K. D. Kang’s research while affiliated with University of Colorado Boulder and other places

Results of recent 2-D kinetic simulations of two kinds of turbulence driven by cold electron beams are presented and discussed. Weak electron beams injected at a boundary drive strong Langmuir turbulence marked by spatially collapsing nonlinear wavepackets and density cavities. The two-stream instability in a magnetized plasma with mobile ions evolves first into phase-space holes which manifest themselves as bipolar electric field structures. Later interactions yield electrostatic whistlers and ion-Bernstein waves.

This talk presents a coordinated experimental investigation and computer modeling of basic plasma processes in simultaneous trapping and acceleration of both positive and negative ions by propagating electrostatic structures such as RFQs. The concept of increasing the current through stacking modules will be discussed. Furthermore, the concept of enhancing the transfer of electrons between positive and negative ions through increasing the rate of encounters in a quadrupole will be presented. Applications to fusion research through the production of neutral and neutralized beams will be discussed. .

Strong Langmuir turbulence (SLT) is marked by density cavities driven by ponderomotive force and by spatial and temporal self-focusing (collapse) of nonlinear wavepackets trapped in these cavities. SLT is a distinctive and fundamental nonlinear state of a plasma which has been driven in laboratory plasmas by weak, nonrelativistic electron beams (1,2,3). In this paper detailed comparisons are made between the results of new 2-D Particle-In-Cell computer simulations and UCLA experiments on beam-excited SLT with the aim of developing a comprehensive theoretical framework for understanding these results. Both beam-trapping (wave-particle) nonlinearities and ponderomotive force (wave-wave) nonlinearities are studied in time and space. Scaling laws are developed for early trapping phenomena and for later cavity evolution.