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We are talking about two very different time scales. The time scale I was discussing was the pinch time. The B-field reduces the electron thermal conductivity in the plasma allowing the plasma to reach a higher temperature for its <1 us lifetime. The ions and neutrons produced in fusion can escape the magnetic field during the pinch. I'm not sure on all the details but some discussion on the conversion involved MHD conversion of the ion energy and a thermal cycle to deal extract the neutron heat. The MHD extraction would be on the pinch time scale but containing MeV ions is not possible in this configuration. The ions run away and get converted to electricity. The neutrons are free to do as they please so they thermalize in a blanket to breed more tritium and extract the heat at high temperature.

The concept is sound in the sense of the physics is proven but the engineering is going to be costly. They are talking about a 40-60 MA machine for the power plant. The designs requires a system with over 500,000 switches operating at 6 MV or more up to 0.1 Hz. The unit modules work but it is a long road to making hundreds of unit modules work together at low jitter.

Sandia is grasping at straws since NIF has come out saying they are the only path to fusion and that was their intention from Day 1. MAGLIF is more psychological than physics driven. If you think about it, what is the first comment made by skeptics such as myself? Fusion has never produced more energy than it took in. If Sandia can show a configuration, practical or not, that can demonstrate physics gain, it will excite people about fusion again. Strong fusion supporters should embrace the potential of breakeven on any concept because it will help the politics of fusion on the large scale. In particular this is a hybrid concept and one might argue that LPP is using a hybrid concept. The argument is simple from there. LPP can do the same as Sandia at lower cost, practical repetition rates and with fewer components.

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MAGLIF is a far cry from Focus Fusion. It seeds a strong magnetic field (~1 T) before a 26 MA pinch implodes a metal cylinder onto the preheated, magnetized DT fuel. The concept is supposed to take advantage of limiting electron heat transport by flux compressing the initial 1 T field to something like 20 T or beyond. The physics of the components are proven but the entire concept has yet to come together. Sandia has been promoting the concept for over a year at meetings. It is about as much a path to a fusion reactor as NIF. Another piece of nice physics that cannot be engineered at a reasonable cost. The system relies on recycling the vacuum transmission lines every shot at a 0.1 Hz repetition rate. It is also a thermal process which does not play to the strengths of a pinch device.

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Milemaster wrote:
Now, we could make this vertical grooves slightly Helical, (Rifled) This will induce a circular motion to the plasma, similar to the effect of the coil and improving the filament formation allowing to manage the plasmoid size.

The other potential problem with the helical geometry is someone has a patent on that approach.

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Your protocol moves into a realm of unexplored territory. Understanding the unknown is the heart of science and I believe experimental work is the key. However, enthusiasm must be tempered by technology; specifically, technology that is in reach. A comment that I’ve heard passed around for years in science is that cutting edge work is funded to attempt one miracle. Your protocol has at least two miracles in it. The first is the operating pressure. I think it is possible to build an experiment to test the pressure component at a few shots per day. The cost is the real issue. To drive a plasma at 50 atm, you would need a substantial capacitor bank. The electrodes could be designed but pressure seals that can operate from 0.03 atm to 50 atm are daunting. I don’t know if the technology exists. If someone knows of such a technology I would love to know about it. It would fix some of my problems in other projects.

The second miracle is operating the pulse power at the repetition rate you are implying. If I take your words in the most favorable way for the pulse power, it is a daunting pulse power system. If one starts with the assumption that you want to fire a pulse as soon as the gas in the pressure chamber is back in equilibrium after the previous shot (~1ms). A capacitor bank can be charged in 1 ms with the correct supply. It is not common but possible. The energy in the bank at a minimum would be the LPP bank or ~60 kJ. Charging the bank then takes an average power of 60 MW (average home is 20 kW so you need the equivalent power of 3000 homes). OK, still not impossible but I don’t want to pay for that electrical substation. Charging and discharging a capacitor bank of this magnitude at 1 kHz has never been attempted. The capacitors exist as do the switches but again cost is significant at $7000 per switch (100+ switches) and $300 per cap (>200 caps).

