Simulations of micro-scale dynamics of discharges in conditioning experiments at the High Voltage Padova Test Facility

As part of the development project for the Neutral Beam Injector for the ITER tokamak, and to support operations of the Megavolt ITER Injector and Concept Advancement (MITICA), the High Voltage Padova Test Facility (HVPTF) is operational in Padova, Italy. The experiments at HVPTF focus on investigating and understanding the phenomena associated with in-vacuum discharge events occurring between high-voltage electrodes, with the goal of mitigating or preventing breakdown occurrences. The facility features a cylindrical vacuum chamber equipped with moveable supports for two electrodes. Two independent power supplies provide voltage differentials up to 800 kV DC, with current and voltage signals being monitored during experiments, as well as the pressure within the chamber and the composition of gases exiting it. Additionally, multiple lines of sight enable the installation of various diagnostics around the chamber, allowing for comprehensive observation of the dynamics of emissions during discharge events across visible, IR, UV, and X-ray spectra. Past works have presented analyses of the X-ray radiation coming out of the chamber, mainly caused by the Bremsstrahlung effect of the electrons travelling in the volume. Different aspects of the experiments were considered, and tentative explanations were brought forward. Some recent advancements in the development of the diagnostic and control systems of the experiment have given the chance to obtain more accurate and reproducible results. At the same time, the realization of a simulation environment, based on the combined use of multiple software has been proposed, for the modeling of the various processes involved in micro-discharges. This paper presents the basis for the development of the simulation model, focusing on the interchange of charged particles and photons between the electrodes. The coefficients required for the balance equation of the model are presented and calculated, with a display of the results.