Observation of MHD-correlated blobs during internal reconnection events in VEST

Abstract

Internal reconnection events (IREs), one of the relaxation events driven by internal magnetohydrodynamic (MHD) instabilities in fusion plasmas, are accompanied by a strongly MHD-correlated blob at the edge in the Versatile Experiment Spherical Torus spherical tokamak. The MHD-correlated blob plays a significant role in the onset and the strength of IREs. Various techniques analyzing visible camera images show correlated waveforms between blobs and magnetic fluctuations, and they produce visualized images of corotating structures of the MHD modes and the MHD-correlated blobs. In the images, a phase drag in the rotations of the two structures initially appears and vanishes on the verge of IREs. IREs maintaining the phase drag, however, leads to a less violent impact in terms of current decrease and magnetic field bursting. In addition, the MHD-correlated blobs are followed by the increasing degree of nonlinear interactions between the internal MHD mode and high-frequency broadband fluctuations (>60 kHz) at the edge. These results suggest that boundary plasmas can impact internally driven relaxation events via MHD-correlated edge phenomena.