Utility of an ion beam probe diagnostic for fluctuation studies in high energy density, magnetically confined plasmas

A heavy ion beam probe has been shown to be an attractive diagnostic tool for measuring fluctuating parameters in high energy density, magnetically confined plasmas. New techniques were developed that provide accurate, simultaneous measurements of the amplitude and phase of both density and space potential fluctuations. The measurements are made with a spatial resolution of approximately 0.1 cm3 and time resolution (currently limited by the electronics) of approximately 10 μsec. The new techniques were used to identify a coherent oscillation in an energetic arc plasma as a Kelvin-Helmholtz flute driven by nonuniform anti E/sub r/ x anti B/sub z/ plasma rotation. The mode was identified by its localization in a region of E/sub r/ x B/sub z/ velocity shear, the relative magnitudes of density and space potential oscillations, and the range of the phase difference between density and space potential oscillations across the plasma. A more rigorous normal mode analysis using the radial wave equation derived from finite Larmor radius fluid equations was performed to predict the radial eigenfunctions of density and space potential using experimental equilibrium parameters. Both predictions and experimental data show the peaking of the instability in the regions of strong fluid shear, as well as the large amplitude and rapid phase shift of the space potential fluctuations. The agreement was excellent considering that a linear symmetric model was used to predict the behavior of a fully developed, asymmetric instability