Observation of Parametric Decay by Use of CO2Laser Scattering from a Plasma

By use of C${\mathrm{O}}_2$ laser scattering, parametrically excited lower hybrid waves were observed in the Alcator C tokamak. The power spectrum had a maximum at $N_{\parallel }\gtrsim 7$, in contrast to the pump-wave spectrum which peaked near $N_{\parallel }\simeq 3$ ($N_{\parallel }$ is the index of refraction parallel to the magnetic field). Ion tail formation near the plasma edge correlated well with the occurrence of parametric decay. The consequence of these phenomena on wave propagation into the plasma interior is discussed.