[asymmetric_implosion]

Joeviocoe wrote:

The switch problem is the materials erode leading to a plasma/vapor of that metal. When the current turns off, the plasma/vapor expand and coat all surfaces. After some number of shots, a thin metal film builds up on the insulating ceramics or plastics that normally hold off the high voltage. The finite conductivity can lead to problems with voltage hold off, excessive leakage current in the switch. These conditions can lead to pre-fire and large jitter.

That is why I mentioned a gas flow system that keeps the material vapor from coating the switch internals. The flow gas must be inert and kept hot to prevent solidification. But that should be doable.

asymmetric_implosion wrote:
Many problems in fission can be mitigated. After fuel is burned it can be reprocessed extracting the useful components and burning the other products in an accelerator. Proliferation is a concern but thorium cycles can by-pass many of those concerns. Ask the Indians about whether they are investing in fusion or thorium fission. The Chinese have a more diverse picture but they are investing in tokameks for their fusion program.

I certainly like the diverse approach to energy that the world has in general. I wish the U.S. was as bold sometimes.
LFTRs and even Travelling Wave reactors are important to fund and develop regardless of any progress on the fusion front.
I was just remarking how improper it was to regard fission as an energy silver bullet when it was the ‘new thing’ on the block.
Focus Fusion may be the holy grail, but that just means it tackles many issues at once. But it won’t save the planet without help. Different reactors for larger marine propulsion or large centralized power needs. Stacking 100 DPFs seems impractical. A 100MW polywell might serve better.

Gas flow is used in these switches and they still have only 5E6 lifetime in the case of spark gaps. Thyratrons cannot flow gas and work correctly. There are other components in the switch I didn’t mention that are required for operation and stagnant gas is a must.

I think the US has a pretty diverse energy investment. Coal is king but we use natural gas, fission, solar, hydro and wind. DOE is supporting all these efforts via numerous funding vehicles and incentive programs. Nuclear is a tough sell in the US for a number of reasons. The main reason is legal. It only takes a few folks to file legal action which can shutdown a plant start up. The Nuclear Regulatory Commission is hearing license requests and new design approvals at a high rate these days and every one is heavily challenged. It takes years to get approval for a design let alone a site license. On the nuclear front, we are operating Gen II reactors in the US. Gen III reactors are going on-line in China while Gen IV reactors like traveling wave and thorium reactors could be licensed before Gen III reactors are built.

In my opinion the Polywell has a longer road to fusion than the PF. There are two sides in the Polywell argument going on in literature and from a pure physics standpoint the Polywell seems difficult. You rely on beam generation which is easily demonstrated at low density. As you increase density, which is required to achieve gain, Debye lengths and plasma fluid effects kick in limiting your ability to produce beams. The PF produces beams in a dense plasma because of it’s instabilities. The Polywell is a steady state device that is unlikely to see the same instabilities.

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willit wrote: maybe it should be tested to get to Q>1 before bickering about materials science. there are a lot of engineers out there that would fight to be on the project once this milestone is met.

run it till it breaks then make that part stronger. repeat.:cheese:

Just sharing information from experience. Q>1 is not required to burn up anodes, tax switches or improve pulse power. Some plasma focus operators don’t aspire to Q>1 but we are working on these problems for other applications. Why not design around a problem before you have it?

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The switch problem is not that the electrodes erode away. That is a problem for the anode in the PF. The switch problem is the materials erode leading to a plasma/vapor of that metal. When the current turns off, the plasma/vapor expand and coat all surfaces. After some number of shots, a thin metal film builds up on the insulating ceramics or plastics that normally hold off the high voltage. The finite conductivity can lead to problems with voltage hold off, excessive leakage current in the switch. These conditions can lead to pre-fire and large jitter. Pre-fire and large jitter were the reason LPP is changing switches. The switches by R.E. Beverly had a poor materials choice internally. Scroll back through the photos on the switches.

