Simulation of spheromak evolution and energy confinement

Simulation results are presented that illustrate the formation and decay of a spheromak plasma driven by a coaxial electrostatic plasma gun, and model the plasma energy confinement. The physics of magnetic reconnection during formation is also illuminated. The simulations are performed with the three-dimensional, time-dependent, resistive magnetohydrodynamic NIMROD code [ C. R. Sovinec , A. H. Glasser , T. A. Gianakon , D. C. Barnes , R. A. Nebel , S. E. Kruger , D. D. Schnack , S. J. Plimpton , A. Tarditi , and M. S. Chu , J. Comput. Phys. 195 , 355 ( 2004 ) ]. The simulation results are compared to data from the Sustained Spheromak Physics Experiment (SSPX) [ E. B. Hooper , L. D. Pearlstein , and R. H. Bulmer , Nucl. Fusion 39 , 863 ( 1999 ) ]. The simulation results are tracking SSPX with increasing fidelity (e.g., improved agreement with measured magnetic fields, fluctuation amplitudes, and electron temperature) as the simulation has been improved in its representations of the experimental geometry, the magnetic bias coils, and the detailed time dependence of the current source driving the plasma gun, and uses realistic parameters. The simulations confirm that controlling the magnetic fluctuations is influenced by the current drive history and by matching the gun current in sustainment approximately to the value corresponding to the eigenvalue in the flux-conserver for the parallel current in a force-free equilibrium.