Development of a Cherenkov probe with high time resolution for runaway electron measurements in the HL-2M tokamak

An energetic particle diagnostic system based on the Cherenkov effect has been developed to detect the characteristics of runaway electrons (REs) in the HL-2M tokamak. The Cherenkov radiator is a cylinder made of sapphire with a diameter of 5 mm and a height of 10 mm. Its surface is coated with molybdenum film with thickness of 5 mu m. The Cherenkov light spectrum which is emitted by REs when they pass through radiator with different energy is simulated by Geant4. Simulation results show that REs with energy above 120 keV can be effectively measured. The Al2O3 radiator is mounted at the end of an adjustable long shaft. The shaft can enter or withdraw from the chamber to optimize the radial position of the radiator. The Cherenkov signal can be transmitted through an optical path to the Silicon Photomultiplier (SiPM) and the sampling frequency of the signal acquisition system is 100 kHz. This probe was installed on the HL-2M device in 2022 and successfully obtained valid data during the autumn experiments. Measurements of the RE losses by the Cherenkov probe have been performed during HL-2M Ohmic discharges. Clear experimental evidences of the enhancement of RE losses during sawtooth instabilities have been obtained by the Cherenkov probe system. A detailed description of the probe system and the first experimental results are reported.