I will try to muster my enthusiasm, but the key questions are what is the budget for such an experiment and where is the money coming from? Just off the top of my head you are talking about $1M just to reproduce the LPP system. The high pressure testing would probably cost another $1-2M/yr in people/overhead for likely two years to build the experiment and test it. A materials budget of $500K/yr would probably be essential after the pulse power and other essential components are in place.

The high repetition rate system could cost 10-100X the FF-1 system in hardware. To run the experiments would probably cost $3M/yr or more in qualified people with overhead to answer the questions. If you envision the experiments on the smaller scale it could cost less, but I can’t see a program of less than $5M over 5 yrs to get the answers you are seeking. The potential pitfall for the PF is that at small scale you are unlikely to form a plasmoid.

If you consider these comments unenthusiastic, please understand that the money is likely the limiting factor. Given the current government funding climate, it is hard to imagine them investing in such an experiment. Private investors are clearly interested (LPP is an example), but this project starts off requiring more funding than LPP has now to complete their first stage. There are a number of serious technical challenges that need to be addressed. None of these are impossible but the return on investment might be poor and the technical challenges might mask the answer you desire.

If a funding stream has yet to be identified, time should be taken to identify a likely customer before sweating out the technical details. I have yet to encounter an investor or gov’t techie with a checkbook that needs >25% of the technical details in the first discussion. Big picture ideas are generally enough before a proposal (gov’t) or business plan (investor). If you are seeking a nights and weekend kind of commitment of people, it might be possible without funding just to fill in some technical details, but a serious set of experiments is out of reach. Most university PF devices fire a few shots per hour at most and privately held machines are harder to access without money.

If you tell me that money is no problem, more technical details can be worked out and a team of solid individuals could be identified to attempt such an experiment. I can provide a list of folks that are active in the community with the skills that you need.

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Oxygen might not be aggressive enough. The beauty of carbon is oxygen turns carbon into CO2 which is easily pumped from the chamber. Boron forms a stable solid oxide. Fluorine can form a boron gas but it is highly toxic and fluorine attacks almost everything. The key question is the etch rate vs the deposition rate. Fluorine is commonly used in semiconductor chamber cleaning so the recipe exists. An question to pose would be using a mixture of BF3 (toxic boron gas) and hydrogen as fuel. The fluorine would attack the electrodes, but it would also keep the boron in gas form. I guess it would be a trade off in which gets you first, boron build up or electrode/chamber erosion.

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Plasma flow along the electrodes is described by a number of models. The most analytically tractable is the snow plow model. The models grow with complexity as detail is added. The most advanced models run on parallel computing systems taking hours to complete a full simulation. This models consider current, geometry, gas pressure and depending on the model, the fraction of the gas that is carried by the plasma. In a plasma focus, only a fraction of mass is actually carried by the plasma. All these models assume breakdown (plasma generation) is possible and meets certain criteria. Generating a plasma at 50 atmospheres is non trivial to say the least. There is a reason that plasmas are generated at low pressure.

The pressure is an important input in the electrode design. For a given set of electrodes and current, there is a unique optimum pressure. One can increase the current to increase the operating pressure but moving from 0.03 atm to 50 atm would require an increase in current from say 1 MA like FoFu-1 to 50 atm without changing the electrodes requires an increase in current by ~31X. To my knowledge, the peak current ever generated in a pulse power device is 26 MA at Sandia National Lab on the Z-machine. The down side is the Z-machine pulse will not work for a PF; it is 100 ns instead of the 1-2 microseconds more commonly used in a large plasma focus. Z stores 20 MJ to generate the pulse. For a PF you would need something like 10X the stored energy to produce a pulse that is 10X longer. If you want to extend the current pulse by 1000x, you need 10,000X the energy of Z or 200 GJ per shot. Power generation would need to be 3X more than the bank energy to be useful. A single shot that produces 600 GJ is not impossible but the system engineering is well beyond our abilities at this time. For perspective, the average power plant operates at 3 GJ per second thermal energy or roughly 1 GJ/s electrical. You could argue that you need to fire only once every 400 s to sustain this rate. The problem would be designing a system that could deal with such a peaked power generation cycle.

