Gamma Ray Imaging of Runaway Electrons on the Compact Toroidal Hybrid Experiment

Abstract

Runaway electrons (REs) represent a significant challenge for magnetic confinement fusion devices due to their ability to achieve relativistic energies and produce substantial damage to plasmas facing components. While runaway electron dynamics have been extensively studied in tokamaks, comparatively little experimental work has investigated their behavior in stellarator magnetic configurations. This thesis investigates the formation, confinement, and evolution of runaway electrons in the Compact Toroidal Hybrid (CTH) experiment using hard X-ray and gamma ray imaging diagnostics.

Hard X-ray measurements demonstrate that REs generated in steady-state CTH discharges are well confined by stellarator magnetic fields and are capable of producing >2 kA RE beams with energies in excess of 10 MeV. The effects of varying the edge rotational transform and central magnetic field strength have also been explored by deconvolution of the RE energy distribution from the HXR spectra. It was determined that increasing the edge rotational transform from 0.09 to 0.185 produced a corresponding increase in the mean energy of the runaway electron from 9.18 +/- 0.67 MeV to 15.60 +/- 0.91 MeV while operating at a field strength of 0.64 T, and there is evidence for enhanced momentum pitch angle scattering at low edge transform.

To further investigate these effects, a physics-informed gamma ray imager (GRI) diagnostic was developed, constructed, and commissioned on CTH. The diagnostic design was numerically optimized for CTH using statistical models of GRI signal benchmarked with CTH data. In parallel, a flexible synthetic diagnostic, GRILib, was developed to aid in the GRI design and to model gamma ray imaging measurements for arbitrary magnetic field configurations.

The combined application of the CTH GRI and GRILib enabled the reconstructed evolution of the runaway electron energy and pitch angle distribution, with an upper bound to the RE energy determined to be 26.5 +/- 5.0 MeV carried predominantly on REs with a pitch angle less than 22.18 degrees. The results presented in this thesis establish that runaway electron behavior in stellarator plasmas is strongly dependent on the magnetic geometry, and that gamma ray imaging is a suitable diagnostic tool for studying runaway electrons in CTH, with experimental access to the RE pitch angle distribution.