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Simulations of sustained HIT-SI operation in the present experimental regime and beyond
| Author: | Izzo V.A. |
| Coauthor: | T.R. Jarboe, P.E. Sieck |
| Institution : | University of Washington |
| Abstract text: | Sustained discharges of the HIT-SI spheromak experiment have been simulated with the 3D MHD code NIMROD. Boundary conditions on the axisymmetric confinement region substitute for the inclusion of the non-axisymmetric injectors, where the fields at the injector openings are presumed to take the form of an RFP operated in spheromak mode. To that effect, applied normal fields have the form of RFP toroidal fields, based on a Grad-Shafranov solution, and the applied voltage is set to produce the injector current needed for an injector lambda corresponding to the injector cross-section eigenvalue.
Plasmas obtained during the initial operating phase of HIT-SI are believed to have a low Lundquist number (S), based on best guesses of plasma parameters. A simulation of HIT-SI operation in this regime predicts no spheromak formation during the sustained phase. When the injectors are ramped off in the simulation (more quickly that in the experiment), the remaining fields relax to form a spheromak. As S is increases beyond the estimated present experimental regime, initial signs of relaxation are observed in the form of the selection of a preferred toroidal direction for the injector fields corresponding to the growth of the n=0 magnetic energy in the spheromak region. Results from simulations from a range of higher S values will be presented.
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