Electromagnetic plasma emission during beam‐plasma interaction: Parametric decay versus induced scattering

Abstract

Two‐dimensional electromagnetic particle‐in‐cell simulations are performed for examination of electromagnetic plasma emission at twice the electron plasma frequency. Electromagnetic “2fp” waves are considered to be excited by nonlinear three‐wave processes in beam‐plasma interactions. In this paper, nonlinear development of an electron‐beam‐plasma instability is studied as an initial value problem. The present simulation result confirmed that electromagnetic 2fp waves are strongly enhanced by the wave‐wave interaction between forward and backward Langmuir waves, which is in agreement with the previous studies. It is also demonstrated that large‐amplitude forward Langmuir waves decay into backward Langmuir waves and ion acoustic waves via a parametric decay and that electromagnetic 2fp waves are also enhanced by the decay of Langmuir waves. However, the growth rate of the electromagnetic 2fp waves due to the parametric decay of Langmuir waves is not as high. It should be noted that induced backscattering of Langmuir waves by enhanced thermal fluctuations of ions cannot be neglected in the excitation of backward Langmuir waves. Hence, low‐noise simulations are necessary to suppress the effect of enhanced thermal fluctuations in the particle‐in‐cell method.