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(EX3/2) High Performance and Stability in COMPASS-D

B. Lloyd¹⁾, L. C. Appel¹⁾, K. B. Axon¹⁾, C. A. Bunting¹⁾, R. J. Buttery¹⁾, P. G. Carolan¹⁾, N. J. Conway¹⁾, G. Cunningham¹⁾, J. Dowling¹⁾, M. R. Dunstan¹⁾, T. Edlington¹⁾, A. R. Field¹⁾, S. J. Fielding¹⁾, S. J. Manhood¹⁾, K. G. McClements¹⁾, H. Meyer¹⁾, A. W. Morris¹⁾, M. R. O'Brien¹⁾, T. Pinfold¹⁾, V. Shevchenko¹⁾, K. Stammers¹⁾, M. Tournianski¹⁾, M. Valovic¹⁾, M. Walsh²⁾, C. D. Warrick¹⁾, H. R. Wilson^1)
 
1) EURATOM/UKAEA Fusion Association, Culham Science Centre,
Abingdon, Oxon, OX14 3DB, United Kingdom
²⁾ Walsh Scientific Ltd, Culham Science Centre, Abingdon,
Oxon, OX14 3EB, United Kingdom

Abstract.  COMPASS-D is a compact, adaptable, D-shaped tokamak equipped with powerful heating (ECRH) and current drive (LHCD/ECCD) systems allowing access to both H-mode (ELMy and ELM-free) and quasi-stationary high beta regimes under conditions of dominant electron heating ( T_e > T_i), negligible external momentum input and no central fuelling. Control and avoidance of neo-classical tearing modes (NTMs) has enabled quasi-stationary high beta ( $\beta_{\mathrm{N}}^{}$ $\sim$ 2, $\beta_{\mathrm{p}}^{}$ > 1) discharges to be sustained for sim 20 energy confinement times and a duration corresponding to 20% of that of a nominal ITER discharge, when normalised to the current diffusion time. Controlled seeding of NTMs by external application of resonant magnetic perturbations has enabled NTM onset criteria to be carefully explored and compared with theory; observed island evolutions follow theoretical expectations. Off-axis lower hybrid current drive (LHCD) has been reliably used to completely stabilise NTMs in high beta discharges. Detailed modelling has shown that the stabilising effect is consistent with a reduction in the stability index $\Delta{^\prime}$, although other stabilisation mechanisms may also contribute. High frequency energetic particle driven instabilities ($\sim$400kHz), which exhibit frequency-sweeping (`chirping'), have, for the first time, been observed with high power ECRH as the sole source of auxiliary heating.

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