I think LPP is on the right track with a physics demo but to do a complicated physics demo you need the correct equipment. FoFu-1 has been hampered by switch problems since its inception. The Raytheon switches, if they are the one I know of, will improve reliability dramatically. They are used on a 1 MA pulse power system that fires at 1 Hz and holds off 200 kV. Look up the linear transformer driver if you want to know more. I’m guessing these are the little brothers but they are good switches. They are talked about at many conferences. A few folks I know use them and they frequently comment on the high quality of the switches.

To my knowledge only a few groups operate their PF device at more than 1 Hz. The SRL group operated a PF using solid state up to 80 Hz at 260 kA. The NTU/NIE group in Singapore can operate two devices at up to 10 Hz. A few others operate at 1 Hz and above (~80 kA and ~300 kA) but they are rare. I’ve heard rumors of an Italian PF at 1 MA and 1 Hz but I haven’t come across any data. They were interested in radioisotope production so they would publish in journals I don’t frequent. I haven’t run across any talks at meetings I usually attend. The group I work in is using thyratron switches for 10 Hz, 60 kA and a type of spark gap for 0.25 MA, 1 Hz. Very few people have interests above ~10 Hz. The beauty of the PF and Z-pinches for most folks is the strong non-linear scaling in yield with current.

Many problems in fission can be mitigated. After fuel is burned it can be reprocessed extracting the useful components and burning the other products in an accelerator. Proliferation is a concern but thorium cycles can by-pass many of those concerns. Ask the Indians about whether they are investing in fusion or thorium fission. The Chinese have a more diverse picture but they are investing in tokameks for their fusion program.

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I am referring to solid state switching in my comments about cost per J stored. It seems a bit odd to use statements like $1/Joule stored when talking about a switch but the pricing goes that way. The caps I mentioned are state of the art for high current applications. Smaller caps used in everyday electronics have ~1E11 cycles. It is a complicated matter why the lifetime is so short for high current but the numbers on the GA website are realistic. I’ve found out the hard way. (see page 2 of this thread about 1/3 the way down to see my interest in this.)

FYI: Caps are mainly solid state devices but some liquid is used in almost any high voltage cap.

There are other power storage methods but none are rep rate compatible. Flux compression generators (explosively driven devices) are one shot and motor type solutions take too long to brake to allow 200 Hz operation. Out there ideas are floated about superconducting magnetic energy storage but I don’t have much faith in them.

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The problem is materials with caps and switches. As an example, gas switches are basically arcs internally. The cathode of an arc erodes material to support the plasma. Gas inside the switch reduces the cathode erosion but some finite amount of metal is removed every shot. This metal is vaporized and redeposits around the switch. Eventually the walls get coated. How do you stop erosion and redeposition? Stopping erosion is impossible so it must be minimized. You pick a material like tungsten or Rhenium (very expensive) but it only buys you a factor of 2-3 in erosion rate. So if you can’t stop erosion, stop it from redepositing in “bad” places. You can try baffles and other tricks but these materials get coated. Once coated, they are partial conductors. The gap between the baffles and the electrodes is reduced thus the switch hold off voltage is substantially reduced. One might ask about operating a switch that does not arc. Ahhh, welcome to the 1930’s and the thyratron. It operates in a diffuse arc mode with little erosion compared to a spark gap but it only last 50X longer than a spark gap at best. I don’t know of a gas switch option that does not operate in some kind of arc or diffuse arc mode. Caps are another story. I believe the failure is driven by magnetic forces at discharge and holding static potential for “long” periods. The switches are going to be the problem moving forward. A number of smart people are working on better switches but lifetime is not in the cards right now. The effort is put into reducing inductance, increasing voltage hold off, transferring more Coulombs and jitter. These are the most important parameters to high current folks. For those that might not know, the Z-machine at Sandia had a problem with jitter of ~3 ns with a 90 ns current rise time. To solve the problem, 6 ft by 6 ft panels of acrylic were inserted into the switches (6 ft by 6ft). To punch through the panels requires a certain amount of energy. This process reduced the jitter to less than 1ns. The PF does not have this strict a jitter (~10 ns will probably be fine), but it gives you an idea of how large current switching is handled. For reference, Z fires something like 200 shots per year. Switches with 1 shot lifetime are more than enough for the cutting edge science done at Z.

