Experimental observation of reverse-sheared Alfven eigenmodes (RSAEs) in ELMy H-mode plasma on the EAST tokamak

Reverse-sheared Alfven eigenmodes (RSAEs) have been observed by using an interferometer and ECE diagnostics in NBI heated ELMy H-mode plasma on EAST tokamak. A typical feature of these modes is a fast frequency sweeping upward from similar to 80 kHz to similar to 110 kHz in hundred milliseconds during which the plasma temperature, density and rotation keeps no change. Only core channels of the interferometer can observe these modes, implying a core localized mode. The ECE measurement further showed that these modes located at about rho = 0.37-0.46, just around the position of q(min) with rho similar to 0.4. These core localized modes are very weak in the magnetic fluctuations measured by mirnov probes mounted at the machine vacuum vessel. A multiple frequency fluctuation component, seemingly the so-called 'grand cascades', was also clearly observed on the ECE signal at rho = 0.46. During the phase, a transient internal transport barrier (ITB) in ion temperature and toroidal rotation was observed and the ITB foot was just close to the position of q(min). A modulation of RSAE frequency by ELM event was observed and this modulation could be attributed to rotation decrease or q(min) increase due to ELM. Further study of these modes in EAST can provide valuable constraints for the q profile measurement and will be important for the long pulse operation.