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I’m not sure you can sustain the current pulse or more importantly the pinch/plasmoid for 1000X the time. The PF is by nature a fast pulse (~100 ns) event driven by instabilities in the plasma. To stabilize the pinch would eliminate some of the physics that makes efficient fusion possible. These instabilities are also driven by temperature of the plasma and the speed the plasma travels. By operating at 50 atm, you are likely to quash the conditions needed to generate fast ions. LPP operating at 80 Torr is a huge departure from normal PF operation of 1-20 Torr. I’ve seen mentions of atmospheric fill pressures in the long term but the jury is still out on an upper pressure limit. Plasma physics might not be kind enough to allow such high pressure fill. If I were to bet I would guess the operating reactor will be sub-atmosphere at room temperature. If heated the ambient pressure could be greater than atmospheric pressure.

Scanning the pressure to optimize radiation yield is commonly done by those that operate PF devices. Models and semi-empirical relations provide a bounding range but the pressure scan is essential to find the real answer.

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The fix is to send out the chamber for cleaning or implementing a cleaning gas mixture involving fluorine or other extremely reactive gas that can volatilize the contaminant. The problem in a reactor environment is the shutdown time to clean. It could take minutes to hours to coat the parts and then hours to clean them. The “on time” for a coal fire plant or nuclear plant is typically rated at more than 90%. That would be hard to accomplish if coating takes hours and the cleaning takes hours. The alternative would be to run parallel fusion “light bulbs” driven by the same pulse power. One bulb is on while the other is cleaned. I don’t know the boron deposition rate but it is probably an important experiment to try in the near term. The reason to try carbon it is more conducting and dealing with it is low risk from a human stand point.

The boron will be a plasma when it is emitted by the pinch. It will plate out onto something before dust forms. No time to nucleate particles when transit times are less than 10 us. That is not to say the coating will not flake off as dust on a longer time scale. Electrostatic collection might be feasible on particles that have fallen off the walls. The real problem is will the insulator get coated. You need a clean insulator for the PF to work correctly.

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Boron can be made conducting as well so same problem as carbon. It can gum up the insulator and ruin the breakdown along the insulator. Any gas that can decompose into a solid has the potential to gum up the process. This is a common problem in the semiconductor industry with silane and large molecules that contain silicon. I think you will get some shots before it becomes a problem but operating at steady state could be challenging. High temperature will only improve the bonding of boron atoms and grow dense thin films.

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50 atmospheres….wow. That is ambitious. Is this a real thing or an idea? I know shock plasma system operate in liquids and at high pressure but what is the advantage in a PF? To my knowledge the highest successful pinches are still well below atmospheric pressure at like 80 Torr. Most folks operated at more like 8 Torr. If real, what kind of current pulse is required?

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Boron fouling might be an issue for the chamber walls, electrodes and insulator. Once the decaborane is converted to a plasma, it will decompose. People observe this in methane PFs. The chemical bonds break in the high temperature, high density plasma. There is no reason for the carbon to grab back four hydrogen atoms. Decaborane will be the same. Boron could coat every surface or form a debris dust. Devices that fire many shots observe dust formation in the vacuum chamber as electrode and insulator material is converted to plasma and then solidifies from the plasma as it cools. Even if the chamber is held at temperature, the boron will still plate out unless there is chemistry that demands for the formation of a gaseous form or boron. I don’t know the answer for sure but I doubt it. If boron readily formed a gas with hydrogen the tokamek guys would already know about it. It was common practice to coat the inside of tokamek with boron for a time.

I find it hard to believe that the fuel will be evacuated between shots if the repetition rate is much greater than 1 Hz. Continuous feed and pump is done for inert gases for high repetition rate PF devices in lithography. I’d imagine a similar strategy would be implemented at the power plant level. Most of the fuel gas could be recycled. It would be interesting to fire a few shots and see how much the gas pressure drops for gases that chemically react and are likely to have solid products. Methane or acetylene seem like good choices as carbon is inert but very messy in terms of vacuum as carbon holds onto everything. Black powder all over the place isn’t very appealing either.

