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Someone told me that it was common to use apple pie and other common familiar foods to explain power generation back in the 1970’s. It probably meant a bit more back then since everyone loved the calorie unit for energy.

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Only a hundred car batteries per NIF shot (in the pulse power). That is better than I expected when you put it those terms. Z is 22 MJ is 6 batteries per shot. I personally prefer the apple pie energy system at ~1 MJ/slice or 6-8 MJ per pie. Some call it pulse power; I call it dessert. 🙂

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Joeviocoe wrote:

but they were using aluminum cans as the metal shell.

Like soda cans?

As delt0r said, they are pretty thick. The Sandia MAGLIF concept uses a similar idea of a thin Be can instead of aluminum to compress a ~100 eV magnetized plasma. As delt0r said, the problem is switches. The other issue is mechanically reloading on a useful time scale. The LANL tests damaged the test chamber badly with Q<<1. They intend to operate at very low rep-rate with high gain but it is a difficult concept to engineer.

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It’s an interesting take on MTF. I know the tests that were going on at LANL showed some promise but they were using aluminum cans as the metal shell. The liquid has some advantages since you can reuse it. Power plant? Who knows but it might show some interesting results.

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I’ve heard stories of the history but the stories are centered on the Mather focus. Like any verbal retelling, it grows a little with each retelling. The condensed version is the Mather PF discovery was an accident when a gas valve stayed open filling the chamber with gas during a thruster experiment. The tech operating the machine noticed a funny current shape. The Fillipov was intentional as an attempt to built a better Z-pinch. Both designs quickly scaled from ~500 kA to 2 MA in the 1970’s. It is not 100% clear why very few machines were ever operated above 2 MA. I know there are some technical issues but I get the feeling it was a political thing. Z-pinches are at the 26 MA level now from the same humble beginnings with their own technical problems studied and overcome. The history I’ve heard suggests Mather had a substance problem and was largely discredited with the funding agencies. Without his support, it was difficult to sustain the research at LANL. Take the history with a grain of salt.

The technical difference between the Fillipov and the Mather is the anode geometry. Both devices use a coaxial (concentric cylinder) geometry. The Fillipov focus has an anode length shorter than the anode diameter. The Mather geometry has an anode radius larger than the anode length. A few machines operate in the hybrid mode where anode radius and length are similar. The Mather seems to show superior neutron output for a given current. I believe there are solid technical reasons for this to be true but I don’t think anyone has proved it.

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Is it worth including a history of the PF in general to explain the differences between then and now? The PF was originally viewed as a path to fusion. That is the basis for most dismissal of the technology as a fusion reactor. People have “too much” experience with it and “know” the outcome. It would be helpful for the techies to see the differences between FF and conventional PF technology.

The about doc is really thorough. I like the comments on the revisions to shorten and clean it up for the average person. I know I said something above about techies but the history might help the average person understand the basic technology exists but it hasn’t been applied in the correct way.

A comment on other applications…x-ray lithography might be less likely than Q>1. Soft x-ray lithography even with a PF (NIE/NTU, SRL, Cymer, AASC) was developed and tested over 20 years ago. Cost models were favorable in many cases and the technology generally worked. It needed more development but the goal was in sight. The problem was a barrier that the lithographers were not giving up on optical/UV techniques. IBM spent a lot of money supporting the develop along with DARPA and it fell on its sword because the folks doing the work would not adopt it. Maybe times have changed but last I looked groups like Intel are pursuing EUV lithography while avoiding soft x-ray techniques like a plague. I guess my concern is you might have a loser out of the gate.

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The temperature that seemed optimum for us was around 300 C. I don’t know the exact cause but I speculate it has to do with the local gas pressure changing the mass carried by the plasma. Our anode heats due to repetition rate. When it is cold, the implosion time (time between current start and minimum in the dI/dt) is ~700 ns. As the run progresses to steady state, the implosion time drops to ~550 ns. At 550 ns, the neutron yield is optimized. Even at the optimum implosion time, the hot anode performed better than a cold anode.

