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epr_talk_final_caspary.pdf2014-08-14 21:35:54Kyle Caspary

Probing the limits on beta and density in the RFP

Author: Kyle J Caspary
Requested Type: Consider for Invited
Submitted: 2014-05-29 11:28:28

Co-authors: B.E. CHAPMAN, J.K. ANDERSON, S.T. LIMBACH, S.P. OLIVA, J.S. SARFF, S.K. COMBS, C.R. FOUST

Contact Info:
UW-Madison
1150 University Ave
Madison, WI   53706
USA

Abstract Text:
RFP-record values of total beta and normalized density have been achieved in MST plasmas with inductive current profile control combined with pellet and neutral beam injection. Total beta reaches 28% and appears to be limited by transport. The density has thus far reached 2.0*nGreenwald. Inductive (transient) current profile control is used routinely in MST to reduce current-gradient-driven tearing fluctuations and stochastic transport. Beta is thereby increased but is well below pressure-related stability limits due to a concomitant reduction in POhmic. With edge-deposited fueling based on gas puffing, the density is also necessarily low, typically at or below 0.5*nGreenwald, in order to avoid confinement-degrading edge-resonant instability. With direct fueling of the plasma core by pellet injection, a substantially larger density is possible, and this leads to a substantially larger POhmic and beta. Neutral beam injection further augments beta with modest heating and a substantial population of 25 keV fast ions. The beta limit is probed with a three-fold variation in POhmic brought about with a density scan. Magnetic fluctuations in the core and edge increase with density, likely leading to increased transport and the observed saturation of beta. Achievement of the maximum density in these plasmas is aided by neutral beam injection. Fast ions from the beam enhance pellet ablation in the plasma core, thereby allowing utilization of larger pellets. While the density in plasmas with current profile control can substantially exceed nGreenwald, the density in standard-confinement plasmas is limited to about the Greenwald value. This was established in MST by colliding large pellets with the plasma-facing wall, providing a controllable and substantial source of edge fuel. In discharges without edge toroidal field reversal, the density limit is observed to be approximately 10% lower.

Characterization: 1.2

Comments:

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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