Effect of electron cyclotron waves on plasma with runaway electrons

Abstract Runaway electrons (REs) generated during tokamak disruptions are a major concern for the safe operation of future fusion devices. The interaction of REs with waves has been proposed as a potential mechanism for their mitigation. This study investigates the effect of electron–cyclotron heating (ECH) on post-disruption plasmas containing REs. O- and X-mode EC waves are routinely used for plasma heating and current drive. However, these modes do not interact directly with relativistic electrons and cannot be injected into plasmas with densities exceeding their respective cutoff densities. In contrast, the internal slow X-mode (sX) can resonate with relativistic electrons and may therefore provide a pathway for their mitigation. We report DIII-D experiments designed to access the internal slow X-mode through O-sX conversion during post-disruption RE plateaus. ECH increased the background-plasma density, doubled the loop voltage, and strongly enhanced the RE synchrotron signal, demonstrating substantial ECH-plasma coupling and increased effective dissipation of the RE channel. Although ECH produced a pronounced response consistent with enhanced RE dissipation, we could not isolate conclusive evidence of resonant sX-RE interaction. Strong heating, ionization, and impurity-redistribution effects appear to dominate the response and may mask a direct wave-particle signature. Nevertheless, the results demonstrate the potential of ECH for RE control through background-plasma modification, with O-sX access offering an additional possibility of direct resonant interaction.