Measurement of fast electron distribution using a flexible, high time resolution hard x-ray spectrometer

A 16 spatial channel hard x-ray (HXR) diagnostic using solid state CdZnTe detectors (active area 10 mm×10 mm×2 mm, 50 mm×20 mm×20 mm packaged) has recently been installed on the Madison Symmetric Torus (MST) reversed field pinch to measure the XR flux from ∼10 to 300 keV. Rather than using conventional pulse height analysis, the shaped output pulses from the detector are digitized using 12-bit, 10 MHz ADCs. The resulting waveforms are then fitting with multiple Gaussians; this allows a fraction of normally discarded pile-up events to be recovered. The technique is cost effective and allows for (a) excellent energy resolution—limited by the detector resolution rather than electronics; (b) dynamic time binning—rather than counting over predefined time bins, x-ray events are recorded as discretely timed events; (c) better noise rejection/pile up detection—achieved by fitting using the full information of the time history and known pulse shape from the amplifier; and (d) simple hardware implementation. The measured HXR energy flux is coupled to the Fokker–Planck code CQL3D to derive the electron distribution function for the fast electrons and infer the diffusion coefficient. The diagnostic has been used to measure diffusion coefficient on MST during improved confinement plasmas and measurements of radial diffusion of electron Bernstein wave (EBW) heated fast electrons are planned.