Ahhh, the cheaper than coal argument. A little history…when fission was the new thing, business models were build around the electricity being free. This was after they demonstrated an actual power generating system. I hope the day comes when fusion is cost competitive with coal. That would be a huge victory. Don’t get me wrong, I like the idea of clean fusion energy. The problem is I’ve been disappointed so many times that I try to approach fusion and other problems as a skeptic and ask questions. I think the LPP approach is as viable as any other fusion concept at this point. I hope they make the breakthrough but it will be a long road after that to a working reactor.

[asymmetric_implosion]

Ran out of space….

The next solution is hot swapping the pulse power system. That is a great deal of waste and sunk cost per unit. Spark gaps at the 20 kV, 25 kA level cost $1000 per unit in the 1-10 per purchase range. If the machine requires something like 20 switches at higher voltage, say 100 kV switch to be safe and 200 kA per switch, you are talking about something like $5000 per switch at 1-10 units. You need something like 100 switches per day so you could probably get them for 25% of the 1-10 unit cost or $1250 per switch. So each day, you need to buy $125,000 in switches. Caps cost about 25% of the switches so you spend $160K per day on the pulse power alone. Thyratrons are $3500 per unit and you need two hundred units every three days. Again, bulk discount to 25% of single unit you only need $60K per day in switches and another $30K in caps or ~$100K per day on the pulse parts alone. A tech to replace these components costs ~$100K per year including overhead like benefits, pension, etc or $272 per day. You will probably need 4-5 techs so they person cost is minimal. The change over time is a minimum of 1 hr per change at 1 change every three days so you get 71 on hours for 1 off hour. The off hour costs you $100K in parts and people. The cost of electricity in CA is $0.15 kW-hr as a sort of average. The reactor generates 5000 kW at steady state for 71 hrs out of 72 hrs possible. That is 360,000 kW-hrs or $54K. You need to more than double the cost of electricity to make this a cost effective venture as I neglected fuel gas, regulatory compliance, operator pay, facility overhead costs, etc with existing technology. This also neglects any supply chain problems, contracts to purchase parts and people to keep the parts flowing. One might argue that clean energy has other benefits so the gov’t will step in but in the end you are still paying for it. I will admit that one could do better than a 75% discount over single unit price but 10% or so is the best I can imagine. That buys you some space ($27 K per day) but you are barely breaking even all things considered.

[asymmetric_implosion]

Dividing up the parts is possible, but you still have finite part lifetimes. Two months of operation is ~1E9 shots.

A common trick in rep-rate pulse power is to use 100 kV caps for a 40 kV application. The reversal (opposite polarity ring in PF) is a key factor in determining the capacitor lifetime. The reversal is usually quoted as a percentage of the maximum voltage. Typical high voltage, high current cap lifetimes with <10% reversal are 1E7-1E8 for GA Series S caps (check out the General atomics high voltage website). Larger capacitors are more like 1E5-1E6 pulse lifetime (GA Series C). This limits you to ~ 1 day operation at 200 Hz with ~1E7 shots per day. Ceramic capacitors can last longer (~1E8) but they are limited to sizes of 10 nF so you will need hundreds or thousands of units. They are also limited to 50 kV. I forgot that high current cap lifetimes were so miserable.