It is important to note that we use SS304 as the anode. SS304 has poor thermal conductivity. We used emission spectroscopy to look for impurities and found none. I want to do a scan with a mass spec on the fuel gas after the run but it’s not in the cards right now. When we switched to a Moly anode, we noticed a drop in yield of a factor of two. Moly has a thermal conductivity about 1/4 copper. We know from measurements that the SS304 has a large temperature gradient relative to Moly. Is that gradient important? Don’t know. We know that all metals take up deuterium to some extent. We fired a number of shots on an anode in deuterium and then switched to argon. We recorded neutrons during the argon run. The temperature might be the optimum to release hydrogen from the metal and allow it to be replaced with D2. Over several runs you go into saturation. Burns reported a similar hypothesis on DT shots. It is hard to see the difference between H2, D2 and T2 using spectroscopy. We’ve found that operating at 300 C is difficult with Moly because it is nearly isothermal and our o-rings suffer from the temperature. Therefore, we actively cool the anode. In the process, our yield went down. The parameter space grows more complex as the temperature increases because the chemistry of the electrodes and possibly the chamber walls becomes important. These are problems addressed in plasma reactors for deposition and etching but they are seldom thought about in the context of a PF. You mentioned sulfur in SS304 in a past conversation…we see no evidence of sulfur. Based upon conversations related to other work, sulfur is just a bad in copper as SS304. In our experiments, the most likely contamination comes from the vaporization of anode metal. The cloud of vapor expands from the anode base and into the pinch region. Is some tiny mass present before the next shot that enhances the neutron yield as is described in lit with high Z noble gases. I know in our experiments with fuel mixing that we could not see any lines of argon additive at the optimum neutron enhancement. The same could be true for Fe, Moly or tungsten. The metal should plate out so I wouldn’t expect you to see this effect firing a few shots an hour or day. It might be that the vaporized metal releases the impurities trapped in it. Our solution to volatile products is a low base vacuum. We don’t start operating until our base vacuum is ~1E-6 Torr. The machine works better at base pressures of ~5E-7 Torr.

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Burning something up has a cost to it as well and redesigning has a cost as well. The cost of choosing bad switches are already well discussed on this board. I’m not suggesting significant time and money be spent on a massive redesign. I agree that at a few shots per day it isn’t worth addressing some problems. It was a suggestion from my experience operating a rep-rate PF that that anode erosion be addressed and the sources that I observed in my experiments.

Vansig: I already presented my solution a few posts back in this thread. The solution is derived from more than 250,000 shots fired on a single anode and the impact it has on the anode. We revised our design and fired another 100,000 shots to find more problems. We are on our third revision to address thermal management and e-beam management. To be honest, the e-beam problem isn’t the hard problem. The hard problem is figuring out if the anode temperature is an important parameter in optimizing the fusion yield of a pinch and if this temperature is useful/allowed for a given application. There is little to no data in published literature about this subject. It might be relevant to achieving Q>1 as we’ve observed increases in neutron output as temperature increased and then it declined after a peak temperature. The increase is 4-5X in some cases.

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Joeviocoe wrote: Would any voltages greater than 60 kV be even relevant to LPP’s DPF?

It depends on the optimum load design for the PF. As I said before, every PF regardless of size has to contend with to 10-20 mOhm of impedance in the flowing plasma. It is an artifact of the self similar physics of the PF rundown. You desire a roughly 100 km/s plasma speed during the coaxial rundown. The geometry is coaxial so the time rate of change of inductance (dL/dt) is 2E-7*ln(b/a)*v_axial. b is the cathode radius and a is the anode radius. For most PF devices, the ratio of b/a is between 1.5 and 2. Thus ln (b/a) is between 0.4 and 0.7. This leads to 8E-3 to 14E-3 Ohm. Some additional impedance always crops up so it is common to use 20 mOhm as the upper limit for machine design. If you use 20 mOhm, you need 60 kV just to drive the coaxial section during the axial rundown at 3 MA. Consider the other impedance like the bank resistance (<10 mOhm), bank inductance (~20 nH), bank capacitance (10-1000 uF) and inductance/resistance of the plasma (time varying). At a minimum you need to double the minimum voltage to achieve your desired current with a reasonable tolerance. Using this data, it is clear you might need more than 60 kV even at the 2 MA level. The last published paper from LPP has data at 1 MA and ~40 kV on the bank [PoP paper]. The scaling to 2 MA is fairly linear unless the cap bank size went up.

I was suggesting a scheme to minimize consumables (switches) by using transformers. To minimize the number of switches, you need to increase the primary voltage. Step downs of 2:1 are known at the 1 MA level but 4:1 would be better. If you require 100 kV to drive the load, then you need 400 kV on the primary. It seems a bit ridiculous at first glance, but >1 MV is very common in large pulse power. The trick is gas switches are used so you have the lifetime problem. The difference is by using a 4:1 transformer you need 1/4 the switches if you have a 400 kV switch (not easy). By replacing consumables with non-consumables (Transformer cores) you might find an advantage in cost in the long term. This approach works well at low currents but it might now work as well at high currents. More to be done.

