Plasma stability
Plasma instabilities described by kinetic theory can contain aspects such as finite Larmor radius (FLR) effects and resonant wave-particle interactions, which is not captured in fluid models such as MHD.
MHD stability is also closely tied to issues of creation and sustainment of certain magnetic configurations, energy confinement, and steady-state operation.
The most fundamental and critical stability issue for magnetic fusion is simply that MHD instabilities often limit performance at high beta.
In most cases the important instabilities are long wavelength, global modes, because of their ability to cause severe degradation of energy confinement or termination of the plasma.
A possible consequence of violating stability boundaries is a disruption, a sudden loss of thermal energy often followed by termination of the discharge.
Ideal MHD instabilities driven by current or pressure gradients represent the ultimate operational limit for most configurations.
This good agreement provides confidence in ideal stability calculations for other configurations and in the design of prototype fusion reactors.
Moderate beta values are possible without a nearby wall in the tokamak, stellarator, and other configurations, but a nearby conducting wall can significantly improve ideal kink mode stability in most configurations, including the tokamak, ST, reversed field pinch (RFP), spheromak, and possibly the FRC.
Progress in understanding the physics of the RWM and developing the means to stabilize it could be directly applicable to all magnetic configurations.
A closely related issue is to understand plasma rotation, its sources and sinks, and its role in stabilizing the RWM.
Resistive instabilities are an issue for all magnetic configurations, since the onset can occur at beta values well below the ideal limit.
The stability of neoclassical tearing modes (NTM) is a key issue for magnetic configurations with a strong bootstrap current.
Although the basic mechanism is well established, the capability to predict the onset in present and future devices requires better understanding of the damping mechanisms which determine the threshold island size, and of the mode coupling by which other instabilities (such as sawteeth in tokamaks) can generate seed islands.
The configuration of the plasma and its confinement device represent an opportunity to improve MHD stability in a robust way.
The benefits of discharge shaping and low aspect ratio for ideal MHD stability have been clearly demonstrated in tokamaks and STs, and will continue to be investigated in experiments such as DIII-D, Alcator C-Mod, NSTX, and MAST.
Neoclassical tearing modes can be avoided by minimizing the bootstrap current in quasi-helical and quasi-omnigenous stellarator configurations.
Kink mode stabilization by a resistive wall has been demonstrated in RFPs and tokamaks, and will be investigated in other configurations including STs (NSTX) and spheromaks (SSPX).
Improved diagnostic measurements along with localized heating and current drive sources, now becoming available, will allow active feedback control of the internal profiles in the near future.
The Electric Tokamak experiment is intended to have a very large driven rotation, approaching Alfvénic regimes where ideal stability may also be influenced.
Localized RF current drive at the rational surface is predicted to reduce or eliminate neoclassical tearing mode islands.
Routine use of such a technique in generalized plasma conditions will require real-time identification of the unstable mode and its radial location.