This is a difficult problem. It will take some creative solutions to keep everything working correctly.

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NIF was built to be Q>1. It’s hard to simulate weapons when Q1 but it is likely through brute force alone. NIF has yet to run an optimized load. They are running test loads to calibrate the simulations and test diagnostics. This is normal practice when bringing up a large experiment. If you don’t return to previously tried experiments you won’t know if you are repeating them.

ITER was designed to be Q>1 but there are a number of technical challenges and political hurdles. The debt crisis in the EU could hamper ITER. Technical challenges include the survivability of the first wall and plasma heating.

As far as p-11B goes, the large science community has taken the perspective that you walk before you run. LPP has taken the same approach; start with D-D or D-T. Admittedly, a tokamek that can use p-11B or any aneutronic fuel is unlikely. Aneutronic fuel may be possible with NIF, but I doubt anyone will try it. NIF isn’t about controlled fusion energy; it is about simulating weapons.

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andrewmdodson wrote: My understanding is that the switch only has to close extremely rapidly… the capacitor bank discharges completely into the reactor, expending functionally all the current and then the system resets for the next pulse…

Why does there need to be reverse conducting? Are you using the same electrical pathway to recharge the cap bank?! I thought that no power had yet been extracted from the DPF?

The switch needs to close rapidly but it doesn’t have to be 1 ns or so. The bank discharges completely but the PF is an underdamped ringing circuit. As a result some current flows back into the cap bank. Some solid state switches can’t handle reverse conduction from the ring. I know SCR type switches turn off when the voltage and current reverse. So what happens to the current that is trying to flow back into the bank? You have created an opening switch device. Opening switch devices are well known for pulse compression applications that need to generate high voltage in a fast pulse. The PF circuit is a charged inductor without an outlet when the switch opens after a forward going pulse. Inductors like to keep current moving so they generate voltage at the expense of stored magnetic energy. A fast closing switch can more than double the voltage left in the PF. The SRL group had the same problem so they implemented a massive collection of fast, high voltage diodes to channel the reverse current safely away from the switches to ground. Most gas switches don’t care about the direction the current flows so they happily ring until the resistance in the circuit converts the energy to heat or the system voltage drops below the sustaining voltage (<1 kV for most gas switches).

In an ideal PF, the reverse current would be small but it doesn’t always work out that way. In a good pinch att 0.25 MA I still see a negative ring of ~70 kA. That is a small fraction of the forward pulse but it could easily take out solid state components if the voltage rings up.

This assumes the voltage at pinch time doesn’t take out the solid state system. When a plasma focus ‘pinches’ the plasma to achieve fusion conditions, the impedance in the circuit increases dramatically. I can’t speak for FoFu-1 specifically, but a 1 MA machine typically generates ~250 kV at pinch time. The voltage is divided back to the bank by series impedance relationships. If the switches are a moderate to large impedance in the circuit which isn’t a bad assumption, they will see a substantial voltage drop. Suddenly you have a 40 kV circuit with a positive voltage discharging to near zero and a 250 kV spike appears at the the pinch. About 50% of the voltage will appear across the switches. Can a 40 kV hold off switch in forward bias deal with a 125 kV voltage spike in reverse bias? I don’t know for sure but I have my doubts.

Don’t hate the solid state guys, hate the universe. I’m pretty sure the solid state guys would love for the rules to be more favorable. 🙂

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Some one posted this link over a month ago. Sorry I don’t remember the thread.

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Solid state technology has come a long way. I remain hopeful that it will work at the currents and voltages desired someday. The problem I see is managing several thousand switches at once in this regime. I’ve looked into it before and the potential down time for the pulse power and cost is a real problem. Even if the switches are managed well on the forward going pulse the pinch voltage can be many times the charge voltage and how is the reversal current going to be dealt with. SRL has worked on the technology since 1992 but they have updated when possible and they are using circa 2005 solid state technology in the last paper. The system operates at up 80 Hz at 230 kA. The switches they use don’t require low jitter because of the saturatable magnetics.

