Fast-detection of hard x-ray emission and correlation with magnetohydrodynamic instabilities on the High-Beta Tokamak-Extended Pulse (HBT-EP).

Understanding the spatial and energy dynamics of runaway electrons is essential to preventing damage to next-generation fusion experiments and pilot plants. In the High-Beta Tokamak-Extended Pulse (HBT-EP) device, hard x-rays (HXRs) are produced through thick-target bremsstrahlung radiation when energetic electrons are deconfined and strike limiting surfaces. This paper describes a new HXR detector system composed of ten scintillator detectors and a visible-light fast camera, which is used to detect fast bursts of HXR emission that correlate with magnetohydrodynamic (MHD) perturbations. Hard x-rays cause spatially uncorrelated speckling on the fast camera, and the appearance of speckles is strongly correlated in time with x-ray bursts. We also observe a threshold below which an HXR signal is insufficient to produce a given speckle signal. Using these detectors, HBT-EP can measure the toroidal and temporal characteristics of HXRs, which are used to infer the characteristics of deconfined electrons. We report a strong correlation between the phase of a rotating n = 1 HXR source and an n = 1 MHD mode, suggesting that MHD activity directly enhances the radial transport of energetic electrons.