Development of a Synthetic Gamma Ray Imager Diagnostic for Runaway Electrons in a Stellarator

The Compact Toroidal Hybrid (CTH) experiment at Auburn University is an ohmically driven stellarator capable of producing up to 10 kA of runaway electron current and multi-MeV x-ray fluxes. A Gamma Ray Imager (GRI) is being developed for use on CTH to enable novel experimental studies of runaway generation in a 3D stellarator magnetic field. The GRI is a lead pinhole camera with scintillators in tightly collimated channels which enables 2D imaging of bremmsstrahlung emissions from the runaway beam. Successful inversion of GRI images can reveal details of the runaway electron momentum distribution, but requires understanding of how the channels sample the magnetic field structure. A new synthetic diagnostic has been developed which incorporates generic 3D magnetic field configurations, and verified using data from the DIII-D GRI. It has been used to study the effects of non-axisymmetry and rotational transform profile on the GRI images. In addition, synthetic GRI images of reconstructed CTH equilibrium have been generated to evaluate potential GRI sightlines.