A fast switch with 1 ns jitter is not necessary for a PF. The typical current rise time is ~ 1us. A few percent jitter is tolerable on a plasma focus. The PF devices I work with have ~5% switch jitter and they work well. In many ways, the shape of the current pulse does not matter for a PF. We use the sinusoidal shape because the pulse power is cheap. A linear current ramp or a fast rise to a flat top current should work as well. Other pulse power architectures can give these shapes but they cost more than folks are willing to pay. Solid state is typically in the regime of more than folks wish to pay. Another issue with solid state for this application is the radiation produced by the PF. One the research scale the switches won’t be affected by x-ray or neutrons. However, on the power plant scale, the solid state would be bothered by radiation. You can make the components so called, rad hard, but that costs even more.

I’m sure this all sounds like I’m dumping on solid state, but these are the concerns of people building the pulse power systems. The promise of solid state pulse power has not escaped anyone. The Army, Air Force, DOE, and many other are pouring money into solid state pulse power technology for applications like radar and microwave generation at high power. The problems appear to be manufacturing limitations at the moment but there are serious physics limitations to solid state technology that are not easily overcome. The problem for LPP is many of these applications require high voltage and only modest currents 100 A to 1 kA. The dream of a 10 kA switch at high voltage isn’t interesting to those funding the technology. DTRA is probably the only US agency that was interested but it faded when physics slapped them in the face. Last I talked to someone who builds both solid state and gas switch technology professionally, he was confident that gas switch technology was the only approach to >100 kA systems with modest repetition rate (~10 Hz) in the 1 us rise time regime for years if not decades.

The rail gap is a gas switch like a spark gap. Rather than use two hemispherical or two cylindrical shaped electrodes, two rails are parallel to each other. A third electrode (trigger) lines between them but out of plane. Drawings are on-line if you look up rail gap. The switch requires a fast rising trigger pulse (4 kV/ns) to operate correctly. Basically, the switch is a multichannel spark gap. With the fast trigger you can set up many channels to carry the current. The arcs jump around the electrode on different shots so you can get reasonable lifetime and low inductance (~20 nH). A spark gap can tolerate a slower trigger as only one arc is produced. The inductance of a spark gap can be quite large ranging from 50 nH to 200 nH per switch. The rail gaps tend to carry higher current than most spark gaps but operate at similar voltages. The rail gap can operate in a flowing gas mode that keeps the switch relatively clean. I use four rail gaps in a PF at the 0.25 MA scale and they are great switches. The lifetime of the switches is ~10^7 shots but you can extend it using refractory electrodes instead of brass which is the standard. The gaps are for sale on line for ~$20K per switch. In a 2 MA machine I would guess you need twelve switches at a minimum. The trigger does cost and it needs some thought but the technology exists to make a 4 kV/ns pulse without too much trouble. I have operated my rail gaps for thousands of shots without cleaning, replacement or problems.

The other option is the thyratron. Thyratons are wonderful switches for ~ 10 kA, 40 kV applications up to 1 kHz. I use a pair in an experiment. They are the easiest gas switch to trigger that I know of. Particle accelerators wouldn’t exist without them. They are still the main stay of SLAC and other large systems so the infrastructure to produce, maintain and operate is in place. For the lab scale PF, the thyratron is costly (~$4000 for the switch as described) and they require auxiliary equipment like heaters which make the switch more like $7500. They have low jitter (~10ns) with almost no effort and can operate with ~2ns jitter for the dedicated pulse power master. The problem is these switches aren’t good with reversal and they carry such low currents relative to the desired peak current. I priced a thyratron based PF at the 200 kA level and it cost nearly $250K in parts. One might argue that $2.5M is not so bad in the context of a 2 MA power plant and buying in bulk helps reduce the unit part cost. I see this an an option moving forward. Switch lifetime is ~10^8 shots so you can probably make a power plant work with ~200 switches. I believe SLAC operates thousands of switches. Again, know how is there.

In the end cost, lifetime, reliability and the operating characteristics will determine the switch choices. If the FoFu power plant operates at ~10 Hz I find it hard to believe that solid state technology would be used. If the plant must operate at 100 Hz or more, get out the check books and prepare for a costly technology development cycle.

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