New insights into how beam-excited instabilities saturate

Abstract

New kinetic simulations are carried out in which low density electron beams are injected at a boundary, as in early UCLA laboratory experiments on strong Langmuir turbulence and in beam-generated Langmuir turbulence in Earth’s electron foreshock. Kinetic simulations specifically addressing these UCLA experiments have not previously been undertaken. These and other simulations reveal the conditions for strong Langmuir turbulence in both the laboratory and the foreshock plasmas. Strong Langmuir turbulence is marked by spatial collapse of coherent nonlinear Langmuir wavepackets. New evidence is supplied for the role of stimulated backscatter of beam modes off ion density fluctuations as well as “break-up” (possibly modulational) instabilities as routes to strong Langmuir turbulence. After early transient evolution, Langmuir wavepackets renucleate in density cavities from previously burnt-out packets. Evolution of the beam in real space and velocity space is studied, with attention to processes such as the Bers “meniscus” effect, trapping of beam electrons, and velocity-space diffusion.