First results of a newly built hard x-ray/soft gamma spectrometer imaging system: on the aspect of plasma disruptions

Abstract A new hard x-r ay and soft gamma-ray spectrometer imaging system (HXS) has been built for two-dimensional measurements of plasma emitted photons in the Experimental Advanced Superconducting Tokamak. The system uses a 2D cadmium zinc telluride detector and integrated electronics, and is as a whole shielded by a tungsten box with a pinhole and tangential to the toroidal field. Three classes of typical energy spectra have been summarized in different experimental scenarios during the past campaigns. After performing tomography calculations, the local emissivity contours have been obtained in different energy ranges, which clearly show asymmetry of the plasma crosssection. The spatial perturbation structure is similar to the magnetohydrodynamic modes with low mode numbers. In particular, the runaway island found by an infrared camera (Jaspers et al 1994 Phys. Rev. Lett. 72 4093) is also measured by the HXS. There exists a reversal population in the energy spectra of both slide-away and strong neutral beam injection shots. It is consistently observed that the count rate is increased in the low-energy range before the plasma disruptions. Calculations in phase space indicate that the accelerated momentum flux can be deflected back to the low-energy region by the large pitch-angle scattering. In the post-disruption phase, the plasma current is not replaced by runaway electrons due to tearing modes or transiently bursting instabilities. This paper constructs the basics for the proper use of HXS for hard x-ray and soft gamma-ray measurements in future investigations of plasma disruptions.