(EX/P3-13) The Influence of Beam Injection Geometry upon Transport and Current Drive in the MAST Spherical Tokamak

R.J. Akers1), L.C. Appel1), P.G. Carolan1), I. Chapman1), N.J. Conway1), G.F. Counsell1), G. Cunningham1), E. Delchambre1), M.R. Dunstan1), A.R. Field1), S. Gee1), P. Helander1), D. Keeling1), A. Kirk1), S. Lisgo1), B. Lloyd1), D.G. Muir1), A. Patel1), R. Scannell1), D. Taylor1), A. Thyagaraja1), M.R. Tournianski1), M. Valovic1), M.J. Walsh1), MAST Team, NBI Team
1) EURATOM/UKAEA Fusion Association, Abingdon, United Kingdom of Great Britain and Northern Ireland

Abstract.  Future Spherical Tokamaks (STs) require a large pressure driven component of plasma current, combined with off-axis auxiliary current for sustaining central safety factors q0 > 1 (thereby avoiding sawteeth). To achieve long pulse (∼5s), discharges with q0 > 1 on MAST, TRANSP simulations indicate that neutral beam current drive (NBCD) from 10 MW of NBI, comprising a combination of off-axis and co- and counter-directed mid-plane beams is required. A number of experiments are being performed in order to confirm modelling predictions. Changes to the beam geometry can be effected by either a) reversing Ip (resulting in counter-NBI) and/or b) moving the plasma vertically either to a Lower (L) or Upper (U) single null diverted (SND) configuration (resulting in off-axis injection). Using mid-plane co-NBI, ELMing H-mode discharges are routinely studied, as are Internal Transport Barriers (ITBs), with ion energy transport reduced almost to the underlying neoclassical level. For sawtooth free operation, however, 2.5 MW of counter-NBI is needed to balance the mid-plane tangential co-NBI. 40-60 keV counter injected deuterium orbits are, unfortunately, only weakly confined by the MAST field (this having been confirmed using I.R. imaging). Nevertheless, counter-NBI results in temperatures rivalling those of co-NBI, highlighting the profound influence the E×B shearing rate has upon plasma performance. Wide ITBs can be routinely produced, with both ion and electron thermal diffusivity reduced to the ion-neoclassical level. Due to the large Bpol/Btor ratio in the ST, NBCD efficiency for horizontally oriented off-axis NBI is predicted to be significantly different if the beam is injected above, rather than below the magnetic axis. Experiments confirm predictions, with sawtooth onset time later and li lower for LSND. In order to carry out current diffusion analysis, an MSE system is being commissioned and new 2D imaging diagnostics are being deployed in order to map high resolution data measured at the ``tokamak" mid-plane (TS, high resolution CXRS, Zeff etc.) to the ``plasma" mid-plane for the SND regime. In this paper we discuss all three injection scenarios and the resulting impact upon energy, particle and momentum transport, together with the interpreted NBCD and changes to the q-profile evolution, comparing and contrasting with data from ASDEX-U and JET where possible.

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