Effect of unstable MHD modes on the confinement of a stellarator plasma

The results of an experimental study of the influence of unstable MHD modes on plasma confinement in an L-2M stellarator are presented. The spectral a nd statistical characteristics of turbulent plasma simultaneously at both t he edge and center of the plasma are investigated. It is shown that at cons tant power of electron-cyclotron heating of the plasma the energy content o f the plasma depends strongly on the strength of the externally applied ver tical magnetic field used to adjust the position of the plasma column. Appr eciable degradation of plasma confinement is observed for values of the ver tical field such that ideal MHD modes become unstable in the greater part o f the plasma column. This in turn is due to the formation of a magnetic con figuration with a magnetic "hump.'' At the same time, in the edge plasma th e instability of resistive-balloon modes grows, and turbulent particle tran sport increases. (C) 1999 American Institute of Physics. [S0021-3640(99)003 06-0].