Enhanced interaction between electrons and large amplitude plasma waves by a dc electric field

A simulation study of the interaction between relativistic electrons and large amplitude plasma waves driven by a dc electric field is performed using a one-dimensional electrostatic particle code. Two distinctly different initial conditions, one with beam electrons and the other with a pump plasma wave, yield qualitatively similar results. Runaway electrons accelerated by the dc field act as efficient converters of the external dc field energy to ac (wave) energy. The momentum distribution develops an elongated tail which sustains electroacoustic modes with frequency less than the plasma frequency. Multiple harmonic branches of the plasma waves are clearly observed. A collective threshold dc field is found beyond which the runaway electrons free fall in the dc field and no two-stream instability occurs. These simulation results are discussed in the light of a recent tokamak experiment where momentum clamping of the runaways is observed