Spheromak energy confinement in sustained and transient conditions

Summary form only given. The most successful approach for generating the spheromak magnetic confinement configuration is through electrostatic helicity injection. A large arc discharge forms the spheromak by stretching, reconnecting, and amplifying a small amount of externally generated poloidal flux. The details of the inherent plasma relaxation processes are being investigated through nonlinear magnetohydrodynamics (MHD) simulation. Our previous work considers the low-pressure limit of MHD and shows that plasma current driven by the electrodes pinches and forms a configuration that is unstable to an n=1 mode. Amplification of poloidal flux results from the saturation of this MHD mode. With continuously applied current drive, magnetic field-line trajectories are chaotic, due to the n=1 mode and nonlinearly generated n>1 fluctuations. When the drive is removed, however, the fluctuations decay faster than the large-scale amplified poloidal flux, and closed magnetic flux surfaces form. Recent computations include temperature evolution with thermal transport coefficients and electrical resistivity that are appropriate for collisional plasmas. The temperature dependencies of these coefficients produce striking effects in the simulated spheromak evolution. The cold edge plasma tends to impede parallel thermal conduction to the wall in sustained conditions, allowing the plasma core temperature to reach tens of eVs. When the drive is removed, the cold edge plasma assists magnetic reconnection, so that closed flux surfaces form rapidly, and core temperatures increase to approximately 100 eV. The finite temperature computations have been performed in conditions that are realistic for the SSPX spheromak at LLNL, and simulation results on magnetic field and temperature evolution are compared directly with laboratory measurements. The comparison highlights the importance of transient effects in partially sustained conditions and helps analyze the beneficial effects of applying- the second voltage pulse.