Fast directional hard x-ray diagnostic for high-frequency energetic-electron fluctuations in the QUEST spherical tokamak.

A fast directional hard x-ray (HXR) diagnostic system has been developed for the Q-shu University Experiment with Steady-State Spherical Tokamak using plastic scintillators coupled to photomultiplier tubes. During intense HXR emission, the detector response transitioned from pulse-counting to current-mode operation, enabling observation of high-frequency fluctuations in the HXR signal. A clear correspondence was observed between HXR and magnetic fluctuations, while directional differences were identified among forward-, backward-, and vertical-viewing configurations. The developed technique establishes high-frequency HXR fluctuation measurements as a new diagnostic of energetic-electron dynamics, opening a new experimental window into wave-particle interactions through direct measurements of energetic-electron fluctuations.