Inside out DPF

[Francisl]

The most common concern about the current dpf design is that the heat build up inside the electrode will be too great for a production machine. What if we move the pinch function outside of the center electrode? We could use the plasma sheath acceleration like a dpf and a pinch like a z-machine. Instead of using the cage of a z-machine we could use the plasma arriving at the center of high voltage high current electrodes to trigger the pinch current.

[asymmetric_implosion]

In the picture you attach, I don’t see how you accelerate the plasma to the speeds of ~100 km/s required of the axial phase of the plasma focus and the even faster speeds for the radial implosion. In the configuration you show, the plasma collides in the axial direction which is parallel to the magnetic field. The plasma density is so low that the particles will fly past each other and never meet the conditions of fusion.

The electrodes are a problem in a production scale machine but heating is only one problem. There are other plasma focus like configurations that have different problems but the electrodes are possibly more agreeable to thermal management. There are concepts of imploding plasma using other plasma current being pursued by a company called HyperV. Take a look. This concept looks something like the HyperV idea.

[Francisl]

The HyperV was the inspiration for the drawing that I made. The drawing is pretty crude and I was trying to explore the idea of having the pinch occur outside of the central electrode. That should allow more power to be applied to the pinch without damaging the electrodes so much.

[asymmetric_implosion]

You have lost all the benefits of a PF and replaced them with all the problems of other fusion concepts. The problem with the HyperV concept and other similar concepts is the limited compression ratio. Using low density mass to push instead of a magnetic field limits your coupling to the strong shock limit for deuterium which is a compression of 4:1. By driving the plasma with a magnetic field one can achieve compression ratios of 10:1 or greater. In this configuration you also limit you ability to produce the instabilities that most consider necessary to produce the fast ions required to drive fusion in a PF. In the case of LPP’s approach with a quantum magnetic field effect, you need the high compression to generate the intense magnetic fields required to achieve the plasmoid.

In defense of hyper V they plan to use high Z gases as a tamper which will allow higher compression ratios but it is difficult to believe they will achieve a large compression ratio. The other problem for HyperV is the lack of support in DOE right now which was the dominant if not sole source of funding. Their big supporter was transferred to another project area and the new project manager isn’t as supportive of their approach. Last I heard, the entire concept was bad news at DOE.

[Joeviocoe]

Seeing how the HyperV concept is already exploring this method, yet is much farther behind the DPF in meeting the criteria for fusion… I wouldn’t see a good reason to want to redesign the DPF at this point.

Electrode wear is going to be a problem, but an engineering problem. Let’s get to the point where we’ve proven Q>1 with fusion and/or aneutronic fusion with pB11… then we can see how much redesign is needed to make it practical for a power device.

Constantly redesigning is time consuming. If the physics prove feasible (hopefully within a year or two), then there will be an increase in attention to all the engineering problems. Electrode wear is not going to be a problem until after they cycle up to several shots in a short time. But I think that they can prove the physics before that.

[asymmetric_implosion]

Q>1 is a necessary but not sufficient condition for a fusion reactor. Electrode failure in a PF has been a problem for many PF applications at repetition rate. Plasma facing in general is a huge area of research. A breakthrough could benefit many applications from high current switching on the grid to fusion. LPP may not have the resources to explore this right now but it is probably worth discussing the issue to see if anyone has any bright ideas.

[Francisl]

I agree that the current designs should continue. If electrode failure becomes a problem then other designs can be considered. I was trying to spark a conversation about other possibilities that may be improvements. A few months ago someone brought up the idea of two dpf units facing each other. That idea didn’t go anywhere. Essentially, that is what I was trying to depict with the pinch occurring in the space between the two dpf units.

An alternative is to use a single dpf with the electrodes designed to maintain the pinch outside of the central electrode.

[asymmetric_implosion]

People have tried two PF’s facing each other on and off since the 1970’s. The tale is one of woe. It led to the design of the hypocycloidal pinch. Rather than two concentric cylinders, the hypocycloidal pinch uses three ring electrodes. The rings are stacked in a cathode-anode-cathode configuration. A plasma is generated by flashing an insulator, like a PF, at the larger diameter. The plasma runs toward the center like a Z-pinch but it turns the corner at the of the rings and implodes. The pinch lifetime was observed to last for 10-100X longer than any pinch device at similar densities (~1E19 /cc). As a general rule, a pinch can hang together longer if the density is lower. A single NASA tech report was written and the idea was abandoned. I don’t know why. I would post the report but it is huge. I’ve wanted to test this idea in more detail as the beam damage and x-ray deposition should be significantly reduced due to geometry. The key problem that I observe is the energy stored in the pulse power is much larger than a PF so the energy released per shot must be larger.

[praos]

May I make a sugestion? Make electrodes in form of thin Be shells then circulate Li trough them. Then Li will act in double role of conductor and coolant with even lower cross-section, cost and toxicity.

Praos

[asymmetric_implosion]

The key question is how thick do the walls have to be to survive. Circulating liquid metal is an efficient cooling approach but Li is a pain. I’ve never worked with it myself but the couple folks that I know that work with it complain about it constantly. I believe high pressure He is the stated cooling method for electrodes because it is easy to clean up. I’ve used water as a coolant on lower power systems and water can be a challenge at times. We looked at metal heat pipe technology and it wasn’t practical for size reasons. Flowing Li might work if the pumps, etc exist. A potential downside is activation of the flowing Li by a flux of neutrons. Helium doesn’t have that problem.

[Francisl]

asymmetric_implosion wrote: People have tried two PF’s facing each other on and off since the 1970’s. The tale is one of woe. It led to the design of the hypocycloidal pinch. Rather than two concentric cylinders, the hypocycloidal pinch uses three ring electrodes. The rings are stacked in a cathode-anode-cathode configuration. A plasma is generated by flashing an insulator, like a PF, at the larger diameter. The plasma runs toward the center like a Z-pinch but it turns the corner at the of the rings and implodes. The pinch lifetime was observed to last for 10-100X longer than any pinch device at similar densities (~1E19 /cc). As a general rule, a pinch can hang together longer if the density is lower. A single NASA tech report was written and the idea was abandoned. I don’t know why. I would post the report but it is huge. I’ve wanted to test this idea in more detail as the beam damage and x-ray deposition should be significantly reduced due to geometry. The key problem that I observe is the energy stored in the pulse power is much larger than a PF so the energy released per shot must be larger.

I think I found two of the articles that you are referring to: INVESTIGATION OF HIGH ENERGY RADIATION FROM A PLASMA FOCUS FINAL CONTRACTOR REPORT NASA Grant and DENSE PLASMA FOCUS PRODUCTION IN A HYPOCYCLOJDAL PINCH.

This led to an article about using the hypocycloidal pinch as a high power trigger switch: Studies of the Plasma Puff Triggering Mechanism of Inverse Pinch Switch. Would this be practical now which is nineteen years later?

[asymmetric_implosion]

The switch is practical. Ideas like the HCP were turned into multi-channel switches like the MMCS switch that is used in some linear transformer driver systems. They are high current, high voltage switches with very low inductance. The problem is the rep-rate is limited to a few Hz at most.

I think the HCP is feasible to build and test. The key unknown is whether it will be able to sustain fusion as long as the x-ray pulse.

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