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Helicity injection current drive in the Pegasus Toroidal Experiment

Author: Aaron J. Redd
Requested Type: Consider for Invited
Submitted: 2009-12-04 16:57:32

Co-authors: J.Barr, M.W.Bongard, R.J.Fonck, E.T.Hinson, D.J.Schlossberg, K.E.Thome, N.Woodruff

Contact Info:
University of Wisconsin - Madison
1500 Engineering Drive, ERB Ro
Madison, Wisconsin   53706

Abstract Text:
Plasma startup and steady-state sustainment without using an Ohmic solenoid is a general problem facing tokamaks. This problem is most acute in small aspect ratio tokamaks, in which the relatively small center stack assembly does not allow the luxury of a significant Ohmic solenoid. The Pegasus Toroidal Experiment is an ultra-low aspect ratio tokamak (A~1.15), dedicated to exploring the physics of low-A tokamaks and the boundary between typical tokamak and spheromak operating spaces. Pegasus uses compact plasma sources (“washer guns”) as point-source magnetic helicity injectors to form tokamak discharges. This non-solenoidal startup technique has produced up to 100 kA of toroidal current in published studies [Battaglia et al, Phys. Rev. Lett. v.102, p.225003 (2009)], and after recent hardware modifications has produced up to 170 kA. The maximum toroidal current is limited by helicity balance and a relaxation limit, with this limit corresponding to the usual lowest-energy Taylor state. This theoretical current limit scales with the TF coil current, the gun-injected current, and the injection geometry, and these predicted scalings are consistent with the results from experimental scans over those parameters. Addition of Ohmic solenoid drive during the gun injection period enhances the helicity injection rate, but the maximum current cannot exceed the relaxation limit, demonstrating that this is a hard limit on the plasma current. Conversely, Ohmic solenoid drive can be used to sustain and enhance the gun-generated plasma, if the Ohmic drive is applied after the gun injection ceases. The current-profile relaxation associated with this helicity injection technique correlates with bursts of low-n MHD activity on the outboard side of the discharge. Planned near-term physics studies include the direct observation of the current-profile relaxation using an internal Hall-effect magnetic probe array and enhancement of the gun drive with shaped electrodes. In the longer term, the relaxation limit model is guiding the enhancement of the gun injection and TF coil power supplies, with the goal of reaching 300-400 kA of gun-driven toroidal plasma current.

Characterization: A3,E3


Princeton University

Innovative Confinement Concepts Workshop
February 16-19, 2010
Princeton, New Jersey

ICC 2010