Dynamics of fundamental electromagnetic emission via beam-driven Langmuir waves

Abstract

The nonlinear process of electromagnetic Langmuir decay, which leads to radio emission near the plasma frequency, is studied for situations in which Langmuir waves are directly driven by an electron beam and indirectly generated via electrostatic Langmuir decays. The electromagnetic Langmuir decay is stimulated by the presence of ion-acoustic waves. An approximate method is devised for studying this emission process with axial symmetry (along the direction of beam propagation) in three spatial dimensions, based upon the Langmuir and ion-acoustic wave dynamics in one spatial dimension. Numerical studies of the fundamental electromagnetic emission starting from electron dynamics are then carried out via quasilinear theory, and the results are explored for illustrative parameters. The evolution of the fundamental transverse waves shows the combined effects of local emission and propagation away from the source. At a given location, the emission rate shows a series of peaks associated with successive electromagnetic decays of the Langmuir waves, which are either driven by the beam or produced by successive electrostatic decays. The emission rate for a given electromagnetic decay decreases with time, following an initial increase. In addition, the emission rate for a specific electromagnetic decay shows approximate dipolar form, consistent with previous analytical work. Consequently, the fundamental transverse waves emitted locally propagate approximately symmetrically in both the forward and the backward directions. Variation of the background electron to ion temperature ratio, beam injection parameters, and angular widths of the Langmuir and ion-acoustic spectra are found to affect the emission rate and, hence, the fundamental transverse wave levels. Furthermore detailed studies show that the wave numbers of the maximum emission rates are also in good agreement with an approximate prediction for simple model Langmuir and ion-acoustic spectra.