Pitch Angle Dynamics and Synchrotron Emission of Runaway Electrons in Quiescent and Disrupted DIII-D Plasmas

Full text: We present the validation of theoretical models for the pitch-angle probability distribution function (PDF) of runaway electrons (RE), through simulations of synchrotron radiation (SR) in DIII-D quiescent and disrupted plasmas for which the energy PDF is known from measurements but the pitch-angle PDF is poorly understood. SR of RE in magnetically confinement fusion plasmas is important because it provides a limiting mechanism of the maximum energy that RE can reach, and because it can be used as a diagnostic to infer parameters of the RE energy and pitch-angle PDFs. Recent studies using the SR synthetic diagnostic showed that SR depends on the RE energy, and more strongly on their pitch-angle PDF. Our simulations of RE in quiescent plasmas recover the typical visible SR in DIII-D when the spreading in the initial RE pitch-angle is less than the predicted by simplified theory that only consider the balance of electric field pinching in pitch angle and collisional pitch-angle scattering. We also present results of simulated infrared SR of RE in DIII-D disrupted plasmas after following their dynamics for tens of ms to find a better estimate for their pitch-angle PDF that takes into account the full-orbit dynamics of RE, SR energy losses, the acceleration of the electric field, the magnetic field geometry, and collisions with the background plasma and impurities through the use of experimental impurity density profiles. Research sponsored by the Office of Fusion Energy Sciences of the U.S. Department of Energy at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, and by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory. Research sponsored by the Office of Fusion Energy Sciences of the U.S. Department of Energy under contracts DE-FC02-04ER54698, DE-FG02-07ER54917. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. U.S. Department of Energy under Contract No. DE-AC02-05CH11231. (author)