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Turbulence-driven shear flow and self-regulating drift wave turbulence in a cylindrical plasma device

Abstract

This dissertation provides an experimental test of the basic theory of the self-regulating drift wave turbulence (DWT)/sheared zonal flow (ZF) system in a cylindrical plasma device. The work is carried out from three approaches: the first explores the statistical properties of the turbulent Reynolds stress and its link to the ZF generation, the second investigates the dynamical behavior of the DWT/ZF system and the third investigates the variation of the DWT driven ZF verses magnetic field strength and ion-neutral drag. A radially sheared azimuthally symmetric plasma flow is generated by the DWT turbulent Reynolds stress which is directly measured by a multi-tip Langmuir probe. A statistical analysis shows that the cross-phase between the turbulent radial and azimuthal velocity components is the key factor determining the detailed Reynolds stress profile. The coincidence of the radial location of the non-Gaussian distribution of the turbulent Reynolds stress and the ion saturation current, as well as the properties of the joint probability distribution function (PDF) between the radial particle flux and turbulent Reynolds stress suggest that the bursts of the particle transport appear to be associated with radial transport of azimuthal momentum as well. The results link the behavior of the Reynolds stress, its statistical properties, generation of bursty radially going azimuthal momentum transport events, and the formation of the large-scale ZF. From both Langmuir probe and fast-faming imaging measurements this shear flow is found to evolve with low frequency (̃250-300Hz). The envelope of the higher frequency above (5kHz) floating potential fluctuations associated with the DWT, the density gradient, and the turbulent radial particle flux are all modulated out of phase with the strength of the ZF. The divergence of the turbulent Reynolds stress is also modulated at the same slow time scale in a phase- coherent manner consistent with a turbulence-driven shear flow sustained against the collisional and viscous damping and the radial turbulence correlation length and cross field particle transport are reduced during periods of strong flow shear. The results are qualitatively consistent with theoretical expectations for coupled DWT- ZF dynamics. The drift turbulence/zonal flow system shows a strong variation with magnetic field and neutral gas pressure. The density fluctuation amplitude, radial particle flux and the absolute value of the divergence of the turbulent Reynolds stress at shear layer and the shear flow are negligible when B̃600G. As the magnetic field is raised, these quantities all exhibit a rapid increase for 600G 700G these quantities increase at a slower rate. The ZF is stronger at lower neutral pressure and weaker at higher neutral pressure, and the density fluctuation amplitude, radial particle flux and the absolute value of the divergence of the turbulent Reynolds stress at shear layer and the shear flow all decrease with an increase of the neutral pressure

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