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jlbarr_epr2014_madisonwi.pdf2014-05-30 12:48:39Jayson Barr

A Predictive Model for Non-solenoidal Startup using Local Helicity Injection

Author: Jayson L. Barr
Requested Type: Consider for Invited
Submitted: 2014-05-30 12:48:02

Co-authors: M.W. Bongard, M.G. Burke, R.J. Fonck, E.T. Hinson, J.M. Perry, A.J. Redd, D.J. Schlossberg

Contact Info:
University of Wisconsin-Madison
1500 Engineering Dr.
Madison, WI   53706
USA

Abstract Text:
Local helicity injection (LHI) aided by outboard poloidal-field (PF) induction is a routine tool for non-solenoidal startup on Pegasus. To date, plasma currents Ip ≤ 0.18 MA have been achieved using up to 6 kA of externally injected current. LHI discharges on Pegasus are created near the outboard injectors with aspect ratio (A) ≈ 5 and grow inward to fill the confinement region at A ≈ 1.3. This extreme change in geometry results in large inductive voltages on the plasma. A predictive, lumped-parameter circuit model for LHI plasma current evolution is in development to guide injector requirements for systems on Pegasus and potentially NSTX-U. The model employs power and helicity balance, using low-A formulations for both the plasma external inductance and equilibrium-field to estimate inductive loop-voltages in the moving plasma frame. Initial results match experimental Ip(t) trajectories within 20 kA using a prescribed plasma geometry evolution. Validation of the model with equilibrium reconstructions of Pegasus discharges is in progress. The ultimately achieved Ip is strongly dependent on the initial Ip attained when the injected current relaxes to a tokamak-like state. This early Ip is consistent with both a Taylor relaxation current limit and a kink instability limit. Helicity injection is the largest driving term in the initial phase of the plasma evolution, but in the later phase is reduced to 20-30% of the total drive as PF induction and decreasing plasma inductance become dominant. In contrast, MA-class startup on NSTX-U and beyond will require operation in the regime where helicity injection drive exceeds inductive and geometric changes when the plasma is at full size. Experiments using upgraded injector systems with high-voltage capability (Vbias ≤ 1.5 kV) are underway to explore plasma behavior and test the model in this helicity-dominated regime on Pegasus.

Work supported by US DOE Grant DE-FG02-96ER54375

Characterization: 1.0,2.0

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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