Overview
The application of resonant magnetic perturbations (RMPs) to plasma discharges in the Mega Ampere Spherical Tokamak (MAST) facility has been observed to induce a plasma pump-out effect. This phenomenon occurred in both L-mode and H-mode plasma configurations. This research investigates the role of neutral fueling in the observed density reduction resulting from RMP application. The study involved a 0-D particle balance analysis to determine neutral fueling rates and the average particle confinement time ($\tau_p$) for the main ion species. A time-dependent global multi-reservoir particle balance (MRPB) model, incorporating atomic and molecular reservoirs, was developed to further explore the influence of neutrals on density changes. Additionally, EMC3-EIRENE modeling was employed to provide insight into potential mechanisms for these changes.
Research Context
Resonant magnetic perturbations (RMPs) are applied to plasma to influence its behavior, and in MAST, their application has been linked to a reduction in plasma density, termed 'pump-out.' Understanding the mechanisms behind this density reduction, particularly the role of neutral particles, is crucial. This investigation focuses on quantifying the changes in neutral fueling and particle confinement under RMP conditions and developing models that can accurately represent the observed plasma response.
Approach
The study utilized a multi-faceted approach to investigate neutral fueling response to RMPs:
- 0-D Particle Balance Analysis: This initial analysis was used to calculate the neutral fueling and the average particle confinement time ($\tau_p$) of the main ion species in MAST discharges. This provided quantitative measures of key plasma parameters both with and without RMPs.
- Time-Dependent Global Multi-Reservoir Particle Balance (MRPB) Model: A detailed model was developed to simulate the particle balance within the plasma. This MRPB model specifically included atomic and molecular reservoirs, allowing for a more comprehensive representation of neutral particle dynamics. The model's capacity to reproduce experimentally measured density reduction and ionization increase was tested, considering either a reduction in $\tau_p$ or a reduction in particle fueling efficiency as potential drivers.
- EMC3-EIRENE Modeling: This advanced modeling technique was applied to investigate the spatial distribution and behavior of neutral particles in the presence of RMPs. The aim was to determine if changes in neutral particle fueling locations, particularly due to altered magnetic field topology, could explain the observed density reduction.
Findings
The research yielded several key findings regarding the impact of RMPs on neutral fueling and plasma density in MAST:
- Increased Ionizations: When RMPs were applied, an increase in ionizations was observed.
- Reduction in Particle Confinement Time ($\tau_p$): The 0-D particle balance analysis indicated a 15% reduction in $\tau_p$ for L-mode discharges following RMP application. A similar reduction in $\tau_p$ was found during the inter-ELM (edge localized modes) periods of H-mode discharges.
- MRPB Model Reproduction of Density Reduction: The time-dependent global multi-reservoir particle balance (MRPB) model was able to accurately reproduce the experimentally measured density reduction. The model also successfully replicated the observed increase in ionizations. These reproductions could be attributed to either a reduction in $\tau_p$ or a reduction in particle fueling efficiency within the model.
- EMC3-EIRENE Indication of Fueling Location Change: Results from EMC3-EIRENE modeling suggested that the observed changes could be attributed to neutral particle fueling occurring in locations characterized by now-opened field lines. These open field lines are a consequence of the chaotic edge-region created by RMP applications.