Observation of rapid frequency chirping driven by runaway electrons in DIII-D

We report the first observation of chirping instabilities driven by runaway electrons (REs) in a tokamak. Energetic particles often drive instabilities through wave-particle resonances. The frequency of an instability can sweep gradually as background plasma parameters evolve or it can change rapidly (known as "frequency chirping") due to nonlinear evolution of the energetic-particle distribution function. Chirping instabilities driven by REs in two distinct frequency bands are observed for the first time in the DIII-D tokamak. In addition to their intrinsic interest, these observations could be of practical importance, as kinetic instabilities that modify the RE distribution function may mitigate damage by runaways in a tokamak reactor.