Particle and wave dynamics in a magnetized plasma subject to high rf pressure

Abstract

The nonlinear response of a plasma subject to an intense, spatially localized rf field has been investigated experimentally. In a uniform quiescent magnetoplasma of density ne≳109 cm−3, temperature kTe≃5 kTi≃1 eV, and magnetic field B0∼100 G, a resonance cone (ω/ωc≃0.1; where ωc is the electron cyclotron frequency) is established which converges away from a circular exciter to reach a maximum amplitude at the remote cone apex. When an rf burst of intensity ε0Erf2/nekTe≲O (1) is applied, a strong density depression (δne/ne≃40%) is formed in the focal region on a time scale (∼1 μsec) short compared with a collision time. Fast ion bursts [(1/2) mvi2≃35 eV≳100 kTi], large amplitude ion acoustic waves, but negligible electron temperature increases are observed. Density and resonance cone field interact nonlinearly and in time develop a state of turbulence. Ion heating (ΔTi/Ti≃100) dominates over electron heating (ΔTe/Te∼2). The direct measurements of the particle distributions with probes and energy analyzers are supplemented by test wave techniques. Small amplitude ion acoustic, electron plasma, and Bernstein waves are propagated through the region of high rf intensity and the perturbed plasma properties are deduced from the test wave behavior. The observed nonlinear collisionless interactions between particles and intense rf field are qualitatively explained by a model involving the ponderomotive force on electrons and acceleration of ions by space charge fields.