Investigation of fast particle driven instabilities by 2D electron cyclotron emission imaging on ASDEX Upgrade

Abstract

Detailed measurements of the 2D mode structure of Alfven instabilities in the current ramp-up phase of neutral beam heated discharges were performed on ASDEX Upgrade, using the electron cyclotron emission imaging (ECEI) diagnostic. This paper focuses on the observation of reversed shear Alfven-eigenmodes (RSAEs) and bursting modes that, with the use of the information from ECEI, have been identified as beta-induced Alfven eigenmodes (BAEs). Both RSAEs with first and second radial harmonic mode structures were observed. Calculations with the linear gyro-kinetic code LIGKA revealed that the ratio of the damping rates and the frequency difference between the first and second harmonic modes strongly depended on the shape of the q-profile. The bursting character of the BAE type modes, which were radially localized to rational q surfaces, was observed to sensitively depend on the plasma parameters, ranging from strongly bursting to almost steady state.