The influence of gas fuelling location on H-mode access in the MAST spherical tokamak

The observation that high-field side (HFS) gas puff refuelling facilitates access to the improved confinement (H-mode) regime on the COMPASS-D and MAST tokamaks prompted a theoretical investigation of the role of the neutral gas dynamics in controlling the edge plasma rotation and radial E-field, E-r. Within the framework of neo-classical theory, higher edge plasma flow, and hence E-r, are predicted when fuelling from the HFS-rather than from the more usual low-field side (LFS)-provided neutral viscosity dominates the transport of toroidal angular momentum. Here, these predictions are compared with experiments on MAST, where the influence of the gas-puff location on the edge E-r profile is measured spectroscopically. An increase in E-r is indeed observed with HFS refuelling in the region where the edge transport barrier forms, provided the neutral density at the LFS is sufficiently low so as not to damp the toroidal flow.