Switches are a bit more complicated. Switches are typically limited by erosion of the electrodes. You can operate a switch below the theoretical current (really a Coulomb limit as the eroded mass is determined by the total coulombs that pass through it) but typically you can only derate by 2-3X which buys a 2-3X increase in lifetime. A typical spark gap switch is good for 1E6-5E6 pulses. Spark gaps are the basis for most high current switches. Thyratron switches are good to 5E7 shots but they can only carry about 10 kA while the spark gap can carry 100 kA with ease. Railgap switches are the spark gaps big brother with multiple conducting channels instead of a single channel and they are limited to ~1E6 shots (Check out Perkin Elmer high voltage and L-3 communication electronic devices). New switches are underdevelopment but the incentive to go beyond 1E6 or 1E7 shots for a high current, high voltage system is very low. Cutting edge high current machines (>1MA) fire less than 1000 shots per year with very few exceptions. Fusion concepts like MAGLIF embrace the pulse power limitations by asking how they can maximize the fusion gain per shot to limit the number of shots. If it works as conceived, MAGLIF would only fire one shot every 10 s and operate at 1000 MW electric. With only ~9000 shots per day you can operate for a year before you have to work on the switches. Once a year shutdown for maintenance is comfortable to utilities that will likely supply the power. Even at 5E7 shots for a thyratron, you are limited to less than 3 days. With a spark gap, you are operating for at most 7 hrs. Pretty miserable having to replace switches 3-4 times a day.

Another problem with these switches is the realistic rep rate. Spark gap type switches are limited to ~10 Hz operation. Active cooling, flowing gas and other needs are likely required. Thyratrons operate up to 1 kHz but they need external heating at 300 W or more per switch to work. Take a 2 MA PF operating at 70 kV (reasonable thyratron limit). You need 200 switches to accommodate the current with no room to grow. That is an additional 60 kW of electrical power to operate the switch. Then you need to trigger 200 switches at low jitter (<10 ns). Each thyratron has a trigger board which needs to be timed. Once timed the switches are pretty good. It might be difficult to find a bad switch/cap module and one is enough to drag you down.

One might argue that solid state technology could be force fit into this problem with the advantage of >1E9 shot lifetime. I believe that a 2MA, 70 kV bank can be built but the actual lifetime and the cost will likely be a problem. Solid state costs something like $1-10/Joule stored at this level. Gas switch technology is more like $0.01-0.1/Joule stored without any effort. A company called SRL built an 80 Hz, 260 kA PF device. It took a true pulse power genius (Rod Petr) to design the bank to operate a 8 kV. The next immediate question is why not operate at low voltage if it is easier? In a plasma focus there is a 10-20 mOhm loss due to the moving plasma that cannot be avoided. To overcome this impedance means a minimum of 20 kV to drive 2 MA. Account for the other losses in the switches, capacitors and bus bars and you are at 40 kV to drive 2 MA. If I am correct, FoFu-1 is at ~1 MA and 40 kV. Increasing the cap bank size does not help as you still cannot get around the 10-20 mOhm impedance in the moving plasma; it simply reduces the impedance of the bank. It will take a great deal of resources to have a pulse power system that operates for 2 months at the 2 MA level without maintenance. It might not be possible with available materials.

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I have hope for the anode because you can use the physics of a highly directional beam to help you. The switches are the problem. I guess it is good for creating jobs and manufacturing to replace switches every few days if not every day. The switches are going to require a heavy investment to realize a 5 MW power plant operating at 200 Hz for more than a few days.

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The direction of the force on the anode is largely 1D but I will let LPP speak about their experiment in detail. I know it was discussed a month or more back when they broke an insulator. I think the force depends on the machine configuration but the shaking comes from magnetic pressure on the electrodes outside of vacuum, switch closure which can be very loud depending on the current in the switch and the plasma shock hitting the vacuum vessel wall. At 60 kA, our switches are nearly silent. The vacuum wall makes most of the noise. Z-pinch machines use more complex pulse power with multiple stages of operation to achieve the 100-200 ns pulse required to drive the Z-pinch.

As far as increasing the duration of the pinch goes, there is one PF-like experiment that demonstrated >1 us confinement. The hypocycloidal pinch, a modified form of a PF, was developed by a researcher at NASA (NASA report TN D-8116). He didn’t use D2 as fuel but found an x-ray pulse that lasted for >1 us. No one picked up the research but it shows promise for stability well in excess of any other Z-pinch device at useful densities. I’d love to build one and test it but the not in the cards for me.

