Low frequency Alfvénic activity in ST40: assessment of stability and likelihood for nonlinear frequency chirping

We present analyses of low-frequency Alfvénic instabilities in the ST40 spherical torus high field plasmas, with simultaneous neutral beam heating due to two sources at 25keV and 55keV. Given the limitation on internal measurements, the plasma profiles for the TRANSP and NUBEAM simulations have been estimated using an integrated analysis of the available line-of-sight integrated measurements and volumetric integrals. NOVA/NOVA-C [Gorelenkov et al, Phys. Plasmas 6, 2802 (1999)]) analysis indicates that the most unstable modes are found to be core-localized n=1 BAAE eigenmodes with mixed acoustic and electromagnetic Alfvénic polarization (Gorelenkov et al, Phys. Lett. A 370, 70 (2007)). The global transport analysis combined with the eigenmodes' stability indicates that BAAEs are strongly unstable and that there is a greater tendency for bursty chirping response as the level of background micro-turbulence decreases in time. Those predictions are shown to be consistent with the experimental data. This finding is interpreted in terms of the suppression of coherent phase-space structures that support chirping when the resonant fast ion dynamics is dominated by frequent stochastic, orbit-decorrelating events.