New constraints and energy conversion efficiencies for plasma emission

Abstract

Radio emission near the electron plasma frequency fp and 2 fp due to electron beams is important in many laboratory and space applications. Langmuir waves produced as a result of these beams undergo various interactions leading to radio emission. Two categories of such interactions are decay processes involving ion acoustic waves and processes involving scattering off thermal ions (STI). In this paper energy conversion efficiencies for STI emission processes are derived. These are then compared with existing expressions for emission via decay. It is found that decay dominates STI processes when decay is able to proceed. Conditions are derived for emission near fp, and 2 fp by decay processes. These depend on whether significant nonthermal levels of L′ or S waves are produced by the initial nonlinear processes. These conditions are determined by comparing nonlinear growth rates with Landau damping rates. It is found that where the ratio of the beam speed to the electron thermal speed exceeds roughly 3–6, L′-waves persist and emission near 2 fp can proceed. It is also found that when beams are sufficiently dense, cold, and fast, S-waves persist and emission near fp involving decay processes can proceed.