Topics in plasma instabilities: trapped-particle modes and MHD

The non-linear theory is now the principal frontier in stability research and has made great advances in the past few years, although it is far from having predictive capability. Of particular interest are Direct Interaction Approximation mode-coupling calculations and introduction of ''clump effects'' which go a long way toward explaining the broad-frequency spectra which are seen experimentally. Perhaps the biggest question mark as to whether our qualitative understanding is at all correct is the recent stellarator experiments on Wendelstein, which showed that, at least in that geometry, drift waves were closely associated with plasma currents. This is in complete contrast to all the linear theory predictions which have, however, only been made for tokamaks. The most dangerous of these predicted microinstabilities, at least in the collisionless limit, is the trapped-particle mode driven by unfavorable curvature. In this paper the author points out that in some proposed tandem mirror schemes these microinstabilities become very similar in their properties to macroscopic MHD flute modes, thereby providing a link between the two classes of instabilities. (Auth.)