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Joeviocoe wrote:

Does the physics of DPF prevent designing a larger device; are there engineering limitations?

Yes and no. The primary limitation encountered to date is the insulator between the anode and cathode in vacuum. Experiments have demonstrated a system without an insulator at 5 MA but the experiments were focused on soft x-ray production. Is the insulator the show stopper to scale up??? I don’t know for sure. Some experiments at 2 MA suggest a solution to the problem but it might drive the fusion yield away from optimum. This is a pressing point in PF physics that is actively researched because scaling up a PF to high current should cost less than a Z-pinch. The problem to date is that PF devices seem to show decreased fusion yield with currents above 1 MA. Z-pinch devices show a fall off at something like 300 kA. See the attached figure with PF devices and radial implosion Z-pinch devices as an example. This is data I’ve compiled from the 1970’s to present from peer-reviewed published lit including data from LPP. If you can scale up the device along the alpha=0.25 curve, you can get more fusion yield per shot. If the fusion yield (DD neutron yield) falls off at 2 MA or more, it seems that a smaller and high rep rate PF is the better option.

I thought that it was higher voltages that brokedown the insulator, not higher current. And that higher current, with higher cycle rates, causes the skin effect to heat the cathode unevenly causing damage.

And that the DPF fusion yields scale higher with increased current ( I^4.7 ) but that 3 MA would be optimal.

Am I completely wrong here?

Voltage is a secondary concern. Insulators for the Z-machine operate at up to 6 MV without any problems. The insulator failure seems to come in three forms: debris build up leading to bad initiation which is current driven by cathode erosion; mechanical failure from shocks due to drive largely by magnetic pressure i.e. current squared; and UV irradiation leading to materials damage and possibly re-strike again dominated by current. Kies has a nice paper on the topic and how to mitigate some of the problems up to 300 kV charge voltages. It is not clear to me if the tungsten pins used by LPP can get around the problems by localizing the initiation a small distance from the insulator. Using a thicker insulator tends to eliminate the problems due to shock unless the cathode and anode plates start to move due to magnetic pressure like on the LPP device but again, Z-pinch folks solved this problem long ago as they are operating at the 20 MA level in Z-machine. The UV problem is the most difficult to deal with as the plasma must flash the insulator. Higher Z gases produce far more UV than hydrogen and deuterium so boron could be a problem. There are alternative ideas like using gas puffs to supply the necessary fuel on the PF axis and let the axial phase be dominated by hydrogen. Again, Kies has a paper on this at the 1.5 MA level for the SPEED-2 PF. Banks exist that could drive PF devices at 10 MA with reasonable rise times. The high risk is getting the insulator to survive more than 1 shot. Everything else can be done at a few shot a day level. Moving to even 10 Hz operation at 2-5 MA has never been done. To my knowledge, the Italians were working on a 1 MA, 1 Hz bank for a PF. I don’t know if the machine was ever built. Discussions have come and gone about using LTD systems to power PF devices but no one has tried to my knowledge. They are 1 Hz, 1 MA systems that will sit in most living rooms. Beyond 1 MA is a few shot a day territory for most pulse power systems.

I won’t speak to an optimum because different people define optimum differently. I don’t know if there is a true optimum for a fusion system. My guess is that there is a mix of cost, engineering risk and possible physics limitations that will decide on how a repetition rate PF will be built. I can tell you this much, a 60 kA PF firing at 10 Hz sounds like an automatic weapon firing. A 3.5 MA Z-pinch causes the floor to move noticeably. A 20 MA Z-pinch feels like a small earthquake. Firing a 2-5 MA PF at 200 Hz will be an experience for those near by without some sort of seismic isolation. I feel for the operator.

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jimmarsen wrote: Does the physics of DPF prevent designing a larger device; are there engineering limitations?

