MHD Modeling of Coaxial Helicity Injection in HIT-II and NSTX
Author: C. R. Sovinec
Requested Type: Poster Only
Submitted: 2011-06-10 12:03:03
Co-authors: E. B. Hooper, R. A. Bayliss, and A. J. Redd
Univ. of Wisconsin-Madison
1500 Engineering Drive
Madison, Wisconsin 53706-1
Coaxial helicity injection (CHI) for current drive has its origins in the alternate / innovative-confinement program. It biases a pair of toroidally symmetric electrodes that intercept the strike points of open poloidal magnetic field to generate linked poloidal and toroidal magnetic flux. For spherical torus (ST) configurations, this DC technique can be used for current startup and initial heating to save inductive Volt-seconds for plasma that has attained some level of electrical conductivity and energy confinement, as demonstrated in the Helicity Injected Torus II (HIT-II) and in the National Spherical Torus Experiment (NSTX) [1,2].
We describe the application of resistive MHD computation to model CHI in both experiments. The necessary modifications to boundary conditions in the NIMROD code [https://nimrodteam.org] are related to those used for modeling the Sustained Spheromak Physics Experiment [3-5]. However, numerical coordination of injector and absorber gaps for STs needs special consideration. Our computations for HIT-II use a simplified model without pressure and are run to steady state for conditions without strong relaxation. Laboratory  and computed results on injector current and plasma current agree reasonably well as toroidal magnetic field and injector flux are scaled . They are consistent with a dimensional estimate from the Grad-Shafranov equation that provides a new perspective on a previously published model based on a current-sheet equilibrium and the magnetic pressure required for the 'bubble-burst' criterion . Numerical solutions of the Grad-Shafranov equation with an assumed current profile also indicate qualitative changes as the predicted criterion is crossed. Our computations of NSTX include temperature evolution, anisotropic thermal conduction, temperature-dependent resistivity, and detailed modeling of the experiment’s CHI capacitor-bank response. Results for the magnetic flux expansion show electron temperatures that are consistent with laboratory measurements, and initial 3D computations show fluctuation activity in the expanding current sheet.
Contributions of CRS and RAB are part of the Plasma Science and Innovation Center and are supported by US DOE grant DE-FC02-05ER54813. Work by EBH is performed under the auspices of the US DOE under contract DE-AC52-07NA27344 at LLNL.
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 R. A. Bayliss, C. R. Sovinec, and A. J. Redd, submitted to Phys. Plasmas (2011).
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