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[asymmetric_implosion]

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.