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eth_epr2014_f.pdf2014-08-14 19:10:01Edward Hinson

Advancing Toward Reactor Relevant Startup via Localized Helicity Injection at the Pegasus Toroidal Experiment

Author: Edward T. Hinson
Requested Type: Consider for Invited
Submitted: 2014-05-30 17:55:12

Co-authors: J.L. Barr, M.W. Bongard, M.G. Burke, R.J. Fonck, J.M. Perry, A.J. Redd, D.J. Schlossberg

Contact Info:
University of Wisconsin-Madison
1500 Engineering Drive
Madison, WI   53726
United States

Abstract Text:
Localized current sources in the plasma scrape-off region inject magnetic helicity and drive toroidal current in a tokamak plasma without an Ohmic solenoid. This Local Helicity Injection (LHI) technique is being developed on Pegasus. Attainable currents are governed by helicity balance and Taylor relaxation. The helicity injection rate, and hence the resulting current drive, is set by the impedance of the high-current density beam in the scrape-off region. This impedance is governed by quasineutrality requirements and is dependent on plasma edge density. NIMROD simulations indicate that injector-driven current streams persist during LHI, with the tokamak-like plasma built up from injected current rings created by reconnection between adjacent turns of the driven helical streams. Observed MHD activity is in qualitative agreement with NIMROD, and includes bursts of n=1 magnetic activity localized to the outboard edge. Anomalous ion heating (Ti~0.5 keV) correlates with this MHD activity, consistent with reconnection heating. Helicity input rate scales as the product of voltage and area of the current source. The size of the injector apparatus at the plasma is thus minimized by operating at the highest possible voltage. This can be achieved using active arc injectors with conical frustum cathode geometry, local scraper limiters, and an array of external guard rings. This technology reduces impurities to negligible levels, enabling voltages >1kV to be applied without deleterious PMI for the length of the discharge. Tailoring of the discharge evolution can provide stable current profiles that can couple efficiently to subsequent current sustainment techniques. For example, LHI-initiated plasmas in Pegasus can transition to stable, OH driven H-mode discharges.

Characterization: 1.0

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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