STABILITY OF TEARING MODES IN FINITE-BETA PLASMAS

The linear stability of classical tearing modes in a finite-beta, cyli ndrical plasma is reconsidered. The different regimes of tearing mode instability are delineated by means of critical Lundquist numbers. The well-known criterion DELTA'>DELTA(c) due to Glasser, Greene, and John son [Phys. Fluids 18, 875 (1975)] defines a critical Lundquist number above which the plasma is stable. However, the physical importance of this Lundquist number is diminished by the fact that there is another critical Lundquist number, which is typically an order of magnitude sm aller, above which the mode changes qualitatively in that the growth r ate scales linearly with resistivity, and eventually becomes indisting uishable from resistive diffusion. The effect of parallel viscosity, w hich reduces the stabilizing coupling to the sound wave, is also consi dered. For large parallel viscosity, it is shown that DELTA(c) tends t o zero. Thus, the stability criterion DELTA'>DELTA(c) for a finite-bet a plasma is simply replaced by the criterion DELTA'>0. These analytica l results are shown to be in good agreement with numerical results.