Development of a GEM based diagnostic for soft x-ray measurements resolved in space, time, and energy at RFX-mod2

This study presents the development of an advanced soft x-ray (SXR) diagnostic for the RFX-mod2 device, tailored for photon energies of ∼1–25 keV. The system will consist of two gas electron multiplier (GEM) detectors designed to enable energy-resolved 1D tomographic reconstructions (i.e. reconstructions of the SXR radial emissivity profiles). Simulations have shown its potential to achieve a spatial resolution of ∼1 cm and to differentiate between thermal and suprathermal plasma components. This capability is particularly valuable for RFX-mod2, where strong magnetic reconnection events are expected to generate significant SXR spectral tails in localized regions. The GEM-based diagnostic will therefore complement the SXR silicon photodiode arrays (SiPh) foreseen for installation at RFX-mod2 [2]. SiPh have been successfully employed for two-dimensional tomographic reconstruction of the SXR emissivity at RFX-mod. However, their operation in current mode allows only measurements of photon flux, lacking energy resolution. In contrast, GEM detectors operate in photon-counting mode. They provide energy-resolved measurements with a temporal resolution of ∼10 µs and an energy resolution of ∼20% at 6 keV. They also offer a high rate capability (∼1 MHz/channel) and high radiation hardness. This paper presents the CAD model of the GEM diagnostic, as well as the custom-made simulations used to drive design choices and develop the energy resolved 1D tomographic reconstruction algorithm.