Effect of rational transform on the ohmically heated plasma in the WIIb stellarator

Abstract

Instabilities of the plasma current have been observed when the transform near the plasma boundary is rational, whose strength decreases with increasing mode number. The t=y₂ instability is sufficiently strong to limit the current attainable. The instabilities do not seem to depend strongly on other plasma parameters such as n and Te which can vary 50% from shot to shot. The variation of instability strength with t could be MHD in character.