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Description
The Neutral Beam Test Facility in Padova, Italy, is the site where the development of the Neutral Beam Injector (NBI) for the ITER tokamak is being carried out. In this context, the Megavolt ITER Injector and Concept Advancement (MITICA), which is the full-scale prototype of the NBI, is in the installation and commissioning phase. The system aims at producing neutrals of 1 MeV energy to be injected in the plasma, through acceleration and neutralization of a negative ion beam. A multi-grid multi-aperture system is designed for the accelerator part, and high voltage holding in vacuum over long distances is one of the critical aspects for the prototype to be functional. Operations can be interrupted by in-vacuum discharges and breakdowns, which can also damage the system. The phenomenology of these occurrences is studied at the High Voltage Padova Test Facility (HVPTF), which hosts a cylindrical vacuum vessel containing two electrodes that can be set at voltage differences up to 800kVDC. During operations, current and voltage are monitored for the two independent power supplies, together with the pressure inside the chamber. Moreover, different lines of sight allow for other observations during the experimental campaigns. The dynamics of the discharges have been observed with different cameras in the visible, UV and IR spectrum ranges. In the latest years, X-ray emissions from the vacuum chamber have been observed and studied as well, first with inorganic scintillators and then also with Gaseous Electron Multiplier (GEM) detectors. A strong correlation has been found between the current signal at the power supplies and the X-ray signal at the detectors, suggesting that the study of this radiation can be helpful in the characterization of the discharges and the development of a mean to mitigate and prevent breakdown occurrences. This paper presents a brief summary of the observations that have been made regarding the X-ray time dynamics on the basis of the GEM detector data, together with the initial development of a multi-environment simulation code aiming at reproducing the main aspects of the observed experimental results.
