Assessment of measurement capabilities of a space and energy resolved diagnostic based on GEM technology on MAST-U tokamak
A Gas Electron Multiplier (GEM) based detector was exploited for the first time on the MASTU tokamak during the 2023 campaign to investigate Soft X-Ray (SXR) radiation (1-20keV) emitted from the plasma. SXR radiation carries essential information on electron temperature and density, impurity presence, and Magneto-hydrodynamic (MHD) activities, making SXR diagnostics standard in tokamak experiments. GEM detectors are a relatively new diagnostic chosen for their resilience to harsh fusion environments and their ability to provide energy-resolved SXR emission images with sub-millisecond time resolution, unlike the standard semiconductor diodes. In this study the GEM detector features a pinhole geometry outside the vacuum chamber and sees the plasma through a beryllium window acting as a low cut-off energy filter. The detector is filled with a ArCO2 mixture, consists of an Aluminized Mylar cathode, three Aluminum-coated GEM foils, and an anode made of a 16×16 matrix of 6mm2 pads for 2D readout. It employs custom GEMINI ASICs (Application specific integrated circuit) for signal readout, enabling photon-counting techniques with Time over Threshold (ToT) analysis on each detector channel, reaching a total maximum rate of 256 MHz. Preliminary findings from the 2023 campaign demonstrate the GEM detector’s effectiveness in identifying MHD instabilities and offering spectroscopy capabilities for plasma event energy characterization, complementing existing SXR camera data in spatial and temporal resolution. These results underscore the GEMbased diagnostic system’s potential in advancing tokamak research for comprehensive plasma characterization and control.