Anomalous fast ion losses at high β on the tokamak fusion test reactor

This paper describes experiments carried out on the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)] to investigate the dependence of beta-limiting disruption characteristics on toroidal field strength. The hard disruptions found at the beta-limit in high field plasmas were not found at low field, even for beta's 50% higher than the empirical beta-limit of beta(n) approximate to 2 at high field. Comparisons of experimentally measured beta's to TRANSP simulations suggest anomalous loss of up to half of the beam fast ions in the highest beta, low field shots. The anomalous transport responsible for the fast ion losses may at the same time broaden the pressure profile. Toroidal Alfven eigenmodes, fishbone instabilities, and Geodesic Acoustic Modes are investigated as possible causes of the enhanced losses. Here, we present the first observations of high frequency fishbones [F. Zonca et al., Nucl. Fusion 49, 085009 (2009)] on TFTR. The interpretation of Axi-symmetric Beam-driven Modes as Geodesic Acoustic Modes and their possible correlation with transport barrier formation are also presented. (C) 2015 AIP Publishing LLC.