Wave-particle interactions in nonstationary plasma

Abstract

Waves have proven to be an immensely useful tool for manipulating magnetically confined fusion plasmas. Our sophisticated understanding of the physics of waves and their interactions with charged particles in such plasmas is aided by the fact that the waves often can be treated as perturbations of an essentially stationary background, simplifying their mathematical treatment. On the other hand, despite the rapid, ongoing advancement of the world's most sophisticated plasma compression experiments, such as NIF (LLNL) and Z (SNL), the phenomenology of wave-particle interactions in the sort of nonstationary plasma environments exemplified by these experiments has gone relatively unexplored.

In plasmas undergoing compression, expansion, ionization, and recombination, embedded waves can have very unusual and possibly useful properties. The introduction of explicit time-dependence into some classic problems in plasma wave physics has revealed the nature in which nonstationary processes impact all stages of a wave's lifetime, including its undamped linear and nonlinear dynamics as well as its collisionless and collisional damping. A number of new insights are revealed, including the discovery of an induced, switch-like collisionless damping mechanism capable of producing prescribed bursts of heat, current, magnetic field energy, and/or voltage in time-evolving plasma; a novel method for optimizing plasma-based acceleration of particle beams; enhanced understanding of the effect of waves on plasma compressibility; and the first numerical confirmation of a new, transparent analytical theory of nonlinear wave dynamics. To describe numerically all these effects, novel particle-in-cell simulations were developed. The findings not only stand to expand on the basic physics of waves in plasmas, but they also point toward potentially beneficial applications in the laboratory.