What causes instabilities in tokamaks?

[chandrahas]

As far as I know, almost all kinds of instabilities are suppressed in a tokamak. The only problem I see is that the poloidal field diminishes as we move further out from the plasma. This configurations hence can't suppress ELM's. But does the current passing through the plasma cause instabilities? I mean, I can't think of any but feel like the plasma current creates instabilities.

• So, I was wondering what kinds of instabilities tokamaks face and what causes them? And does plasma current play a role in instabilities?

Thanks

 

[IDNeon]

At slide 32 it begins a discussion of instabilities. http://cds.cern.ch/record/1344490?ln=en

The two primary instabilities appear to be:
1) Ideal
2) Resistive

From the lecture, the ideal instability produce field line bending and the resistive tear and allow reconnection of magnetic field lines where from an earlier part of the lecture discusses "magnetic islands" form.

The causes seem to be either the twisting of field lines or the breaking of field lines.

I hope this launches you in the right direction and maybe will help kick-start more discussion.

I always look forward to The_Wolfman's insights on this topic.

 

[chandrahas]

Oh. Ok.. sorry took me a while to respond, thought this was dead.

 

[chandrahas]

So I am watching what you referred to, but why don't we use alternating electric and magnetic fields to confine the plasma rather than static magnetic fields? I mean like a paul trap but a modified verson of it to hold in both electrons, deuterium, tritium. Is this Idea flawed in anyway? Wouldn't this eliminate many instabilities?

 

[IDNeon]

So I am watching what you referred to, but why don't we use alternating electric and magnetic fields to confine the plasma rather than static magnetic fields? I mean like a paul trap but a modified verson of it to hold in both electrons, deuterium, tritium. Is this Idea flawed in anyway? Wouldn't this eliminate many instabilities?

Well stability seems to be primarily dependent upon the aspect ratio and triangularity of the plasma first and foremost.

I'm trying to understand all this myself from start to finish so it helps to discuss this, my first glance into the question you asked seems to relate to the quantitative results of plasma fusion which dictate that aspect ratio and triangularity of the plasma is important.

In a paul trap the geometry of the fields seems to be completely different, so the question may not be "why not use..." but may rather be "can the paul trap be used instead?" in which case it looks like the tokamak idea would be unrelated because of how the plasma is stable and fusing within this "deltoid" ring parallel with the toroidal field.

I'm starting to read about the complex reasons that aspect ratio and the deltoid (triangular) shape of the plasma matters, it has a lot to do with the bannana, plateau and pfirsch-schlueter regimes that form the currents(?) within the plasma that the species diffuse into.

So these structures are all integral to the fusion process, and doesn't seem to be produceable from the paul trap. The alternating current likely can't sustain or produce these structures or this type of confinement?