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Measurement of high-frequency, small scale density fluctuations in improved confinement RFP plasmas

Author: James R Duff
Requested Type: Poster Only
Submitted: 2014-05-30 12:58:36

Co-authors: L. Lin, W.X. Ding, D.L. Brower, B.E. Chapman, J.S. Sarff, D.R. Carmody, P.W. Terry, D.J. Den Hartog, L.A. Morton

Contact Info:
1126 Vilas Ave #2
Madison, WI   53715
United States

Abstract Text:
In standard RFP plasmas, core transport is governed by magnetic fluctuations associated with global tearing modes. Using Pulsed Parallel Current Drive (PPCD), these global tearing modes are significantly reduced, and smaller-scale electrostatic fluctuations may become more important to electron thermal transport for these high-beta, improved confinement plasmas. On the MST RFP, an 11-chord far-infrared laser-based interferometry diagnostic, with ~8 cm spacing between chords, is used to measure electron density fluctuations with (k_⊥ p_s) ≤ 1.3. A fast magnetic coil array is also employed, which includes an 8 point poloidal array and a 32 point toroidal array. High-frequency (>50 kHz) small-scale (n>15) density and magnetic fluctuations have been observed in the edge of the high-performance PPCD plasmas, where density and temperature gradients become larger during the PPCD cycle. These fluctuations are clearly distinct from tearing and have amplitudes that correlate with the density gradient and electron beta. Recent theoretical work shows that drift-wave-like instabilities are unstable to the higher temperature and density gradients observed in PPCD plasmas. The linear and nonlinear gyrokinetic computational studies using parameters found in PPCD plasmas suggest TEM and ITG modes may be the dominant instabilities, and that microtearing modes may be present in high plasma beta conditions. These studies have looked at the dependence of these modes on parameters like plasma beta and collisionality. The RFP in general, and the MST in particular, is well suited to explore plasma beta scaling given the large dynamic range of plasma beta (2-26%) found in the device under various parameters. Correlation of the measured density fluctuations with plasma parameters such as density and temperature gradients in high beta plasmas will serve to identify the drive and contribute to validation of gyrokinetic codes.

Characterization: 1.2


Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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