|icc2006_reconnection.pdf||2006-02-21 12:43:00||Bick Hooper|
Magnetic Reconnection in the Spheromak: Physics and Consequences
Author: Bick Hooper
Submitted: 2005-12-22 12:03:28
Co-authors: B. I. Cohen. D. N. Hill, H. S. McLean, C. A. Romero-Talamás, R. D. Wood, C. R. Sovinec (U. Wisconsin)
Lawrence Livermore National Lab.
PO Box 808
Livermore, CA 94550
Reconnection plays two roles in the spheromak: magnetic topology changes by conversion of injected toroidal flux into poloidal flux  and magnetic surface closure in a slowly decaying spheromak. Results from SSPX are compared with NIMROD. Voltage spikes on the SSPX gun during spheromak formation are interpreted by comparison with simulation as reconnection across a negative-current layer close to the mean-field x-point. The layer is driven by the n=1 (toroidal) mode; a simulation without n=2-5 modes is semi-quantitatively the same as one with n=1-5. Field lines are chaotic during these events, resulting in rapid electron energy loss to the walls and the low Te, < 50 eV, seen in both experiment and simulation during strong helicity injection. Reconnection also forms closed surfaces (both axisymmetric and low-n islands) during decay on the L/R time when the edge current is sustained by the gun. In SSPX this results in good energy confinement, with Te up to 350 eV ; information about the magnetic field structure can thus be inferred from the Thomson scattering profiles of electron temperature. The topology changes resulting from flux closure are not reflected in the impedance of the axisymmetric gun. In the simulation, closure can occur between voltage spikes if they are sufficiently far apart in time. In the n=1 (only) simulations the structure of the closed surfaces during decay show the n=1 structure more distinctly than those in the simulation with several modes. Up to 21 toroidal modes have also been examined; no qualitative changes result, demonstrating that the fundamental spheromak MHD physics is captured by the n≤5 simulations. We also compare the influence on NIMROD simulations of temperature evolution in SSPX due to changing Z_eff=1 to Z_eff=2.3, consistent with CORSICA energy transport simulations and SSPX data. Possible experiments and paths forward for the spheromak are examined in the presence of the constraints imposed by the reconnection physics.
* Work performed under the auspices of the U. S. DOE by U. California LLNL under contract No. W-7405-Eng-48.
**Work performed in collaboration with D. N. Hill, H. S. McLean, C. A. Romero-Talamás, R. D. Wood, B. I. Cohen, C. R. Sovinec
1. E. B. Hooper, et al., Phys. Plasmas 12, 092503 (2005).
2. H. S. McLean, invited paper at DPP2005 and Phys. Plasmas, to be published; R. D. Wood, et al., Nucl. Fusion 45, 1582 (2005).