Yes and no. The primary limitation encountered to date is the insulator between the anode and cathode in vacuum. Experiments have demonstrated a system without an insulator at 5 MA but the experiments were focused on soft x-ray production. Is the insulator the show stopper to scale up??? I don’t know for sure. Some experiments at 2 MA suggest a solution to the problem but it might drive the fusion yield away from optimum. This is a pressing point in PF physics that is actively researched because scaling up a PF to high current should cost less than a Z-pinch. The problem to date is that PF devices seem to show decreased fusion yield with currents above 1 MA. Z-pinch devices show a fall off at something like 300 kA. See the attached figure with PF devices and radial implosion Z-pinch devices as an example. This is data I’ve compiled from the 1970’s to present from peer-reviewed published lit including data from LPP. If you can scale up the device along the alpha=0.25 curve, you can get more fusion yield per shot. If the fusion yield (DD neutron yield) falls off at 2 MA or more, it seems that a smaller and high rep rate PF is the better option.

Attached files

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Joeviocoe: I operate three plasma focus devices at the 60 kA, 140 kA and 0.5 MA level (typically 250 kA at rep rate). Our experiments are focused on non-energy applications of the plasma focus such as active nuclear interrogation, lithography and basic plasma physics related to what is known as high energy density physics in support of larger experiments for our customers. I work for a small business like LPP but we have a much larger scope of plasma applications including thin film coatings, plasma/ion thrusters and plasma diagnostics that are applicable to many areas of plasma physics. Though our customers have specific needs I try to take some time to work on larger problems like neutron yield scaling (don’t buy the 4.7 scaling law, there are many different explanations and supporting data for each. Published numbers exist from 3.5 to 5.5 in the power law. I have my own take on it but I am finishing up the paper on it so I will leave it there). My work involves repetition rate plasma focus devices (>0.1 Hz) at scales ranging from 60 kA (10-100 Hz) to 0.5 MA (~1 Hz). I have encountered gas lifetime issues, materials issues and repetition rate pulse power problems including switch lifetime and switch properties for specific configurations.

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Anode erosion does not have to be a show stopper. As with any system, go in knowing it could be a problem and design for it. In my case, I didn’t consider e-beam a significant problem as my low rep-rate sources (~0.1 Hz) didn’t have any problems. Operating at 1 Hz all day for 10+ days showed a problem. We pulled an anode after 250,000 shots and found a deep bore hole in the anode that nearly broke out of our vacuum vessel. We thought refractory materials or ceramics would solve the problem. Ceramics were a miserable as they erode into gases and other things that made matters worse. We tried alumina and boron nitride. Refractory metals did a bit better but they were not the knight in shining armor we needed. The real goal should be to spread the e-beam out far from the pinch region (inches if not feet) and let the e-beam impact a specifically designed beam dump that will handle the e-beam. Our experiment does not allow us to do this so we have taken other steps that will work in the short term. Our next move is to implement a more reliable solution like magnetic deflection onto a beam dump. This should give us anode lifetimes that are on the order of our switch lifetime (~1E7 shots). For the purpose of illustration, our current solution is good for 50K to 100K shots. We need 100X increase in materials lifetime to make our source viable.

The LPP source could do better with plasmoid confinement of the e-beam but a switch that operates at 200 Hz for >1E8 shots and high voltage, high current is currently unavailable and again for materials reasons might not be possible. The solid state option is difficult to imagine working well at the 2-3 MA level with 50-100 kV on the bank. It is not impossible but you only get one case to screw up. Solid state will last forever (>1E9 shots if not 1E10 shots) if it is treated right but one bad step and it is done. It is my opinion that the switches will determine the tolerable lifetime for the other components.

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Most of the anode material loss seems to come from the e-beam damage on the bottom of the anode. The outer anode walls tend not to erode and in some cases they will actually increase in diameter as cathode material will deposit on the anode faster than the anode material erodes. I’ve done tests with copper anodes and stainless steel cathode rods. The anode looked like stainless steel in less than 1000 shots on the outer diameter at 140 kA. The cathode erosion is probably more substantial on the inner diameter but there is plenty of cathode material to burn so it is a minor problem. I’ve burned up plenty of anodes and still use the same cathode rods.

