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Effects of the current boundary conditions at the plasma-gun gap on SSPX performance.

Author: Roman A Kolesnikov
Requested Type: Consider for Invited
Submitted: 2012-12-21 13:44:48

Co-authors: L.L. LoDestro, H.S. McLean, W.H. Meyer, L.D. Pearlstein

Contact Info:
LLNL
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Liveromore, CA   94550
USA

Abstract Text:
The Sustained Spheromak Physics Experiment (SSPX) was a toroidal magnetic-confinement device without toroidal magnetic-field coils or a central transformer but which generated core-plasma currents by dynamo processes driven by coaxial plasma-gun injection into a flux-conserving (FC) vessel. Final results of the SSPX program were reported in [1, 2]. A primary purpose of the experiment was to explore and possibly reach conditions for high magnetic-flux amplification. It was observed that the poloidal magnetic field (B_p) exhibits a strong peak as the drive increases (at lambda_gun>lambda_FC). Candidate explanations for the decrease of B_p at higher drives include poor density control, presence of large-amplitude n=1 MHD modes, as well as capacitor-bank limitations. Alternatively, the results might be due to the location of the outer strike-point in the upper gun; it intersects the copper wall near tangentially, or, as it continues to rise, stainless steel and then the insulator at near normal incidence. Either of these latter cases would be expected to release significantly more impurities, which would in turn adversely affect performance. To investigate this we added proper insulator boundary conditions, which set lambda=0 on flux contours intersecting insulating material at the plasma-gun gap, to the free-boundary equilibrium Corsica code. Using this option we perform new reconstructions of extended-formation SSPX equilibria and look for correlations between the position of the outer strike-point and the peak B_p. Initial results indicate that, as lambda_gun is raised, the outer strike-point reaches the insulator (or stainless steel) at the peak B_p. Also, using proper boundary conditions results in a substantially better fit between experimental data and computational results. Thus we observe a significant correlation between the outer strike-point being on the insulator (or stainless steel) and the observed limit to magnetic-field generation.

[1] H. S. McLean, 50th Annual Meeting of the APS, Division of Plasma Physics, Dallas, TX, November 17-21, 2008.
[2] E.B. Hooper, et al., Plasma Physics and Controlled Fusion 54 (2012) 113001.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

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