Experimental Studies of Plasma Heating due to the Parametric Decay of a Large Amplitude Pump Into Lower Hybrid Waves.

Plasma heating of a high density (omega _{rm pe}/omega_{ rm ce}>=q 1), low temperature (T _{o} =.2eV) linear plasma by the parametric decay instability of lower hybrid waves excited in a parallel-plate geometry is studied experimentally for omega/omega_{rm lh} = 3 and 10. The electric field distribution is modeled considering the higher-order modes produced in this geometry. A method of measuring bulk electron temperature in the presence of an oscillating potential is developed and validated. Electron heating results show that the parametric decay instability in this geometry is strongly driven and strongly convectively damped, resulting in very rapid electron heating which saturates in a time of 20-30omega^{-1}. Bulk electron heating consistent with a recent theory of parametric decay of a large amplitude driver near the lower hybrid frequency is also observed. Control of the instability by use of a finite bandwidth driver is also studied, and the results show that the threshold of the parametric decay instability in this geometry can be increased by a factor of 3 by a noise-modulated driver with Delta omega/omega =.7. These results are contrasted with earlier studies.