Anode in center of a magnetic confinement fusion device?

[MartinFick]

TL;DR Summary

Would adding an anode in the center of a magnetic confinement fusion device such a zpinch, stellarator, or tokamak, help create greater compression by forcing the positive ions outwards away from the anode against the magnetic field pinching them together, effectively creating a hollow tube of plasma (centered on the anode) instead of a cylinder?

I have been reading about the theta pinch and how it is very stable, but it sounds like it cannot compress the plasma enough. https://en.m.wikipedia.org/wiki/Pinch_(plasma_physics)#The_θ-pinch

I also have read about fusors https://en.m.wikipedia.org/wiki/Fusor#:~:text=A fusor is a device,the center, they can fuse. And one of if the obvious problems with accelerating ions towards cathodes is that if they hit the cathode they will be absorbed.

So it had me wondering if you could use electric fields to repel positive ions in one direction while using magnetism to push them is the opposite direction. This way, the ions would never reach the cathode and get absorbed, and possibly you could pinch the ions further then with magnetism alone?

 

[Alex A]

Hi @MartinFick, welcome to PhysicsForums.

The ugly answer no one wants to hear is that if something so simple was going to work someone would have tried it by now. Some variation of this almost certainly has been tried.

Two observations - the plasma will have positive ions but also free electrons. Those electrons will move to neutralise that field and they have much less mass. So the electric field will act on the electrons much more efficiently than the positive ions. The original concept of a fusor was to create a zone of negative potential which hydrogen nuclei could fall into and fuse. It worked very very badly and the modern design has the polarity reversed. Creating a negative well by firing electrons into a zone is almost certainly less efficient than just firing the nuclei at each other because of how much lighter and faster electrons move. A modern style fusor with a negative grid also doesn't work this way though, with the fusion happening mostly away from the central poissor.

My second observation is that the natural instinct to apply compression is to have a field pressing one way and another field pressing another way. Except what usually happens is the forces cancel out and the total force experienced by the particles would be difference between the fields. For a tiny distance, that net force will be insignificant. There is a more general problem with inverse square fields in that they tend to cancel out completely when you try to build a container out of them.

So I would expect lower compression, and like with a fusor, better results theoretically with the polarity reversed except some other problem will be more wrong and nature will defeat it in practice. My understanding of fusion power research is that the last 50 years have been this again and again and again as every clever idea hits fundamental limits of materials and scale.

 

[BVirtual]

Rotating plasma is highly abrasive to metal, and any anode in the 'center' will face a material loss issue. At this center the density of the plasma can reach 50 to 100 times of the density of lead. A "center" anode has little chance of surviving, before shutdown is required to replace the anode. Good thought though. For other plasma devices such a strategy might be more successful. Particularly where the plasma is not so abrasive against the anode surface, due to its position.

A key issue is "where" is the center? Any anode placed at the "center" will change the location of the "center." Plasmoid behavior includes repelling external fields at the surface of the plasmoid, by creating counter eddy currents in the plasmoid surface, so external fields do not enter the main body of the plasma. Lower frequency fields can partially penetrate. Even lower can get all the way through. Higher frequency fields are reflected. A new anode will reshape the plasmoid, presenting a new surface to the anode fields, and the anode fields will likely be repelled. All the current mathematics for rotating plasma would have to be redone, decades worth of computer simulation would have to be redone.

External DC fields will entirely reshape the plasmoid shape facing the source(s) of the DC field. External magnetic fields will likely create inside the plasmoid internal magnetic fields that repel the external field right at the new plasmoid surface. Such a plasmoid is called a magnetic plasmoid. There are other ways of forming magnetic plasmoids.

Each plasma chamber has its own unique math model, and rarely are properties "shared" in a way the algorithm for one chamber can be used for other chambers. Each plasma scientist must for each chamber design write new math models/algorithms for computer simulation for that one chamber. It is a real burden, slowing down the path to fusion power.