Spatial Structure of Fast-ion Driven instabilities inferred from Soft X-ray emission in the MegaAmp Spherical Tokamak *

Mode structure is obtained for bursting fast-ion driven MHD instabilities in the Mega Amp Spherical Tokamak (MAST) from inversion of soft X-ray (SXR) emission measurements. SXR emissivity fluctuations associated with the modes are isolated using cross-correlation with Mirnov coil measurements. Comparison of measured frequencies, toroidal mode numbers and structure with theory show that the bursts include fishbones and another type of mode whose time-dependent structure are consistent with expectations for energetic particle modes (EPM). The frequency of the bursting modes is observed to decrease with time, from just below the toroidal Alfvén eigenmode (TAE) frequency to just above the plasma rotation frequency. The structure measurements show that the radial location of peak fluctuation amplitude of individual bursts get closer to the plasma core when the mode frequency approaches the plasma rotation frequency. Although the frequency of some of the observed modes are closer to the TAE frequency than the rotation frequency, the location of these modes disagrees with the results of TAE simulations carried out with the MISHKA ideal MHD linear eigenmode code. This highlights the need for  models with physics beyond ideal MHD to accurately describe and predict these modes. The mode structure measurements will be compared with linear simulations using the Gyrokinetic Toroidal Code (GTC) [1]. In future work, the measured and simulated fluctuation peak locations could be used as initial input to energetic particle transport models, such as the kick model.

[1] Z. Lin et al, Science 281, 1835(1998)