Parametric Excitation of Low Frequency Waves in ICRF-Produced Plasmas on GAMMA 10

In high power ICRF experiments on the GAMMA 10 tandem mirror, plasmas with a strong temperature anisotropy are produced when the cyclotron resonance layer exists near the midplane of the central cell. Saturation of the diamagnetism is sometimes observed when the heating ICRF power is increased. In the plasmas with the strong temperature anisotropy, Alfvén-ion-cyclotron (AIC) modes are spontaneously excited. Low frequency (LF) waves which have differential frequencies between the heating ICRF waves (6.36 MHz) and the AIC modes (5.5 – 6.0 MHz) are also detected. At the midplane of the central cell, azimuthal mode structures are measured and the heating ICRF waves of m = +2, the AIC modes of m = -1 or 0 and the LF modes from m = -2 to m = +2 are detected. These experimental observations suggest the heating ICRF waves with several azimuthal mode numbers branch into the AIC modes and the LF waves. Although axial mode structures are also measured, it has not yet been confirmed that they satisfy the matching conditions. A possibility of the parametric decay of the heating ICRF waves to the AIC modes and the LF waves is discussed.