Experimental Studies of Multi-Species Pure Ion Plasmas PDF Download

Author: Matthew Affolter
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Languages : en
Pages : 87

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Book Description
Cyclotron modes are studied on rigid rotor, multi-species ion plasmas confined in a Penning-Malmberg trap. Collective effects and radial electric fields shift the cyclotron mode frequencies away from the "bare" cyclotron frequencies for each species s. These frequency shifts are measured on the distinct cyclotron modes (m = 0, 1, and 2) with sinusoidal azimuthal dependence. We find that the frequency shifts corroborate a simple theory expression in which collective effects enter only through the ExB rotation frequency and the species fraction, when the plasma is radially uniform. At ultra-low temperatures, these plasmas exhibit centrifugal separation by mass, and additional frequency shifts are observed in agreement with a more general theory. Additionally, quantitative measurements of the plasma heating from short resonant cyclotron bursts are found to be proportional to the species fraction. These cyclotron modes are used as a diagnostic tool to estimate the plasma composition, in order to investigate the damping of Langmuir waves due to inter-species collisions. Experiments and theory of this collisional inter-species drag damping are presented. This also provides the first experimental confirmation of recent theory predicting enhanced collisional slowing due to long-range collisions. Drag damping theory, proportional to the collisional slowing rate, is in quantitative agreement with the experimental results only when these long-range collisions are included, exceeding classical collision calculations by as much as an order of magnitude. At large wave amplitudes, Langmuir waves exhibit harmonic generation, non-linear frequency shifts, and a parametric decay instability, which are experimentally investigated. The parametric wave-wave coupling rates are in agreement with three-wave instability theory in the dispersion dominated oscillatory coupling regime, and in the phase-locked exponential decay regime. However, significant variations are observed near the decay threshold, including slow, oscillatory growth of the daughter wave. These experimental results have motivated wide-ranging theory and simulations, which provide both insights and puzzles.