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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

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    $\begingroup$ Free energy drives systems unstable in order to remove the free energy (e.g., excess currents). $\endgroup$ – honeste_vivere Apr 19 '17 at 12:50
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Well, that is quite a big and important topic in fusion research. One way to categorize all the possible instabilities is their origin, i.e. what is responsible for driving them. We have basically two mechanisms there:

  1. The plasma current
  2. The plasma pressure

In both cases it is their gradient which can drive a number of instabilities. (Note that the current driven instabilities are something you usually do not have to worry about in a stellarator.)

In order to identify unstable or stable regimes, you usually deform (via calculations or simulations) the equilibrium by a small quantity and then calculate the resulting change in the potential energy: if the potential energy decreases the system is unstable, if it increases it is stable (like a ball which either sits on top of a mountain or in a valley: a small change in its position will lead to the ball rolling down all the mountain or just returning to its position). Doing such a stability analysis then yields a number of unstable modes ("modes" because you expand the deformation in modes), deformations of the equilibrium against which the plasma is unstable and that therefore grow.

Since you seem to be more interested in tokamaks, the current driven instabilities can be further separated by either assuming a perfectly conducting plasma with no resistivity or taking into account a finite resistivity (ideal modes vs. resistive modes).

The edge localized mode (ELM) that you mentioned is an example where both, current and pressure gradient are responsible for their occurrence.

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  • $\begingroup$ but how does plasma current create instabilities? $\endgroup$ – Chandrahas Apr 19 '17 at 13:28
  • $\begingroup$ @Chandrahas whenever you have strong gradients, i.e. strong changes of a quantity over a small spatial distance, this can trigger instabilities. Like when you try to make a mountain of sand at a beach: if it becomes too steep, an avalanche process will start. $\endgroup$ – Alf Apr 19 '17 at 13:38
  • $\begingroup$ @Chandrahas well, I realize that I might have misunderstood you, let me give it another try: the strong current in the tokamak is responsible for the magnetic field confining the plasma. If you have instabilities in the current, this changes the magnetic topology and thus the confinement. $\endgroup$ – Alf Apr 19 '17 at 13:44
  • $\begingroup$ Since, we use a transformer, doesn't it produce predictable and precise currents? Is the instability in the current due to unpredictable plasma behavior? $\endgroup$ – Chandrahas Apr 20 '17 at 6:18
  • $\begingroup$ @Chandrahas on a large scale, yes, quite predictable. On a smaller scale, difficult... The current is carried by the particles in the plasma and with some small scale variations (fluctuations) lead to small changes in the resistivity which then in turn influences the current profile. $\endgroup$ – Alf Apr 20 '17 at 7:35

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