Energy-Sensitive X-ray Cameras for Thermal and Non-Thermal Plasmas: A 20-Year Detector Technology and Plasma Physics Journey

Soft and hard x-ray diagnostics are foundational tools in high-temperature plasma and fusion research, enabling measurements of MHD stability, energy confinement, impurity transport, runaway electron dynamics, radiated power, plasma-wall interactions and disruptions. Traditional tomographic systems provide space-resolved but energy-integrated measurements, limiting access to local plasma parameters due to the lack of energy resolution. This tutorial traces two decades of progress in energy-sensitive x-ray diagnostics, highlighting the transition from single-energy filtered arrays to multi-energy, higher-resolution systems. One-dimensional arrays using metal filters, GEMs, SDDs, scintillators, fiber optics, PMTs, APDs, and Si diode arrays can now provide time-resolved measurements of local x-ray emissivity across multiple energy bands, with sub-ms temporal and sub-cm spatial resolution. To expand diagnostic capability significantly, a novel multi-energy x-ray camera has been developed and deployed at C-Mod, MST, WEST and TCV. It features independently adjustable energy thresholds for each "smart" pixel, enabling unprecedented mapping flexibility on a 2D sensor. Energy-threshold trimming and photon-energy calibration allow selective inclusion or suppression of radiative recombination and line emissions from medium- to high-Z species, enabling inference of a dozen plasma parameters from broadband or line-emission x-ray data alone. These systems enable direct emissivity measurements while also delivering real-time estimates of plasma centroid position, electron temperature, radiated power profiles, and indirect inference of impurity densities and effective charge. New applications include monitoring tungsten flushing during sawteeth, diagnosing W-UFO events, detecting non-Maxwellian anisotropies, fast electron losses near strike points, and the evolution of runaway electron populations. Finally, their integration with real-time control paves the way for non-magnetic equilibrium reconstructions and fast-acting RE alarms—pointing to a broader diagnostic paradigm for future fusion devices.