As delt0r has said, there are other alternatives like the LVA. Each system has merits and problems. It will come down to cost and lifetime in the end. If the LPP’s cost model works for 1 month operation, then you need 5E8 shots between shutdown. Solid state can do it if it doesn’t fault. In fact it could possibly run for a few months which leaves the anode as the limiting factor. State of the art solid state pulse power for a PF is at 260 kA, 8 kV and 80 Hz. It is a big leap to 3 MA, 60 kV and 200 Hz but one that might be necessary.

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The Sandia LTD’s use gas switches and have rise times of 100 ns at 1 MA per module. They operate with +/- 100 kV on the input and give 100 kV on the output with 2:1 current enhancement [Mazarakis, PRSTAB 12 050401 (2009) for 0.5 MA LTD with 100 ns rise, new work was presented last year at the 1 MA level]. The switch lifetime is poor in the SNL LTD and solid state is better in that respect, it becomes a problem of the number of units and triggering the switches. The little I’ve run across on solid state LTD technology is modest currents (100 kA) but it takes many switches per module to get the ~1 us rise times that matter. The voltage is pretty low per module at 2-5 kV. I’m not saying it won’t work but it will require a milliion or more switches. Can you trigger a million plus switches at low jitter? I think so. Can the solid state switches survive the pinch voltage at ~ 3 MA, which is 750 kV to 1.25 MV, when it feeds back to the pulse power (inductively divided of course)? I don’t know. Is the cost of the front end more than the gas switch solution over the long run? Probably not.

I don’t have the ref for it but I am familiar with the work, people use what are called load current multipliers (LCM) on ~1 MA Z-pinches to optimize the matching between the load and the source. The LCM is nothing more than a 2:1 current step up transformer on 100 ns machines. Our PF system operates at 700 ns using a 6:1 current step up transformer. The transformer cores were like $1000 per unit. We use a total of twelve cores. The Thyratron switches we use cost $3500 per unit with another $4K for the controller box per switch. It was a huge cost savings to use the transformer solution and reduce consumables. It fires at 10 Hz as long as the anode can survive. Admittedly, the V-s product for a 3 MA system would be significant by comparison but the cost may not be as bad as you think. The problem again is switches.

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The one bummer with the LVA is the switch count. You have many switches and if their lifetime is too short you have a huge replacement cost. Last I saw a Sandia LTD it had something like 40 switches in it. You need three to five modules so the switches could be a killer. Just throwing it out, but we use a voltage step down transformer to operate the primary at high voltage and low current while operating the secondary at high current and reduced voltage in a small PF. The big advantage is we can drive 60-80 kA using two thyratron switches (~10 kA each) on the primary. It minimizes the switch count (i.e. consumables) but the cost of the cores is significant. In the case when you are replacing switches frequently, it might be a path forward. The real problem is the primary voltage at high current. You need something like 60 kV just to drive the coaxial rundown section. So you need more like 100 kV on the secondary. For a reasonable reduction in switch count you want a 3:1 or 4:1 stepdown. A 400 kV primary is not impossible….

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Well, the patent is more complicated than just an arc. That is the simple story. I agree it is a bit overkill but it was granted. To be fair it has to operate in conditions that are not agreeable to the bench top so that is really the innovation.

The size of the caps haven’t hampered the inductance of PF pulse power. A typical cap has a low inductance of 20-50 nH per unit with several units in parallel (~10). The switches are typically the big inductance hogs at 20-300 nH per unit but they are that value regardless of the cap geometry in any practical case. Some large PF systems like DENA have a bank inductance of <5 nH by massively parallel architecture. My three PF devices operate at ~20 nH in the bank with three different types of switches and three very different architectures. In our sources, the switches and caps typically contribute only 5-7 nH (lots of little things in parallel). Most of the inductance comes from bus bars near the load that must accommodate the transition from air to vacuum. It's not universally accepted but the idea has floated around the PF community that you need some finite inductance in the bank to make the PF work at optimum. The value of inductance goes up with current. It does not agree with power transfer theorems but to accommodate certain plasma conditions to confine and sustain the pinch. The inductance relationship I see thrown around most frequently is the pinch inductance should match the axial phase inductance. The bank inductance should match the axial phase inductance. I don't know if this is the true optimum but it shows up consistently over the years since the 1970's. The pinch inductance is going up with current and in most cases the axial phase inductance is going up with current. Therefore, the bank inductance is going up to sustain the ratio. I can tell you this much, the PF-1000 machine has a low source inductance at 1.8 MA and it performs as well as FoFu-1 at 900 kA. By any pinch scaling law, PF-1000 should be up by more than 10X over FoFu-1.

