The occurrence of strong electrostatic turbulence under various experimental conditions

Abstract

The Langmuir waves of a plasma can be excited through resonant interactions with a particle beam or an electromagnetic beam. For these excitations, there exist three types of phenomenology for the turbulent behavior. (1) The weak interaction regime, after a quasi-linear stage, exhibits a tendency to accumulate waves on large scales through mode couplings for which no dissipation takes place. However, intense long waves undergo a self-modulation instability, which produces a set of localized wave packets. If the beam-plasma interaction occurs along the lines of a strong magnetic field, these solitary waves are stable at some level of approximation. (2) If the interaction takes place in a moderate or negligible magnetic field, the wave packets collapse towards a point singularity in agreement with Zakharov's prediction and burn out by giving their energy to suprathermal electrons. This is the strong turbulence regime, where nonlinear self-focusing dominates over the dispersion effect. The resulting collapse produces intense spikes in the electric field at the Debye scale. This is favorable for spectroscopy involving the turbulent Stark effect. Details of the physics are reviewed, with emphasis on the various expected spectra including the distribution of suprathermal electrons that could be compared with spectroscopic data. (3) There is also an “ultrastrong” regime that has been investigated and is characterized by spikes that are so intense that the local electrostatic energy is greater than the internal energy. Spectroscopic data for this regime have recently become available from a relativistic beam-plasma experiment done at the University of California, Irvine.