Electron Cyclotron Mode Conversion in Plasma with Relativistic Electrons*

Recent experimental results on DIII-D indicate a direct interaction between mode-converted EC waves and relativistic electrons which result in a measurable change in the intensity of emitted synchrotron radiation. In future devices such as ITER and SPARC these runaway electrons (REs) are expected to carry MA's of current and MeV's of energy. While the injection of high-Z impurities collisionally lowers the energy of theses REs, recent experiments have shown that this technique does not scale to machines with higher stored energies. Therefore, work is underway to explore collisionless techniques which are capable of mitigating post-disruption RE beams. Typical EC heating O and X-modes do not interact directly with relativistic electrons due to their low phase velocity nor can they be injected into plasma above the cutoff density. However, a unique opportunity exists to convert the "free space" O-mode into an internal plasma slow-X mode (a.k.a Z-mode). Recent experiments on DIII-D demonstrated access to this mode conversion in plasmas with 100s of kA's of RE current. These mode-converted waves were observed to immediately increase synchrotron radiation from the RE beam hinting at a direct interaction between the mode-converted waves and the relativistic electron population.