(EX/9-1) Evolution of the pedestal on MAST and the implications for ELM power loadings

A. Kirk1), G.F. Counsell1), G. Cunningham1), J. Dowling1), M. Dunstan1), H. Meyer1), M. Price1), S. Saarelma1), R. Scannell2), M. Walsh1), H.R. Wilson3
 
1) UKAEA Fusion, Culham, United Kingdom of Great Britain and Northern Ireland
2) Department of Electrical & Electronic Engineering, University College Cork, Association EURATOM-DCU Ireland
3) University of York, Heslington, York YO10 5DD UK

Abstract.  Studies of pedestal characteristics and quantities determining ELM energy losses in MAST are presented. New results from high temperature pedestal plasmas will be presented which have collisionalities one order of magnitude lower than previous results. The pedestal widths obtained in these low collisionality plasmas are found to be in better agreement with banana orbit scalings than previous high collisionality plasmas, suggesting that banana orbits can only play a role in determining the minimum width when the collisionality is sufficiently low. A stability analysis performed on these plasmas shows them to be near the ballooning limit and to have broad mode structures suggesting larger ELM energy losses, which are observed. The larger losses observed at low collisionality are consistent with measurements from various experiments which indicate that there is an ordering of the ELM energy loss with collisionality with increasing ELM size at low collisionality. The energy transported from the pedestal by an ELM can be examined in terms of the convective particle loss of electrons and ions and the conductive losses of electron and ion energy. During an ELM on MAST the fractional change in density pedestal is effectively independent of the pre-crash pedestal conditions with 4% of the pre-ELM particles being expelled by the ELM, indicating that convective losses are not affected by the pedestal conditions. However, as in other devices, the ELM energy loss increases at low collisionality because the fraction of conducted energy increases. In this paper new results from MAST on the evolution of filamentary structures during ELMs will presented. Using the new insight gained from these measurements the following model for ELM energy losses is constructed: for the first 50-100 microseconds filaments remain near to the LCFS, rotate toroidally with the plasma and are aligned with the field lines. During this period 50-75% of the total ELM particle and energy loss occurs due to an increase in transport across the perturbed field lines associated with the filaments. After this time the filaments detach, accelerate radially away from the LCFS and their energy and particle content are subsequently lost by parallel transport along open field lines to the targets. It will be shown that this can explain both the convective and conductive losses observed.

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