Commissioning of a Gamma Ray Imager for the Compact Toroidal Hybrid Experiment

The Compact Toroidal Hybrid (CTH) experiment at Auburn University is an ohmically driven stellarator capable of producing up to 10 kA of runaway electron (RE) current and multi-MeV x-ray fluxes. Previous work had determined that runaway electron energies are strongly dependent on the |B| and rotational transform profile, but was limited by poor localization of the viewing volume. A Gamma Ray Imager (GRI) has been constructed for use on CTH to enable enhanced studies of how runaway electrons are affected by 3D field shaping. The GRI is a lead pinhole camera with scintillators in tightly collimated channels that enable 2D imaging of bremsstrahlung emissions from the runaway beam. A commissioning process has been undertaken to quantify the imager signal-to-noise ratio and background spectra. Validation of the GRI has been performed by comparing the GRI signal from multiple field configurations with previous measurements on a single channel instrument. Signals from multiple GRI sightlines have also been measured to determine the sensitivity to viewing volume. In addition, a novel synthetic diagnostic has been developed to model the GRI response to changes in experimental parameters, including prescribed runaway electron distribution functions.