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(EX/D1-2) Exhaust, ELM and Halo Physics using the MAST Tokamak

G.F. Counsell1), J-W. Ahn1), R.H. Cohen2), A. Kirk1), P. Helander1), R. Martin1), D.D. Ryutov2), A. Tabasso1), H.R. Wilson1), Y. Yang3)
1) Euratom/UKAEA Fusion Association, Abingdon, United Kingdom
2) Lawrence Livermore National Laboratory, Livermore, CA, USA
3) Institute of Plasma Physics, Hefei, P.R.China

Abstract.  Scalings for the SOL width on MAST extend the parameter range of conventional devices but confirm a negative dependence on power flow across the separatrix. In L-mode and at ELM peaks, >95% of power to the targets arrives to the outboard side. Peak heat flux densities rise by a factor 2$ \sim$6 during ELMs and are accompanied by a shift in the strike-point location but by little change in the target heat flux width. Energy loss per ELM as a percentage of pedestal energy and pedestal collisionality appear uncorrelated, possibly because ELMs on MAST are dominated by convective transport. Modelling shows that parallel gradients in the magnitude of the magnetic field in MAST may drive strong upstream flows. Broadening of the target heat flux width by divertor biasing is being explored as a means of reducing target power loading in next-step devices and has facilitated halo current measurements using series resistors. Halo currents are always less than 30% of plasma current and the product of toroidal peaking factor and halo current fraction is $ \sim$50% of the ITER design limit. Varying the series resistance demonstrates that the VDE behaves more as a voltage source than a current source.

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IAEA 2003