Sustained Spheromak Physics Experiment (SSPX): design and physics results

Abstract

The Sustained Spheromak Physics Experiment (SSPX) was a high-temperature (Tₑ up to 0.5 keV) spheromak formed by coaxial helicity injection (CHI) and with plasma duration of a few milliseconds following the high-current formation stage. Clean walls and low impurity operation were obtained by a combination of baking, discharge cleaning and titanium deposition on the walls, allowing the generation of high-quality plasmas. Resistive-magnetohydrodynamic simulations, benchmarked to the experiment, were used to elucidate the physics. The detailed characteristics of the nφ = 1 toroidal mode associated with CHI were determined as was the physics of the nonlinear current drive and magnetic reconnection that formed and sustained the spheromak. If the helicity injection rate was reduced following formation the plasma became relatively quiescent and magnetic surfaces formed. The measured thermal diffusivity in the core was as low as ∼1 m² s⁻¹. However, reconnection events during buildup or sustainment of the plasma current by CHI were found to open magnetic surfaces throughout the plasma allowing rapid energy loss to the walls. As a result, experiments and simulations in SSPX found no path to simultaneous sustainment by CHI and good energy confinement. Additional physics results are also presented in this review.