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2011_08_19_romero_talamas_mcx_presentation_at_icc_v02.pdf2011-08-19 10:56:14Carlos Romero-Talamás

Plasma isorotation, differential rotation, and stability at the Maryland Centrifugal Experiment

Author: Carlos A. Romero-Talamás
Requested Type: Consider for Invited
Submitted: 2011-06-10 16:11:11

Co-authors: R. C. Elton, W. C. Young, R. Reid, and R. F. Ellis

Contact Info:
University of Maryland
ERF, Bldg. 223
College Park, Maryland   20742
United States

Abstract Text:
The Maryland Centrifugal Experiment (MCX) is a plasma confinement concept based on inducing supersonic rotation and velocity shear in a magnetic mirror to stabilize large-scale magnetohydrodynamic instabilities. This is achieved by biasing an internal axial electrode to produce a radial electric field, E, such that an azimuthal ExB flow is produced (B is the externally-applied magnetic field). Ideally, plasma in a given magnetic flux surface will isorotate, i.e., it will rotate at the same angular speed anywhere on that surface. In a non-ideal situation, plasma may differentially rotate along flux surfaces, caused by non-uniform resistivity in the axial direction (i.e., the E field does not propagate uniformly along flux surfaces).

In this work, we present the results of rotation measurements using Doppler spectroscopy of He+ ions (468.54 nm) at two distinct azimuthal planes along the axis. A 1-m Turner-Czerny spectrometer with 10 simultaneous views and a high-speed camera were used to radially resolve the velocity profiles at each axial location. The data shows that plasma isorotates on the outer flux surfaces, but not on the inner flux surfaces (close to the center electrode). The differential rotation is used to formulate a new paradigm that explains high frequency (hundreds of kHz) and high amplitude (20 – 50% of average) plasma voltage oscillations not previously seen in MCX. It is hypothesized here that differential rotation leads to twisting of the magnetic field, which in turn leads to inductive voltage spikes. Since indefinite twisting of the magnetic field is untenable, and even minor field twisting creates a flow component in the axial direction, some plasma must be ejected during the inductive changes before the field resets itself through reconnection. High-speed images were obtained of the regions where B is maximum, and show plasma ejection from the confinement surfaces in timescales comparable to those inferred from differential rotation and the voltage oscillations. Data from B-dot probes placed close to the outer rotating plasma surfaces is analyzed in the context of the present hypothesis. Results from these analyses, as well as ideas on how to mitigate this instability, are discussed.

Work supported by the U.S. Department of Energy. We thank Prof. H. R. Griem for his advice in spectroscopic temperature measurements.

Characterization: A1,D1

Comments:
Please place together with other MCX presentations.

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011