MHD Stability of the Plasma in the End System of AMBAL-M

We discuss MHD stability of the plasma in the completely axisymmetric end system of AMBAL-M and suggest a physical model to interpret the experimental results. Measurement of the radial plasma pressure profile in the semicusp using a local magnetic probe allowed estimation of the stability safety factor of the end system plasma which occurred to be greater than 3÷4. Gas puffing into the semicusp increases the plasma pressure in this region and hence enhances stability. To explain the observed MHD stability of the end mirror plasma when the MHD stabilizer — semicusp was switched off and the average field line curvature was unfavorable, a model was proposed which assumes that the plasma at the periphery had an electric contact with a limiter. As a result, the potential of flute perturbations vanishes at the plasma periphery. In this case finite Larmor radius effects may stabilize the most dangerous first (global) azimuthal mode because of nonlinear dependence of plasma perturbations on radius.