Can Alternating Current Sources Stabilize Z-Pinch Fusion?

[Colin LeMahieu]

Hey everyone, I've been kicking around this idea for fusion for a while and thought everyone could check my understanding. The idea is to create a sustained z pinch using alternating current sources in parallel.

https://docs.google.com/document/d/1aG-fdXXnEN387YbLcKPWRLR2ygJ38McLgqCZEpNz2-o/edit?usp=sharing

It'd be interesting to know if there are issues that are blatantly impossible.

 

[Astronuc]

What would happen when the current cycles through zero?

Not sure how one would use a set of AC currents in parallel.

 

[the_wolfman]

For your set up it's important to consider how the frequency of the driver compares to the Alfven frequency of the plasma. Magnetically confined plasma equilibrate on time scales comparable to the Alfven frequency.

If you drive the AC at a frequency slower than the Alfven frequency, then the Z-pinch evolution can be modeled a progression through a series of quasi-equilibria. As Astronuc hinted at when the current cycles through zero you will completely lose confinement. Your plasma will likely quench. And it's best to think of the next current cycle as a distinct Z-pinch. People have studied similar pulsed configurations as a means of operating a "steady state" fusion power plant.

On the other hand it's not exactly clear what will happen if your operate the current at a frequency that exceeds the Alfven frequency. You might be able to achieve macroscopic force balance in some average sense, but the plasma particles will no longer be confined to magnetic field lines. This will greatly reduce the particle and energy confinement. It is counterproductive and not ideal for a fusion reactor.

If you're interested in plasma physics/nuclear fusion you need to move beyond a physical mode based on the interaction of individual particles. An important aspect of plasma physics is the collective behavior that arises from the interaction many particles. In your white paper you make multiple erroneous statements that mostly stem from an over reliance on the individual particle mode. For instance there is no longitudinal force in a Z-pinch (you call in a current force), in steady-state the pinch force is balance by the plasma pressure not the coulomb force, and at no point is the pinch force overcome the coulomb barrier. If you're interested I can recommend several textbooks. Chen's book "Introduction to Plasma Physics and Controlled Fusion" is a standard introductory textbook.

 

[Colin LeMahieu]

Thanks for taking a look guys, admittedly my writeup is simplistic since my background is in software. I'll read that book since it would certainly step up my understanding.

I amended the writeup to clarify something I thought was especially lacking in a section called "Plasma tolerances and instabilities". The hope was to draw attention to the proposed significant increase in gap between plasma and the surrounding container. My hope was using this method would avoid needing to solve plasma instabilities at all by losening tolerances to a point where they don't cause a problem.

Let me know if that amendment helps or if I dug myself in even deeper ;) Again, thanks for looking!

 

[the_wolfman]

Plasma physicists have been studying instabilities for 50-60 years. The kink mode in a Z-pinch is perhaps the easiest instability to model and it's fairly well understood. We also have a lot of experience shining laser light into magnetized plasmas. We often use the light reflected from laser pulses to diagnose the conditions inside the plasma. I can tell you will absolute certainty that shinning a laser down the center of a Z-pinch will not stabilize the kink.

It was a cleaver leap to try to apply to result from the atmospheric discharge to the Z-pinch. Unfortunately the conditions in the two plasma are very different. For starters Z-pinches typically have 100 times more current (if not a lot more) than the atmospheric discharges. The drive for the kink increases with the current so Z-pinches are that much more unstable than the atmospheric discharge. Secondly in the paper you cited earlier, the "stabilization" of the atmospheric discharge is due to the formation of a high conducting current channel in atmosphere. Z-pinches are formed in near vacuum conditions.

There are know ways to stabilize the Z-pinch. The first is to apply an axial magnetic field turning the Z-pinch into a screw pinch. The second into apply an axial flow shear. There's actually an exciting experiment, the ZaP flow experiment, at the University of Washington that is exploring this second option.