Coherent Radiation in Overdense Plasmas Interacting with Energetic Electrons of Different Velocity Distributions

According to the standard scenario of plasma emission, the escaping radiations are generated from the nonlinear development of the kinetic bump-on-tail instability, by a single beam of energetic electrons interacting with an overdense plasma. The primarily-excited beam-Langmuir mode and its further scattering over ion-related disturbances are suggested to be critical to the radiation process. Yet, non-beam distributions of energetic electrons, such as the ring (or ring-beam), DGH, loss-cone, horseshoe-like, etc., also exist broadly in space and astrophysical plasmas. They may drive wave modes distinct from those in the beam-plasma system. The corresponding radiation process could be distinct from that described by the standard scenario. To clarify this, we conducted particle-in-cell simulations to investigate the nonlinear response of an overdense plasma disturbed by energetic electrons of velocity distributions (VDs) changing from beam-like to ring-like. Efficient excitations of both the fundamental (F) and harmonic (H) plasma emissions are found for all the VDs investigated here, yet the kinetic instability, the wave modes excited, and the F/H radiation process are different. Details of these differences will be overviewed in this report.