Speaker
Description
The safe and controlled exhaust of heat and particles from magnetically confined fusion plasmas requires having dynamic information about the system. On present-day reactors, this information is obtained using system identification experiments, which involve perturbing the gas injection rate and observing the Scrape-Off-Layer (SOL) response in the frequency domain. However, this strategy is cumbersome for future reactors, as accidental disruptions or reattachment of the SOL plasma could lead to significant damage to the device. Therefore, numerical dynamic models of the exhaust plasma, validated against experiments, are needed. To this end, we present a multi-sine gas injection perturbation approach that uses time-dependent SOLPS-ITER simulations, with a grid extending to the vessel wall, to quantify the plasma response. Comparison to an existing set of experimental data on MAST-U is done to validate this approach. The simulation grid was generated using the Grid Optimization and Adaptation Toolbox (GOAT), and both the Advanced Fluid Neutral (AFN) model and the kinetic neutral model Eirene available in SOLPS-ITER are evaluated. In steady state, it is possible to simulate divertor states ranging from attached to deeply detached using both fluid and kinetic neutrals. The comparison in the frequency domain shows that the AFN SOLPS-ITER simulations predict faster response times to divertor fuelling perturbations than the experiments at low frequencies, likely in part due to the fluid approximation used for the neutrals. Simulations employing time-dependent Eirene for kinetic treatment of neutrals show good agreement with the experimental data throughout the measured frequency range of around 9 to 30 Hz. Simulations with Eirene using the Quasi-Steady-State (QSS) approximation result in different dynamic behaviour due to the non-linear coupling between the plasma solver B2.5 and Eirene. Proper time-dependent kinetic treatments of the neutrals is therefore essential for accurately simulating plasma evolution in response to changes in gas injection.