Electromagnetic radiation from beam–plasma instabilities

The generation of electromagnetic radiation from the unstable electrostatic waves of a beam–plasma system is investigated by means of computer simulation. A two-dimensional model is employed in which the plasma is treated as a finite system and in which the effects of a continuous beam inflow are included. For the parameters of the simulation (nb/n0 ∼1%, an essentially nonrelativistic beam, and no external magnetic field), the radiation is concentrated near the plasma frequency, is emitted preferentially perpendicular to the beam, and is polarized with E parallel to the beam direction. Radiation at the second harmonic is found to be weak. The production mechanism is found to be the scattering of Langmuir waves from ion-acoustic fluctuations, and the observed radiation level agrees well with the theoretical predictions for such a three-wave coupling. The results of the numerical experiments are compared with the recent laboratory experiment of Whelan and Stenzel [Phys. Rev. Lett. 47, 95 (1981)].