Parametric instabilities associated with intense electron cyclotron heating in the MTX tokamak

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Abstract
The paper presents a survey of possible parametric instabilities in the upcoming Microwave Tokamak Experiment (MTX) at the Lawrence Livermore National Laboratory in which an intense pulsed freeelectron laser will be used to carry out electron cyclotron heating in the Alcator C tokamak. Potentially the most dangerous instability is stimulated Brillouin backscatter by electrostatic ion cyclotron modes or quasi-modes for ordinary-mode heating at the fundamental harmonic or extraordinary-mode heating at the second electron cyclotron harmonic. Large reflectances may occur for coherent microwave sources. In addition, extraordinary waves at the amplitudes considered here can be non-linearly absorbed by parametric decay processes at the resonance layers where the pump frequency equals the second electron cyclotron harmonic and where it equals twice the local upper hybrid frequency. Finite-bandwidth effects can significantly inhibit parametric instabilities. Density fluctuations induced by drift wave turbulence at the plasma surface, with amplitudes as observed in earlier Alcator C experiments, are expected to spread the electron cyclotron wave packets but may be of insufficient amplitude to suppress Brillouin backscatter instability.

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