Application of transient CHI plasma start-up to future ST and AT devices

Employment of non-inductive plasma start-up techniques would considerably simplify the design of a spherical tokamak fusion reactor. Transient coaxial helicity injection (CHI) is a promising method, and is expected to scale favorably to next-step reactors. However, the implications of reactor-relevant parameters on the initial breakdown phase for CHI have not yet been considered. In particular, successful handover to steady-state heating and current-drive sources requires that the transient CHI discharge reach a certain minimum temperature, which in turn puts a stringent limit on the allowable gas pre-fill. On the other hand, insufficient pre-fill could prevent breakdown at the beginning of the CHI phase. In order to meet these potentially conflicting requirements, we evaluate CHI breakdown using an extension of the Townsend avalanche theory often applied to analyze Ohmic startup. The model predicts the duration and location of breakdown based initial vacuum conditions: field line connection length, gas pressure, electrode bias, and, if applicable, loop voltage. The predictions of this model have shown good agreement with observations of breakdown in NSTX for both Ohmic and CHI experiments [Hammond et al., Nucl. Fusion 58, 016013 (2018)]. In addition, the model provided an explanation for a CHI failure mode which was previously not well understood, in which breakdown occurred in an unexpected and undesired part of the vessel. We now apply this model to concepts for next-step reactors, determining the feasibility of breakdown and analyzing the risks of parasitic arcing in scenarios for solenoid-free startup. Scenarios for both spherical tokamak and advanced tokamak configurations are studied, with supporting simulations using the TSC code, to compare breakdown and current ramp-up conditions for different aspect ratios.