Development of AXUV diagnostic system on the EXL-50U spherical torus

An absolute extreme ultraviolet (AXUV) photodiode array system has been developed and tested on the EXL-50U spherical torus for radiated power measurement. The system utilizes two 16-channel AXUV cameras to provide 32 lines-of-sight (LOS) covering the plasma poloidal cross-section with a spatial resolution of 3–5 cm. A temporal resolution of 2 μs enables the observation of fast transient phenomena. Double-layer shielding combined with single-point series grounding is employed to suppress any electromagnetic interference and oscillation. To enhance the mechanical precision and thermal stability, a monolithic 3D-printed camera housing with an integrated cooling chamber was fabricated, and a nanotube-carbon absorber was deposited on the inner surface to suppress stray light. A foil bolometer was co-installed with the AXUV array for cross-comparison and both systems show consistent temporal evolution. The diagnostic provides an effective tool for estimating radiated power and analyzing magnetohydrodynamics (MHD) instabilities, such as sawtooth oscillations. In typical discharges, the q = 1 surface position is inferred at ρ = 0.15 through tangent flux surface interpolation, and the radiated power fraction is estimated to be approximately 17%.