Design and Optimization of a Gamma Ray Imager for Runaway Electron Studies in the Compact Toroidal Hybrid Experiment*

The Compact Toroidal Hybrid (CTH) experiment at Auburn University is a current-carrying stellarator capable of generating up to 10 kA of runaway electron current. A previous study characterized the runaway beam via its relativistic bremsstrahlung emissions using a system of scintillators, observing gamma ray energies in the range of 500 KeV to 10 MeV. It was determined that increases in the magnetic field strength and rotational transform would  decrease the runaway energy by a factor of 4 and alter the spatial emission distribution.

To overcome limitations of the scintillator system, a Gamma Ray Imager (GRI) has been designed for use on CTH. The GRI is a lead pinhole camera consisting of scintillators in tightly collimated channels that provide 2D imaging of bremsstrahlung emissions on a poloidal plane of CTH. These images can reveal valuable insights into the runaway electron momentum distribution due to the strong anisotropy of the radiation.

Ray tracing routines have been used to optimize the GRI design by creating a synthetic model of the diagnostic and varying key design parameters. It has been determined that a desirable resolution can be achieved using a 20 cm focal length, with an 8 mm pinhole and 6 mm diameter collimator channel.

In addition, simulations of runaway electron transport within non-axisymmetric CTH magnetic fields have been performed using the Kinetic Orbital Runaway Electron Code (KORC), a fully kinetic particle tracer code. Combined with the synthetic GRI diagnostic, runaway populations and associated gamma ray images have been simulated for a range of CTH magnetic configurations.