HV Discharges Monitoring at HVPTF through X-ray Spectroscopy: Fine Time Calibration Through Microdischarges Analysis

Neutral Beam Injectors (NBI) will be a critical part of the heating of the plasma in future fusion devices. For this reason, a prototype for ITER NBI, called MITICA, was built at Consorzio RFX in Padova, Italy. One of the challenges posed by MITICA is holding in vacuum the unprecedentedly high potential differences (between 200kV and 1MV): in order to study this problem, the High Voltage Padova Test Facility (HVPTF) was set up. The facility consists in a vacuum chamber containing two replaceable electrodes that can achieve a voltage difference up to 800 kV. During operation the supplied voltage, the current flow, the vacuum pressure and the residual gas composition are monitored and archived for offline analysis. Along with the diagnostics integrated in HVPTF, a series of radiation detectors was installed (through a collaboration with ISTP-CNR); such detectors are used to monitor the X-rays produced through bremsstrahlung by free charges accelerated in the vacuum and slowed down on the electrodes. Two scintillators installed in 2019 (a LYSO and a LaBr) were joined by a Gas Electron Multiplier (GEM) in 2022. The detectors produced spectroscopic data for more than 200 experimental sessions. Past works detailed the analysis performed on the spectroscopic data paired with the analogic data from HVPTF, along with perspectives for bremsstrahlung X-ray spectroscopy being a suitable monitoring device for discharge control. This work expands the job performed in the past, highlighting a critical issue on the synchronization of X-ray and analogic data acquired in the past and detailing the implementation of a fine calibration algorithm to tackle the issue.