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co_injection_of_helicity_and_plasma_in_chi_simulations_of_nstx.pdf2014-09-02 17:36:03Edwin Hooper

Co-injection of helicity and plasma in CHI simulations of NSTX

Author: Edwin B. Hooper
Requested Type: Poster Only
Submitted: 2014-05-28 13:38:53

Co-authors: C. R. Sovinec, R. Raman

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PO Box 808
Livermore, CA   94551

Abstract Text:
Previous resistive MHD (NIMROD) simulations of NSTX CHI startup plasmas [1, 2] assumed a constant-density background; in the experiment helicity is injected into vacuum, with plasma and gas flowing from the injector slot on the bottom plate. We report initial results from simulations extended to include plasma injection into a low density, vacuum-like plasma. Extraneous currents in the background plasma are minimized by density-dependent artificial radiation. Helicity and plasma flow from the slot at the ExB velocity due to the applied voltage. Development of the injected flux bubble and subsequent closure of flux surfaces following the removal of the injector voltage are similar to simulations of helicity injection into a constant density plasma. The impact of impurity radiation has been extended in these models: Radiation is used to achieve agreement with the temperature during experimental plasma buildup. Plasma evolution during and following flux-surface closure is sensitive to radiation, indicating that a rapid decay of impurities following injection will help maintain a slowly-decaying, high-quality plasma in future current-drive experiments. Previous simulations including non-axisymmetric modes found a (toroidal) n=1 mode with current-pinch characteristics in the current layer near the surface of the injected flux bubble [3]. In constant density simulations this had little effect on flux-surface closure resulting from resistivity. In preliminary, density-injection simulations the mode is much stronger so it will be necessary to re-examine non-axisymmetric contributions to flux surface closure. Work performed under the auspices of the U.S. Department of Energy under contract DE-AC52-07NA27344 at LLNL.
[1] E. B. Hooper, et al., Phys. Plasmas 20, 092510 (2013). [2] F. Ebrahimi, et al. Phys. Plasmas 20, 090702 (2013); Phys. Plasmas 21, 0566109 (2014). [3] E. B. Hooper, et al., Bull. Am. Phys. Soc. 56(12), 255 (2011).

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