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cch_icc_811.pdf2011-08-16 16:25:18Chris Hegna
icc_cchpaper.pdf2011-08-16 12:53:11Chris Hegna

Healing of magnetic islands by plasma flow

Author: Chris C Hegna
Requested Type: Consider for Invited
Submitted: 2011-06-08 14:42:55


Contact Info:
University of Wisconsin
1500 Engineering Drive
Madison, WI   53706

Abstract Text:
Recent experiments from the Large Helical Device (LHD) indicate that the plasma flow can play a primary role in "healing" vacuum magnetic islands in stellarators. The observed elimination of magnetic islands tends to occur at low collisionality and high plasma beta. A model explaining this phenomenon is developed reminiscent of 'mode locking/unlocking' physics of tokamak and RFP experiments. The theory describes transitions between two asymptotic solutions, a state with a large nonrotating island and a state where rotation shielding suppresses island formation. Transitions between these two states are governed by coupled
torque balance and island evolution equations. In conventional
stellarators, neoclassical damping physics plays an important role in establishing the flow profiles. The balance of neoclassical damping and cross-field viscosity produces a radial boundary layer for the plasma rotation profile outside the separatrix of a locked magnetic island. The width of this boundary layer decreases as the plasma becomes less collisional. This has the consequence of enhancing the viscous torque at low collisionality making healing magnetic islands occur more readily in high temperature conventional stellarators. Detailed comparisons with LHD data indicate that the theory correctly predicts both island growth and healing are possible with the disparate plasma responses distinguished by a sharp boundary in a parameters space defined by the plasma beta and collisionality at the rational surface.

Characterization: A5


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