Characterisation of Detached Plasmas on the MAST Tokamak

The mitigation of the steady-state heat loading to plasma-facing components in the divertor structures is an essential prerequisite for the operation of next-generation conventional and tight aspect-ratio Tokamaks. Currently, the most attractive means of reducing this heat loading is the so-called “detached” regime, which is the primary focus of this study. This thesis describes work that was carried out on the MAST tokamak to accurately characterise the plasma conditions within divertor plasmas, and to test the validity of those measurements. To assist in this study, an interpretive code, OSM, has been upgraded with additional numerical schemes to ascertain the dominant mechanisms governing parallel scrape-off layer transport including flux expansion, dynamic viscosity and cross-field drift motion in attached conditions. It has been found that the invocation of flux expansion, parallel viscosity and diamagnetic drift motion assist the OSM numerical scheme to converge in the presence of (static+dynamic) parallel pressure gradients often observed between the outboard midplane and lower outer divertor leg. It has also been found that the ion pressure, which is currently unknown, could resolve the observed pressure discrepancy. Line-of-sight spectroscopy of high-n Balmer emission lines has been used to ascertain line averaged Te and line integrated ne during the detached phase of MAST discharges with high spatial resolution (≈ 7mm) to determine electron static pressure profiles along flux contours. Furthermore, narrow-bandwidth imaging spectroscopy of Dα, Dγ, CII and CIII emission during the onset and sustainment of detachment has been carried out with high temporal (5kHz) and spatial (≈ 3mm) resolution. These data have been combined with ion flux measurements to the divertor target plates using embedded Langmuir probes (6-9mm radial resolution) and upstream Thomson scattering measurements of ne, Te. The data was input into the OSM code, coupled with the kinetic neutral transport code EIRENE, to check the experimental data set for internal consistency. The experimental data collected has been used to reconstruct the plasma conditions within the detached divertor leg, heavily constrained by experimental data. EIRENE calculations with re-constructed plasma conditions are able to reproduce experimentally observed Dγ/Dα line emission ratios to within a factor of 2 within the recombining region