Speaker
Description
Wave–particle interactions have emerged as a promising pathway for directly affecting runaway electron (RE) dynamics and enabling new mitigation strategies in tokamaks. Recent experiments on the DIII-D tokamak explored both externally launched and self-generated waves that resonantly interact with the RE population.
Experiments utilizing externally launched electron cyclotron (EC) waves investigated a novel mitigation scenario based on OXB mode conversion. When O-mode waves were launched at the optimal angle relative to the magnetic field, a strong density enhancement formed near the predicted conversion window, indicating successful generation of the slow-X mode. Application of EC power during the RE plateau modified the loop voltage and synchrotron emission, consistent with possible changes to RE dynamics.
More recent experiments focused on externally launched helicon waves, which were observed to drive significant pitch-angle scattering of runaway electrons in the low-density Quiescent Runaway Electron Regime (QRE). These interactions produced a momentum-space vortex structure that imposed a new upper limit on RE energies, in agreement with kinetic modeling and theoretical predictions. The results provide direct experimental evidence that helicon waves can strongly modify relativistic electron phase-space evolution.
In addition to externally launched waves, self-excited runaway-driven whistler waves were observed to resonantly interact back on the RE population. Fast visible camera measurements revealed a predator–prey-like relationship between the wave activity and RE synchrotron emission, where wave growth leads to enhanced pitch-angle scattering and synchrotron damping of the RE beam, followed by re-acceleration and renewed wave growth. These observations provide insight into nonlinear wave–particle feedback mechanisms that may play an important role in regulating RE dynamics.
Together, these experiments advance the understanding of fundamental wave–particle interactions in RE plasmas and demonstrate their potential relevance for disruption mitigation in future devices.
Work supported by US DOE under DE-FC02-04ER54698, DE-SC0021622, DE-FG02-07ER54917, DE-AC52-07NA27344, and DE-SC0022270