POLOIDAL SHEAR-FLOW EFFECT ON IDEAL INTERCHANGE INSTABILITY

Poloidal shear flow, driven by the radial electric field, decreases the beta limit determined by the ideal interchange mode in a heliotron/torsatron configuration with unfavourable curvature. The reduction factor is estimated as [1-(k(theta)V(E)')2/(k(parallel-to)'V(pA)2] for localized modes, where k(theta) is the wave number in the poloidal direction, k(parallel-to) is the parallel wave number along the magnetic field, v(E) and v(pA) are the poloidal shear flow and the poloidal Alfven velocities, respectively, and primes denote the radial derivative. For DELTA > L(E), the growth rate of the m = 1 mode becomes smaller than that for v(E) = 0; however, the stabilizing effect disappears and the poloidal shear flow enhances the growth rate for DELTA < L(E) (DELTA is the radial mode width for v(E) = 0 and L. is the scale length of shear flow).