In the context of magnetic confinement fusion experiments, a Gas Electron Multiplier (GEM)-based detector has been exploited at the MASTU (Mega Amp Spherical Tokamak) spherical tokamak in the 2023 and 2024 experimental campaigns to study the Soft X-Ray (SXR) radiation (0.1-20keV) emitted from the plasma. GEM detectors have gained popularity in recent tokamak experiments due to their resilience in harsh fusion environments compared to traditional semiconductor diodes. Additionally, they provide energy-resolved SXR emission images with sub-millisecond time resolution, a capability lacking in diodes. As a result, the adoption of GEM-based detectors has surged in recent years, with numerous installations on tokamaks like FTU, KSTAR, EAST, and WEST, both as 1D array or 2D matrices.

 The GEM detector presented in this study is installed in front of a beryllium window that acts as a low energy filter, with a pinhole geometry outside the vacuum chamber. The detector is composed of an Aluminized Mylar cathode, followed by three GEM foils. It features a 2D readout anode made of a 16x16 matrix of 6mm^2 square pads. The detector integrates custom GEMINI ASICs for signal readout, allowing photon-counting techniques with Time over Threshold (ToT) analyses on each detector channel, reaching a total maximum rate of 256 MHz. The gas mixture employed varies between the two consecutive experimental campaigns: ArCO2 and NeCO2. 

Preliminary findings from the 2023 and beginning of 2024 campaigns showcase the effectiveness of the GEM detector in identifying Magnetohydrodynamic (MHD) instabilities, aligning well with existing SXR camera data in terms of spatial and temporal resolution. Additionally, the detector offers spectroscopy capabilities for discerning energy characteristics of plasma events, and the estimation of the local electron temperature. These results highlight the potential of the GEM-based diagnostic system in advancing tokamak research, paving the way for comprehensive plasma characterization and control.