Beryllium should do well compared to copper on a per unit mass basis by comparing the heat of vaporization. However, copper and beryllium are very similar on a per mol basis.

One could make the anode central hole very deep so it enters into a secondary vacuum chamber to manage the e-beam by spreading it out so that it cannot vaporize the beam dump area. The key problems to date have been the small expansion of the e-beam (~3 deg half angle) so if you let the beam grow naturally, it could take meters before you get to a point where the beam cannot vaporize the beam dump. Most experiments don’t have that kind of space. The alternative to is allow the beam into a small vacuum region well away from the anode and use magnetic fields and electric fields to spread out the beam. The key concern is to keep these fields far from the pinch region so it doesn’t screw up the PF dynamics. It is possible to do the second on most experiments but few groups have serious problems with anode erosion because they fire so few shots before changing the anode.

The plasmoids ability to contain the e-beam needs to be demonstrated before one can seriously speculate about the reduction in damage. It is probably prudent to plan for the non-plasmoid e-beam dose and know that a reduced beam is easier to handle.

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I’m familiar with the Iranian work and I completely disagree. I’ve reviewed some of their work it is fair at best. I’m not suggesting established groups are the only way to go, but they tend to keep an archive of the work that has impact and cite it. That is just the jumping off point. The NTU group is a large reason for the international growth of PF devices so they are linked into most of the international groups as they have trained many students that go out and start groups such as the one in Pakistan and I know some students from Iran trained with NTU. One of the newer groups at Kansas State has a NTU alum leading the group. The spread of technology tends to come from a few groups in the beginning and spread as the students graduate and move on to their own activities. New comers certainly pop up such as the UNLV group but they also tend to focus on different applications.

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Pinch/plasmoid break up is being neglected. The e-beam can escape the pinch during breakup of the plasmoid. The damage can be substantial and it might be even worse if the electrons are high energy. You can superheat a larger volume of metal and erode more mass. It might be possible to reduce the beam current but eliminating it will be a problem. I believe the e-beam confinement by the LPP model is due to extremely large magnetic fields in a toroidal geometry. This works well until the plasmoid falls apart. Then you have hot electrons running around everywhere.

200 Hz operation is problematic for many reasons. We are finding that operating above 5 Hz is not intuitive as it was up to 5 Hz. We are thermally managing a Mo anode to control the temperature, but temperature control alone is not enough. The e-beam erosion can do two things: release gases trapped in the metal like hydrogen, oxygen and other common gases, erode the anode material into a vapor that cannot plate out before the next shot. High Z vapor like tungsten or Molybdenum seems to help the neutron yield as was reported for high Z inert gas/deuterium mixes. The current thinking on our experiment is at high repetition rate the vapor density is beyond the optimum mixing ratio which depresses the fusion yield. LPP might be able to push beyond 5 Hz if the e-beam is suppressed but plasma erosion will eventually supply enough material corrupt the gas and spoil the fusion gain. Flowing the gas a modest rates is not enough to maintain a clean environment. Even the lithography folks operating at 80 Hz had some problems with this. The solution they found was to increase the anode size above the optimum and operate with reduced yield. If I understand LPP’s operating regime, it relies on small anode diameters and high pressure. Increasing the anode size for a fixed current requires operating at lower pressure. The problem is somewhat constrained due to the PF physics.

Relying on governments to get this working is a joke. Resources are key and private resources would be better suited to drive this forward. I don’t know if a global collaboration will come to pass if successful, but the problems are materials limited. All fusion concepts have materials problems and they are not addressed because most fusion folks have a plasma background rather than a materials background. I agree that more can be done but materials are going to be a limiting factor in any fusion device. I’ve done a small materials survey and no material I’ve run across can withstand the e-beam and/or plasma erosion without sacrificing grams of material for the 1E7 shot operation at 100 Hz and 60 kA.

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