Nonlinear plasma simulations of the Sustained Spheromak Physics Experiment demonstrate the role of transient effects in establishing a toroidal magnetic structure that confines internal energy. Magnetohydrodynamics modeling with temperature-dependent transport coefficients compares well with experimental measurements and shows that the second current pulse improves confinement by keeping the q profile from falling below the value of 1/2, suppressing resonant m = 1, n = 2 fluctuations.

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. The simulation results are tracking SSPX with increasing fidelity ͑e.g., improved agreement with measured magnetic fields, fluctuation amplitudes, and electron temperature͒ 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.

Nucl. Fusion 39, 863 ͑1999͔͒, progress has been made in understanding the mechanisms that generate fields by helicity injection. SSPX injects helicity ͑linked magnetic flux͒ from 1 m diameter magnetized coaxial electrodes into a flux-conserving confinement region. Control of magnetic fluctuations ͑␦B / B ϳ 1% on the midplane edge͒ yields T e profiles peaked at Ͼ200 eV. Trends indicate a limiting beta ͑␤ e ϳ 4%-6%͒, and so we have been motivated to increase T e by operating with stronger magnetic field. Two new operating modes are observed to increase the magnetic field: ͑A͒ Operation with constant current and spontaneous gun voltage fluctuations. In this case, the gun is operated continuously at the threshold for ejection of plasma from the gun: stored magnetic energy of the spheromak increases gradually with ␦B / B ϳ 2% and large voltage fluctuations ͑␦V ϳ 1 kV͒, giving a 50% increase in current amplification, I tor / I gun . ͑B͒ Operation with controlled current pulses. In this case, spheromak magnetic energy increases in a stepwise fashion by pulsing the gun, giving the highest magnetic fields observed for SSPX ͑ϳ0.7 T along the geometric axis͒. By increasing the time between pulses, a quasisteady sustainment is produced ͑with periodic good confinement͒, comparing well with resistive magnetohydrodynamic simulations. In each case, the processes that transport the helicity into the spheromak are inductive and exhibit a scaling of field with current that exceeds those previously obtained. We use our newly found scaling to suggest how to achieve higher temperatures with a series of pulses.

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