Assessment of a Space and Energy Resolved Diagnostic Based on GEM Technology on MAST-U

A gas electron multiplier (GEM)-based detector was utilized for the first time on a spherical tokamak, MAST-U, during the 2023 campaign to investigate soft x-ray (SXR) radiation (1-20 keV) emitted from the plasma. GEM detectors, chosen for their resilience to harsh fusion environments and their ability to provide energy-resolved (E-res similar to 25% at 8 keV) SXR emission images (with a spatial resolution of few centimeters) with sub-millisecond time resolution, are a relatively new diagnostic compared to standard semiconductor diodes. In this study, the GEM detector features a pinhole geometry outside the vacuum chamber and observes the plasma through a beryllium window. Filled with an ArCO2 mixture, the detector consists of an Aluminized Mylar cathode, three Aluminum-coated GEM foils, and an anode made of a 16 x 16 matrix of 6 mm(2) pads for 2D readout. It employs custom GEMINI ASICs (Application Specific Integrated Circuits) for signal readout, enabling single photon-counting techniques with Time over Threshold analysis on each detector channel, for a maximum rate of 1 MHz per channel. Preliminary results from the 2023 campaign highlight the GEM detector's ability to complement existing SXR camera systems by adding energy-resolved information to the spatial and temporal data. Case studies demonstrate the detector's capability to capture Magnetohydrodynamic instabilities, such as Snake instabilities, while utilizing its energy-resolved measurements to analyze plasma events, including Internal Reconnection Events. Additionally, the GEM detector enables the estimation of Electron Temperature in Maxwellian plasmas from SXR measurements. These findings underscore the potential of the GEM-based diagnostic system to enhance the understanding of tokamak plasmas by providing simultaneous spatial, temporal, and energy-resolved insights.