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 lambda (Te) is typically 5-8 times broader than the density scale length lambda (ne), 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 (nu* similar to 50) whereas the outer SOL is marginally collisional (nu (+) similar to 7). The peak power density is about 4-5 times higher (similar to1 MW m(-2)) on the inboard side, which has a SOL power density scale length similar to 10 times narrower (similar to0.5 cm) than the outboard. The total power flow was approximately up-down symmetric, whilst the in-out power asylnmetry was observed to be P-in : P-out similar to 1 : 6.5, exceeding the ratio of similar to1:3 for the inboard and outboard separatriu surface areas. A total of about 2/3 of the power entering the SOL reached the strike points (300 kW out of P-SOL similar to 450 kW). (C) 2001 Elsevier Science B.V. All rights reserved.