IAEA Fusion Energy Conference 2010

Proceedings of the 23rd IAEA Fusion Energy Conference
Daejeon, 11-16 October 2010

Organized by the International Atomic Energy Agency
and hosted by the Government of the Republic of Korea

IAEA-CN-180

(EXS/P2-07) Nonsolenoidal Startup and Plasma Stability at Near-Unity Aspect Ratio in the Pegasus Toroidal Experiment

R.J. Fonck1), D.J. Battaglia2), M.W. Bongard1), E.T. Hinson1), A.J. Redd1), D.J. Schlossberg1)
 
1) University of Wisconsin, Madison, WI, United States of America
2) Oak Ridge National Laboratory, Oak Ridge, TN, United States of America

Abstract.   Initiating and growing the current channel without a central solenoid is a critical problem facing the spherical tokamak, and may be relevant to future tokamak reactors. In the Pegasus ST experiment, small plasma arc current sources are placed in the periphery as DC helicity sources, which may offer a technically attractive means of creating an ST plasma. A model invoking null formation, helicity balance, and Taylor relaxation describes the plasma formation via localized current injection at the plasma edge. Experiments on Pegasus have confirmed elements of this model, and demonstrated achievement of Ip∼0.17 MA for a gun-injected current of Iinj≤4 kA. Projection to higher performance and larger experiments are enhanced by the observation that passive electrodes are as effective as the active arc sources once the tokamak discharge is formed. Plasmas evolved using the edge helicity injection generally have hollow j(R) profiles, with sufficient internal magnetic shear to mitigate large-scale tearing modes. This and more flexible field control has expanded the operating regime past the previous limit of Ip/ITF≤1 to Ip/ITF > 2, or IN > 10. Operation at A∼1 offers high Ip at very low BT, giving high values of jedge/B∼1 MA/m2-T, where peeling instabilities are excited. These instabilities show filaments that accelerate while moving radially outward, similar to that seen with Type-I ELMs. The mode amplitude increases with increasing peeling drive, jedge/B.

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