Experimental Observation of an Unstable Mode in Theoretically Ideal MHD Stable Plasmas Confined in the L-2M Stellarator

In net-free current plasmas that are confined in the L-2M stellarator, an electromagnetic mode having thresholds in the plasma density and pressure was observed. The characteristic frequency of the unstable mode was 70–90 kHz, and the direction of its rotation pointed toward the direction of ion diamagnetic drift. The instability was observed at n(0) > 1.5 × 1013 cm-3 and 〈β〉 > 0.14%, where n(0) is the plasma density averaged over the central chord and 〈β〉 is volume-averaged ratio of the gas kinetic pressure to the magnetic pressure. This phenomenon cannot be described within the theory of resistive interchange modes. Possible reasons for the occurrence of the observed electromagnetic mode are discussed using analytical estimations.