Formation of transport barriers in the MAST spherical tokamak

In the Mega Ampère Spherical Tokamak (MAST) plasmas have been generated with internal (ITB) or edge (ETB) transport barriers. ITBs were achieved in both the electron and the ion energy channel. In the presence of an ITB in the ion energy channel, transport analysis shows that the ion thermal diffusivity, χ_i, is reduced to almost neoclassical values while the ITB persists. The widely tested criteria for ITB formation ρ*_T = ρ_s∂ ln T/∂R > ρ*_ITB > 0.014 (ρ_s: Larmor radius at sound speed) obtained from dimensional analysis of JET discharges is easily exceeded on MAST. Even without the evidence of an ITB ρ*_T > 0.014 often applies, showing that this criterion in its current form is not generally applicable. ETBs are most easily formed in MAST if in a double null divertor configuration the discharge is vertically balanced, so that both X-points are almost on the same flux surface (CDND), and if the plasma is refuelled from the high field side mid-plane. The H-mode threshold power, P_thr = 0.5 MW, in connected double null diverted (CDND) is only about half of that in a similar disconnected discharge with the ion VB drift towards the X-point on the last closed flux surface (LDND). Pthr scales between lower double null diverted (LDND) and the single null diverted configuration with the plasma surface area on MAST.