Plasma frequency radiation in tokamaks

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Abstract
Radiation from relativistic runaway electrons is considered as a source for plasma frequency radiation in tokamaks. Two specific mechanisms, Cerenkov emission and radiation produced by nonlinear coupling of plasma and acoustic waves, are studied. In many cases the Cerenkov emission provides a reasonable spectral fit. It can also be used to measure the runaway current, and to estimate the runaway cutoff velocity. The nonlinear emission is found to be negligible unless the acoustic waves are enhanced by about two orders of magnitude above the thermal level (or the plasma waves correspondingly enhanced above their superthermal level). Some observations, though, indicate the need for significant nonlinear (or other) emission in addition to the Cerenkov emission. For some typical Alcator data, the Cerenkov model gives a runaway current of 2.5% of the Ohmic current, and a cutoff energy of 1.5 MeV.