Validation of runaway electron models using synchrotron radiation measurements and full-orbit simulations

We present recent progress on the validation of pitch angle distribution (PAD) models of runaway electrons (RE). The study is based on the comparison of synchrotron radiation (SR) measurements in DIII-D and synthetic signals computed using the Kinetic Orbit Runaway electrons Code that incorporates full-orbit dynamics as well as the geometry and spectral resolution of the camera. We focus on DIII-D quiescent plasma #165826 for which spatial, temporal, and energetically resolved RE distribution functions are available. The PAD models of interest are based on Fokker-Planck (FP) descriptions that neglect orbit effects and balance collisional scattering with pinching due to electric field. We use Proper Orthogonal Decomposition data-mining methods to guide the comparison of experimental and computational diagnostic signals. It is concluded that spatial effects play an important role and that phase-space models overestimate the decay rate of the PDA which affects its width. We also show that the FP PAD model ceases to be an equilibrium distribution when spatial degrees of freedom are taken into account. At high energies the computed PAD significantly deviates from the model PAD, a result consistent with observed discrepancies between Fokker-Planck models and DIII-D measurements.