Verification of the standard theory of plasma emission with particle-in-cell simulations

TL;DR

This study uses large-scale particle-in-cell simulations to verify the standard plasma emission theory, confirming fundamental emission at low beam densities and clarifying the nature of observed magnetic components.

Contribution

The paper provides the first large-scale PIC simulation verification of the standard plasma emission theory, addressing previous computational limitations and controversies.

Findings

Significant fundamental emission occurs at low beam densities (≤ 0.01).

The intensity of most modes decreases as the beam density increases.

Transverse magnetic components are linked to Langmuir turbulence, not F emission.

Abstract

The standard theory of plasma emission is based on kinetic couplings between a single beam of energetic electrons and unmagnetized thermal plasmas, involving multi-step nonlinear wave-particle and wave-wave interactions. The theory has not yet been completely verified with fully-kinetic electromagnetic particle-in-cell (PIC) simulations. Earlier studies, greatly limited by available computational resources, are controversial regarding whether the fundamental emission can be generated according to the standard theory. To resolve the controversy, we conducted PIC simulations with a large domain of simulation and a large number of macroparticles, among the largest ones of similar studies. We found significant fundamental emission if the relative beam density is small enough (say, $\le$ 0.01), in line with earlier study with a much-smaller domain; the relative intensity (normalized by the total initial beam energy) of all modes, except the mode associated with the beam-electromagnetic Weibel instability, decreases with increasing relative density of the beam. We also found significant transverse magnetic component associated with the superluminal Langmuir turbulence, which has been mistakenly regarded as evidence of the F emission in earlier study. Further investigations are required to reveal their origin.