Observation of island-like structures in synchrotron imaging emitted by runaway electrons on EAST

This paper reports on the experimental observation and numerical reconstruction of island-like structures in synchrotron imaging emitted by runaway electrons (REs) on Experimental Advanced Superconducting Tokamak (EAST). During low-density ohmic discharges with external resonant magnetic perturbations (RMPs), island-like structures were observed in the synchrotron radiation emitted by REs. Through simulations based on relativistic guiding-center motion equations and a cone radiation model, it was found that these island-like structures in synchrotron imaging are related to the island-like spatial distribution of REs. The island-like spatial distribution might be caused by the sticky regions in the stochastic field. Appropriate projection is necessary for island-like structures in synchrotron imaging. The study provides insights into the behavior of REs in perturbation magnetic fields and offers a sign for the existence of sticky regions within stochastic fields. Besides, discrepancies between simulation and experimental results suggest that non-linear plasma response should be considered. This research improves our understanding of RE behavior in perturbed fields and has implications for the development of effective RE mitigation strategies in large-scale tokamaks.