Parallel flow in the scrape-off layer is a major area of interest in tokamak research, impacting on impurity
transport, tritium retention and H-mode access. The work presented here is the first major investigation
of SOL flow in the Mega Ampere Spherical Tokamak (MAST), using a Gundestrup probe specifically
designed for the task.
The results of a parameter scan in poloidal field, Bѳ, and temperature, T, of parallel velocity at the
outboard mid-plane are presented, and the results and scalings compared to B2SOLPS5.0 simulations of
MAST and a simple analytical model, in order to identify the relative importance of drift mechanisms
(such as Pfirsch-Schluter and E × B) for driving parallel flow. The results show the predicted linear
scaling with temperature and poloidal field strength, but also suggest a density dependence.
Another major are of interest is the discovery in recent years of coherent filamentary structures that are
radially convected through the L-mode SOL. These filaments are believed to contain sharp gradients in
temperature, density and plasma potential, complicating probe analysis. An investigation to characterise
the intermittency of the MAST SOL, it’s dependencies on poloidal field strength, density or temperature,
and the impact of the filaments on probe measurements was also carried out, and a probe was built to
further investigate the structure and dynamics of the filaments. Based on these experiments a method
for resolving the flow in the filaments and background plasma was developed and applied in the flow
experiments described above. It is found that the parallel Mach numbers are lower in the filaments than
the ambient plasma in the far SOL — suggesting either ion temperatures are at least on the order of 4
times the electron temperature — or parallel flow velocity is substantially lower in the filaments than in
the background plasma