Description
Tomography based on soft X-ray diodes has been widely used in all magnetic devices for nuclear fusion studies for 50 years, enabling 2-D reconstruction of total X-ray emissions in the 2-30 keV energy range with good time resolution. Gas Electron Multiplier (GEM) detectors were developed and used in many tokamaks (FTU, NSTX, MAST-U, WEST, EAST, KSTAR) for 20 years for 2D noise-free imaging, at high sensitivity and with energy resolution. It was then possible to detect soft X-ray (SXR) emissions across different energy bands and to spatially visualize emissions from high-Z impurities, such as W, Mo, and others. The new class of tokamaks (ITER, BEST, SPARC, STEP), aiming for significant fusion power production and, further, a burning plasma, poses severe constraints: high neutron irradiation and a higher energy range, up to 80-100 keV. To address these new challenges, studies were conducted in four areas: neutron irradiation compatibility, detachment of front-end electronics (f.e.), its shielding, and extension of the energy range. Measurements of the functionality of the bare GEM gas detector (without, e.g., f.e.) under 14 MeV neutron irradiation were performed at the FNG facility in Frascati and revealed that the detector can withstand up to 107 n/s cm2. For example, f.e. can be displaced up to 2 meters away using micro-coaxial shielded cables without significant degradation of energy and time resolution. To extend the energy range up to 80-100 keV, the GEM detector was used with a side entrance. The GEM detector has been designed in this case to have 1-D spatial resolution with 16 lines of sight (l.o.s.) and to operate with a Kr or Xe gas mixture to detect X-ray photons up to 80-100 keV. This detector has been used also for X-ray spectroscopy of laser produced plasmas at Gekko XII (Japan) and at INO-CNR Pisa (Italy) femtosecond laser facility. Finally, a genetic algorithm applied to Monte Carlo simulations has been implemented to optimize the materials and layer thicknesses to shield the remote f.e. electronics and the GEM detector as well. These developments have been supported in part by ITER.