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caspary_icc_2011_talk.pdf2011-08-24 10:46:48Caspary Kyle

Pellet injection into axisymmetric and 3D helical plasmas in the MST RFP

Author: Caspary J Kyle
Requested Type: Consider for Invited
Submitted: 2011-06-10 13:33:03

Co-authors: The MST RFP Group

Contact Info:
University of Wisconsin - Madison
1150 University Avenue
Madison, WI   53706
United States

Abstract Text:
In toroidally axisymmetric MST plasmas with reduced magnetic tearing fluctuations, deuterium pellet injection has allowed the Greenwald density limit to be exceeded by more than 50%, and it has driven total beta up to 26% with a pressure gradient that exceeds the Mercier criterion. In spontaneously occurring plasmas with one very large tearing mode, a 3D helical magnetic topology emerges in the plasma core, and pellet injection triggers a rapid change in the growth rate of the dominant mode and brings about the emergence of density gradients.
Plasmas with reduced magnetic fluctuations are brought about by inductive current profile control, a well-established technique on MST. In the absence of the direct core fueling made possible with pellet injection, these plasmas are limited to a density less than 30% of the Greenwald density limit. But with pellet injection, the Greenwald limit has been exceeded by more than 50% with an absolute density as high as 8e19 m^-3. In these high-density plasmas, total beta has reached 26%, a record for the RFP, and the pressure gradient exceeds the Mercier criterion. Simulations in toroidal geometry with the NIMROD code reveal that both pressure-driven tearing and pressure-driven interchange modes are linearly unstable, representing a new regime for the RFP in which current-driven tearing modes are usually dominant.
Plasmas with a single large dominant mode emerge spontaneously in hot MST plasmas achieved with large toroidal current and low density. This mode is that resonant nearest the magnetic axis, and the mode grows to such a large amplitude, up to 8% of the equilibrium field strength, that it envelops the original magnetic axis and forms a stellarator-like magnetic equilibrium in the plasma core with a new helical magnetic axis. The evolution of the central magnetic topology is measured directly in MST with a Faraday rotation diagnostic. This is the same "single-helical-axis state" observed recently in the RFX-mod RFP in plasmas with large toroidal current. Injecting pellets into these plasmas as the dominant mode is growing often causes a substantial increase in the mode's growth rate. In addition, density gradients emerge following pellet ablation. These gradients, which imply locally reduced particle transport, are measured directly with a novel differential interferometry diagnostic. In separate experiments using MST's new 1 MW tangential neutral beam injector, the dominant mode is somewhat suppressed with the injection of fast particles.

Characterization: A2,A5

Comments:

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