COLLISIONLESS M = 1 TEARING MODE

The instability of a magnetically confined plasma against macroscopic modes is analyzed in collisionless regimes where magnetic reconnection occurs because of finite electron inertia and the ion gyroradius rho-i replaces the skin depth d as the width of the mode boundary layer. Growth rates gamma-/omega-A approximately (d/r(s)(rho-i/d)(2/3 are found, with omega-A the Alfven frequency and r(s) the radius of the reconnecting surface. For typical JET parameters, gamma-1 approximately 50-100 mu-s, which compares favorably with the observed instability growth time of internal plasma relaxations.