Characterization of the Plasma Response to Injector Frequency in the HIT-SI3 Spheromak Experiment

Abstract

The Helicity Injected Torus - Steady Inductive 3 (HIT-SI3) was a spheromak experiment at the Univeristy of Washington HIT-SI laboratory that used three injectors running in AC to inject magnetic helicity into a copper flux conserver in the form of induced voltage and magnetic flux. This form of spheromak generation and sustainment, known as Steady Inductive Helicity Injection (SIHI), was used on a previous experiment known as HIT-SI (Helicity Injected Torus - Steady Inductive). This method holds promise as a means of sustaining a steady-state spheromak for long enough to generate fusion power, however the properties of SIHI-driven spheromaks are still not well characterized. In particular, studies of the previous experiment HIT-SI revealed the choice of injector frequency finj could heavily influence spheromak characteristics, and experimental evidence from HIT-SI3 discharges showeda similar dependence. In this work, studies on both experimentally measured data from HIT-SI3 and simulation data have been performed to definitively characterize the differences between spheromaks formed with different injector frequencies. Biorthogonal Decomposition was used as a tool to examine experimental data from HIT-SI3 and helped to distinguish important behavior, in particular the presence of n = 2 fluctuations not associated with the spheromak equilibrium or imposed by the injectors. Experimental data from the magnetic probe sets in HIT-SI3 was compared across different injector frequencies and used in the 2D equilibrium fitting software PSI-Tri to reconstruct the magnetic field profile and estimate beta. The 3D xMHD software NIMROD has been used to simulate HIT-SI and HIT-SI3 in past studies, and was used to simulate experimental HIT-SI3 shots at the injector frequency of 58.5 kHz by applying the injector waveform from the experiment as a boundary condition in the simulation. NIMROD was then used in a set of extended simulations at a variety of injector frequencies, including ones not used or used very little on HIT-SI3, to establish trends in calculated quantities such as injector impedance and beta, the ratio of plasma pressure to magnetic field pressure and an important parameter for determining the potential power of a fusion reactor configuration. These trends are compared to the previous experiment, HIT-SI, in the hope of finding general behaviors that scale predictably with injector frequency and can be extended to future SIHI devices.