Set of electromagnetic instabilities observed in Mercier stable plasmas of the L-2M stellarator

Abstract

Electromagnetic instabilities that have been experimentally observed at the L-2M zero net current stellarator and their theoretical analysis are discussed in this paper. We report the structure of the magnetic surfaces and the numerical calculation results. Under experimental conditions, the ideal internal MHD (magnetohydrodynamic) modes are stable. However, the external ideal peeling modes, which start to increase only if the plasma pressure gradient exceeds some particular threshold value, are unstable. If the peeling mode is stable, an electromagnetic mode, which starts to increase only if the plasma density and pressure exceed some particular threshold values, is observable. The characteristic frequency of the unstable modes was in the range of 70–90 kHz, and the direction of its rotation coincides with the direction of the ion diamagnetic drift. An instability was observed at n(0) > 1.5 × 1013 cm−3 and at 〈β〉 > 0.14%, where n(0) is the plasma density averaged over the central chord and 〈β〉 is the volume-averaged ratio of the gas kinetic pressure to the magnetic pressure. This phenomenon cannot be described within the framework of the theory of resistive interchange MHD modes. Analytical estimates have been performed within the framework of two fluid magnetohydrodynamics, and the calculations and experimental data are in reasonable agreement.