Fast directional hard x-ray diagnostic for high-frequency energetic-electron fluctuations in the QUEST spherical tokamak
R. Ikezoe, Y. Shirai, S. Kawase, T. Onchi, K. Hanada, M. Hasegawa, H. Idei, T. Ido, T. Kinoshita, Y. Nagashima, T. Kawate, L. F. Delgado-AparicioA 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.