A study of ion acceleration, asymmetric optical pumping and low frequency waves in two expanding helicon plasmas

A Study of Ion Acceleration, Asymmetric Optical Pumping and Low Frequency Waves in Two Expanding Helicon Plasmas Xuan Sun This work concerns measurements of parallel ion flow, optical pumping, and low frequency waves in expanding plasmas generated by two different helicon plasma sources. The measurements confirm numerical predictions of the formation of a currentfree double layer in a region of diverging magnetic field. With laser-induced fluorescence (LIF), the double layer structure in both helicon plasma sources was investigated through measurements of the bulk parallel ion flow speed. Both double layers have a total potential drop of 3-4kTe and length scales smaller than ion-neutral mean-free-path. A stronger double layer, with a potential drop of ~ 6kTe, was created in a uniform magnetic field region with a plasma limiting aperture plate. During the investigations of ion acceleration in expanding plasmas, a new phenomenon, asymmetrical optical pumping (AOP) due to the acceleration of ions in magnetic field gradient, was observed. The signature of AOP is a difference in the LIF emission amplitude from a pair of Zeemansplit ion states. A model that reproduces the dependence of the AOP on magnetic-field and ion-velocity gradients is described. With magnetic fluctuation probes, low frequency, transverse, electromagnetic waves were also identified in the expanding helicon plasma. The wave is localized to the vicinity of the maximum plasma density gradient and appears only at low neutral pressure. Based on the scaling of the wave frequency and amplitude with magnetic field strength, the wave was identified as the resistive drift Alfvén wave.