Measurements of the ponderomotive force including sideband mode coupling effects and damping rates

Measurements of the interactions of waves in the ion cyclotron range of frequencies (ICRF) with flute interchange modes are presented. Interactions between the applied l = 1 radio frequency (rf) wave and an m = -1 flute mode give rise to sidebands above and below the rf frequency with mode numbers of l = 0 and l = + 2, respectively. The contribution of the sideband terms to stability are shown to cancel 40% of the direct ponderomotive contribution. This is less than the 90% predicted by theory [Phys. Fluids 30, 148 (1987)] for an applied l = + 1 rf wave above the ion cyclotron frequency with a large separation between the plasma and the vacuum vessel. Measurements of the linear growth and damping rate of the flute instability in the presence of rf are in good agreement with that calculated by considering only the curvature-driven instability and the ponderomotive force. The growth rate of the magnetohydrodynamic mode is consistent with the primary force acting on the plasma being the curvature force. This method allows a determination of the net stabilizing force on the plasma.