Diagnosis of Strong Beam-Plasma Turbulence Conditions from Electromagnetic Emission

Abstract

Electromagnetic emission from strong beam-plasma experiments displays scaling with the beam-instability growth rate ${\ensuremath{\gamma}}_{b}$. We relate this to the underlying turbulence through a simple model balancing rates of electrostatic energy transfer from the beam to the plasma. A transition from subsonic to supersonic turbulence appears as a break in the scaling of emitted power with ${\ensuremath{\gamma}}_{b}$. The implied values of the electrostatic-wave energy density obtained by measurement of the scattering of the relativistic beam agree qualitatively with the model, and yield $W\ensuremath{\equiv}\frac{{E}^{2}}{4\ensuremath{\pi}\mathrm{nkT}}\ensuremath{\cong}0.01 \mathrm{to} 1$. The scattering waves apparently have phase velocities near the beam velocity.