The role of momentum transfer in the detachment front response to power transients for reactor scale tokamaks

We have investigated the response of detached plasmas to multi-ms timescale power transients using 1D simulations of the scrape-off layer on a scale and parameter range relevant for reactor-scale tokamaks, such as the Spherical Tokamak for Energy Production (STEP). Simulations were performed using the fluid code Hermes-3, employing the solver CVODE for robust time integration. Including or disregarding the momentum received by the neutral gas via charge exchange reactions in simulations is found to dramatically affect the detachment front’s response. In the analysis, motion of the ionisation front of a detached plasma is determined by two distinct mechanisms; a fast initial pressure-dominated response as the neutral cloud is rapidly compressed, and a slow burn-through dominated one as the cloud is ionised. The former is only observed if the momentum transfer to neutrals is kept. With transfer of momentum to the neutrals disabled, the plasma only interacts with the neutrals through ionisation and recombination. Thus, the front speed during the transient is determined by the rate of burn-through of the neutrals only. Simulations are compared to a recent semi-empirical reattachment model (Henderson et al 2024 Nucl. Fusion 64 066006), where the best qualitative agreement is seen for cases without neutral momentum and for power rises of (Formula presented) (Formula presented) their baseline power. The difference in these agreements are encompassed by the range of potential fits provided by model parameter choice.