Experimental investigations of the SOL plasma in the MAST tokamak

Initial target plasma parameters have been obtained using Langmuir probe ar rays in MAST DND plasmas. A clear in-out asymmetry was observed in the targ et parameters. All plasma parameter scale lengths were significantly elonga ted on the outboard side due to strong poloidal flux expansion. The electro n temperature scale length lambda (Te) is typically 5-8 times broader than the density scale length lambda (ne), suggesting a significant difference b etween heat and particle diffusivities in the SOL. A rough estimation of th e SOL collisionality indicates that the inner SOL may be collisional (nu* s imilar to 50) whereas the outer SOL is marginally collisional (nu (+) simil ar 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 simi lar to 10 times narrower (similar to0.5 cm) than the outboard. The total po wer flow was approximately up-down symmetric, whilst the in-out power asyln metry was observed to be P-in : P-out similar to 1 : 6.5, exceeding the rat io 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 point s (300 kW out of P-SOL similar to 450 kW). (C) 2001 Elsevier Science B.V. A ll rights reserved.