The charged particle beam to electricity component of the generator is very well tested already when you have a single charged particle species flying through it. I see two real concerns: can the x-rays be converted efficiently with the onion and does the ions produced by the plasma focus travel alone?

The x-ray issue is largely straightforward to test if the onion design exists on paper. The charged particle beam issue is more complex. Firing an accelerator alone isn’t enough. You need an intense ion current of >100 kA passing through something like 100 Torr of gas for tens of centimeters. There is some evidence in published literature that the ions are not traveling alone; they have electron partners. I don’t mean they are neutral in the sense of an atom but rather the heavy ions are pulling electrons along for the ride. If ions and electrons travel together, the net current is zero. With net zero current, you can’t use the transformer based technique proposed by LPP to convert the ion beam to electrical energy. Even if you get a current, what fraction is neutralized before you reach conversion location? If every He ion drags along one electron, you’ve cut the beam current in half. Breakeven suddenly got harder. A test with a conventional particle accelerator is hard because the currents are typically limited to less than 100A and passing ions into a gas cell for the test is challenging.

[asymmetric_implosion]

I hope it useful but enjoyable is a nice sidelight. Our first machine and FoFu-1 were designed by the same person, well, between the switches and the plasma load. Our first machine has been operating since 2006 so we have a few more years “practical” experience. We spent time burning contacts and revising. I know the pain all too well. My experience is limited to 0.5 MA so effects unique to larger machines are beyond my experience at the moment. I hope that will change next year.

Our interest is rep-rate PF operation so we had to focus on making the source run at 1 Hz. Contacts are a key part of the problem. We run our small PF at 10 Hz for hours so there is another level of problems that had to be addressed. This develop was evolutionary instead of revolutionary so it took us quite a few years to get everything running just OK. We hope to continue our PF work to make the source exceptional for our customers.

[asymmetric_implosion]

Gold is a great choice to minimize oxide. We didn’t want to deal with plating gold and it turned out we didn’t need to.

Our anode plate is SS304. Our anode is SS304. We use a small copper gasket (no silver plating) between the two surfaces. The anode contact diameter is 0.75″ and 30 mil thick. At 300 kA which is our normal upper limit of operation in rep rate mode, that translates to 0.63 MA/cm^2. We’ve fired a 450 kA shots at low rep rate without any problems or ~1 MA/cm^2. I’ve run over 20,000 shots on that contact without issue at 300 kA. We use a single 1/4″ screw to hold the anode. Our cathode contacts are far larger, more like 4″ with 30 mil thicknesses using SS304 on silver plated Al. We’ve also done Cu on SS304, W on SS304 and Moly on SS304 up to 400 kA/cm^2 when the harder metal has the contact.

I haven’t done the pressure calculations since the contacts are working well.

[asymmetric_implosion]

We seldom have problems with stainless steel contacts. We typically connect SS304 plates to silver plated aluminum and copper with little trouble. If you want the SS304 surface to behave you could have it nickel plated. We considered nickel plating at contacts between SS304. Another approach is brazing copper onto the SS304 which makes a good contact at the SS304 copper interface and make the current contact between two interfaces of silver plated copper.

Using stronger bolts does not guarantee better pressure. The strength is determined by the strength of the threads into what I hope is the steel.

If the arc is always in the same location you might have a low spot on one of the surfaces. A nice face cut could help. You need to make sure the steel bites into the copper by more than the machine tolerance for the flatness. We always have a perceptible (by touch) indention even after the first contact. We’ve changed cathodes, anodes and other pieces over and over without any problems.

[asymmetric_implosion]

Our experience with indium contacts was they tended to melt and solder the parts together.

If you can share, what is the size of the contact (thickness and diameter if circular)? You might need to move to a larger diameter contact or a contact with more surface area and many more bolts. I know we doubled the bolts in our tightening pattern at one point and it dramatically improved our contact reliability. I think the preferred strategy is to use many smaller bolts rather than a few larger ones. This leads to a more uniform contact like a conflat flange. Are you tightening the bolts in a pattern that leads to smooth and uniform draw-dwon on the contact? If you are arcing at one location, it can be due to a bad tightening pattern.

The other question that might be relevant is the inductance of the contact area. Are you sure that inductance isn’t the problem at the contact and the current is jumping the contact in favor of air or fuel gas leading to discharge.

[asymmetric_implosion]

Good luck with Indium wire. We found it to be a pain. Also, it activates pretty nicely with neutrons.

Rather than wire I suggest thin walled copper tube. It yields easily, it’s cheap and it has none of the problems of Indium.

[asymmetric_implosion]

The main problems with high current pulses are contacts at interfaces between pieces and insulation. Insulation can and should last for long periods when properly designed. I operate a plasma focus that has a similar electric field in the gap between the anode and cathode as FoFu-1. It has operated for nearly 1 million shots without any problems. The more common problem is the interface between two pieces. When you think about low voltage, low current connections, simple contact between the metals is adequate. The ohmic loss in the contact is small compared to the rest of the circuit so little voltage is dropped across the contact (10 kA/cm of length for a contact that is only 0.01 inches wide.

The long shutdown is probably due to switch problems. The contact issue typically takes a couple weeks to resolve at most. The new switches are probably slow to fab (16 weeks is not unusual). Once at LPP, they must be installed and tested. It takes time to get the switch jitter down to tolerable levels.

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Depending upon the simulation there are test cases that have analytical solutions that can be compared to. Sadly, the most common test is to take experimental data and compare. I say sadly because the limited shot rate and the ability of diagnostics to resolve all the key parameters is limited. My observation of high density plasma simulation codes on devices like plasma foci is to keep patching the code until it gives you the same answer as the experiments within reason. Even after these comparisons, the codes don’t always conserve all the necessary parameters like momentum, energy and mass.

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BSFusion wrote: Multiphysics

In what context? How do you go from using a fluid assumption to a kinetic description when you have to track extra variables you didn’t track before? You have to make assumptions on the transition. Depending on the problem the assumptions are reasonable. The plasma focus does not lend itself to a nice transition. I know a group at Livermore is working on a pure kinetic simulation and it takes days of computational time on something like 100 processors and they model only the last 100 ns of the implosion. Fluid-kinetic hybrids have yet to demonstrate agreement with experimental data. MHD models have generally been better when everyone knows they don’t work. Another group in the UK is working the problem on the plasma focus. I’m not familiar with Dr. Dumas, but the Imperial College group has a solid computational base and a 3D MHD code as a starting point.

[asymmetric_implosion]

I’m having trouble with the link as well.

From the description it seems interesting, but 2D, two-fluid MHD simulations of the plasma focus have been around since the 1970’s (D. E. Potter). The problem moving forward seems to be how to go from a fluid approximation to a kinetic calculation. The pinch conditions don’t lend themselves to fluid approximations. Some recent work by Dale Welch in Physics of Plasmas describes full kinetic simulations of a Z-pinch implosion. In addition to the usual complications of the particle transport, is the radiation from the boron going to create problems due to opacity? Kinetic simulation is hard and radiation transport added to kinetic simulation is beyond state of the art in published lit.

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