Current profile control studies on MAST

One of the main operational aims of the MAST experiment [1] and the proposed MAST upgrade is to investigate possible mechanisms to control the q-profile and drive off-axis current. Experiments were carried out to determine the extent to which the q-profile may be modified using two different approaches, transient and steadystate. Transient effects during the plasma current ramp-up phase were investigated with the aim of developing a start-up regime that can later be used as a target plasma for non-inductive current drive or to access advanced modes of operation such as the hybrid or improved H-mode. The most significant effect in this case was observed when early Neutral Beam Injection (NBI) was applied to the fast current ramp-rate start up plasmas causing reversed magnetic shear and the plasma current to ‘pile-up’ off-axis. In steady-state experiments, in which off-axis NBI was studied, results indicate that broadening the fast ion deposition profile by off axis neutral beam (NB) injection helps to avoid harmful plasma instabilities and significantly extends the operational window of MAST. Long pulse (>0.65s) H-mode plasmas were achieved with plasma duration limited only by present machine and NBI engineering limits. In order to match the experimentally observed neutron rate and stored energy a low level of anomalous fast ion diffusion (Db~0.5m s) is required. The introduction of the fast ion diffusion broadens the Neutral Beam Current Drive (NBCD) profile and degrades the relative contribution of NB driven current from ~40% to ~30%. To obtain direct measurements of the current profile, a multi-chord MSE diagnostic has been commissioned on MAST and is currently delivering first results in order to confirm the off axis location of the NB driven current.