The LVA is a linear voltage adder?? The high current pulse power equivalent is the linear transformer driver (might be same thing with different name). Modules that operate at 1 Hz are currently available with 100 kV output at 1 MA into a matched load. You might need 3-5 of these units in parallel. Sandia National Lab is handing them out to universities for numerous studies. I know U. Michigan and U.C. S.D. have modules. The key questions are the scale up of the PF physics at ~3 MA with source inductance. The LTD is a low inductance source so it might not be able to drive the additional inductance that might be needed to optimize the pinch without significant loss. Time will tell.

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Sorry if you think I’m having a different conversation.

I was just doing the math on storing energy in an inductor to drive the pinch. My employer has a patent on inductive energy storage system for powering arcs. You basically use a DC supply to charge the inductor and then close a switch. You get the voltage needed to breakdown the arc and drive the current for like 100 us. That is my very limited background with inductive energy store (mainly from lunch room chats) so I was taking it from that perspective.

POS are crap above 1MA but some do work at the <1MA level. A machine at NRL, Hawk, uses a POS with reasonable success instead of a water line. They drive 700 kA in 200 ns or less.

I’m badly out of touch with compulsator tech, but I thought you need to brake the compulsator to extract energy from it. I take your comment to mean it is not true. I was also under the impression that 2 MA was beyond a compulsator with fast pulses like 10 us. Can you recommend any reading material so I can get caught up?

I’m not sure energy density is the biggest problem in pinch type pulse power systems. I think folks would use smaller pulse power if it was available but I don’t think anyone in the pinch community sees that as a driving area of research. Most of the problems seem to focus on faster rise times and load design.

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If you charge an inductor with the necessary energy (~100 kJ) at the drive current (~2MA) you need 50 nH. That is much less than I thought. I was thinking something like 1 uH. This adds to my argument not to do math in my head. It would be difficult to couple a 1 uH load to the <100 nH pinch but 50 nH has some hope. This is one problem with plasma opening switches. You would generate a high voltage spike at switch opening well in excess of the necessary drive voltage of ~40 kV. You might be saved by the fact that the gas will breakdown and dissipate the voltage before the switch is overwhelmed. Still at 100 kV switch is required and if it's an opening switch you are doubly screwed because it has to stay closed before suddenly opening. Solid state is the only answer. Diamond will not work because you cannot keep it on. SiC has some hope but a 100 kV switch is a challenge. Turn off is far worse than turn on. Another question is how to charge the inductor with 2 MA to start with. You would still require a cap bank because I don't know of a slow pulse 2 MA source to charge the inductor. I guess a fly wheel could do it for low rep rate operation like they used to do on Tokameks. I guess you could charge many units in parallel but you wouldn't want to see the source inductance get too low because you couldn't match to the load.

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delt0r wrote: Well no i can’t *because* there are no fast opening switches. In small scale well yea, every switch mode power supply uses inductors and the main energy store which makes them far more compact than a transformer/ripple cap combination. But the physics is straight forward and power density is pretty good compared to caps. Current at the MA level is a little more challenging without superconductors. But then MA is not trivial with caps either.

But if you can get a solid state switch with high current and high standoff. There is a strong case to use magnetic storage rather than caps.

A large slow inductor will not be able to efficiently feed a fast pulse system like a plasma focus. It is better suited for tokameks. Over a decade of effort was put into the so-called plasma opening switch. The idea was to charge an inductive energy store and switch it to the load. The problem with inductors and fast pulses is the coupling. Plasma opening switches have an efficiency limit of 25%. The switch limits the efficiency to 25% when the switch is perfect. Marx technology has an efficiency limit of 33%. Regular RLC banks can do better depending on the match to the load.

Diamond is unobtainable as a practical switch at the moment. The material is mature enough to be used as a switch but the trigger is a problem. Diamond cannot be doped n I believe so you need to trigger it with UV photons, e-beams ,etc. The switch is only on for as long as the photons are present when operating at high voltage stand off. You need a high power UV laser below 200 nm, a flash lamp or particle beam system. The military invested heavily in diamond and it fell on its sword. For someone that’s saying I’m being negative, again, I’m speaking from experience. My employer worked on diamond switching for a decade or more. The trigger is the show stopper right now. A laser could be built to do the necessary triggering but it would be high power and high rep rate. Not a good combination.

SiC holds a great deal of promise but I don’t think it will reach the necessary voltage without stacking the switches in series which is the problem with Si switches. When you run switches in series you run the risk of jitter and the voltage showing up across one switch. A 20 kV switch does not like having 40 kV across it for long. This is the problem of solid state. Gas switches self break above their rated voltage and recover. Gas has not organization to it so who cares if it arcs. In fact, the switches use arcs to carry the current. Solid state switches arc under self break and solid materials don’t recover. There are tricks to avoid this problem but it takes someone steeped in solid state pulse power technology to make it work. Even the masters stay away from high voltage, high current because of the risk and cost.

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