Experimental investigations of the SOL plasma in the MAST tokamak

Initial target plasma parameters have been obtained using Langmuir probe arrays in MAST DND plasmas. A clear in–out asymmetry was observed in the target parameters. All plasma parameter scale lengths were significantly elongated on the outboard side due to strong poloidal flux expansion. The electron temperature scale length λTe is typically 5–8 times broader than the density scale length λne, suggesting a significant difference between heat and particle diffusivities in the SOL. A rough estimation of the SOL collisionality indicates that the inner SOL may be collisional (ν*∼50) whereas the outer SOL is marginally collisional (ν*∼7). The peak power density is about 4–5 times higher (∼1 MW m−2) on the inboard side, which has a SOL power density scale length ∼10 times narrower (∼0.5 cm) than the outboard. The total power flow was approximately up–down symmetric, whilst the in–out power asymmetry was observed to be Pin:Pout∼1:6.5, exceeding the ratio of ∼1:3 for the inboard and outboard separatrix surface areas. A total of about 2/3 of the power entering the SOL reached the strike points (300 kW out of PSOL∼450 kW).