Interpretation of runaway electron synchrotron and bremsstrahlung images

TL;DR

This paper explains the crescent shape in runaway electron synchrotron images in DIII-D, highlighting the effects of anisotropic radiation, spectral range, and runaway distribution, and introduces synthetic diagnostics to interpret measurements.

Contribution

It combines kinetic modeling with synthetic diagnostics to better understand runaway electron images and proposes that additional physical processes influence runaway energies.

Findings

High-field side synchrotron radiation can be a million times stronger than the low-field side.

Runaway electrons likely have lower energies and larger pitch-angles than kinetic models predict.

Synthetic bremsstrahlung diagnostics can simulate the spatial distribution observed by DIII-D's Gamma Ray Imager.

Abstract

The crescent spot shape observed in DIII-D runaway electron synchrotron radiation images is shown to result from the high degree of anisotropy in the emitted radiation, the finite spectral range of the camera and the distribution of runaways. The finite spectral camera range is found to be particularly important, as the radiation from the high-field side can be stronger by a factor $10^6$ than the radiation from the low-field side in DIII-D. By combining a kinetic model of the runaway dynamics with a synthetic synchrotron diagnostic we see that physical processes not described by the kinetic model (such as radial transport) are likely to be limiting the energy of the runaways. We show that a population of runaways with lower dominant energies and larger pitch-angles than those predicted by the kinetic model provide a better match to the synchrotron measurements. Using a new synthetic bremsstrahlung diagnostic we also simulate the view of the Gamma Ray Imager (GRI) diagnostic used at DIII-D to resolve the spatial distribution of runaway-generated bremsstrahlung.