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The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER Neutral Beam Injector. Evidences of Electron Stimulated Desorption of gas released from anodic surfaces have been observed during the onset of high voltage Direct Current vacuum discharges between stainless steel electrodes, separated by vacuum gaps of a few millimeters and with voltages of about 200 kV. The count of x-ray photons, produced by the accelerated electrons impinging the anode surface and observed as breakdown precursor in the 40% of the cases, was analyzed together with the images taken by an ultra-high speed camera during the onset of breakdown events. Electron stimulated desorption of neutral gas from the anodic surface appears to be a candidate for the voltage breakdown. The dynamic of the observed phenomena has been compared with the results obtained by a numerical model. The latter assumes a Townsend like discharge which develops following the emission of gas from the anode surface. The comparison shows good agreement between the experimental data and numerical simulations.

The High Voltage Padova Test Facility (HVPTF) is an experimental device for investigating High Voltage Direct Current insulation in vacuum, in support of the realization of MITICA, the prototype of a neutral beam injector for ITER. Inside a high vacuum chamber, two stainless steel electrodes, separated by a few centimetres gap, can achieve a voltage difference up to 800 kV. During the conditioning process of the electrodes, current micro-discharges (MD) and associated X-rays are observed, along with a global increase of gas emission (H2 and CO2 are detected by the Residual Gas Analyser). During the last five years, different X-rays detectors have been installed on HVPTF, with the aim to investigate the physical processes behind the conditioning.In this contribution, the fine dynamics of the micro-discharge phenomenon is studied with the purpose to sketch a possible physical interpretation of this mechanism. In particular, the time evolution, during the electrode conditioning, of the MD fine dynamics seems to be related with the increasing asymmetry between the MD current peak values collected by the positive and the negative electrodes. The possible (increasing) contribution of the vacuum chamber, operating as a third electrode, is investigated, by means of a toy model: the different current contributions collected by the two electrodes are evaluated, in particular those generated by secondary electron emission. The rate between anode and cathode currents evaluated by the toy model is in good agreement with the experimental one.

The high voltage Padova test facility (HVPTF) is an experimental device for the study of HV insulation in vacuum, in support of the realization of the prototype of a neutral beam injector for International Thermonuclear Experimental Reactor (ITER), named megavolt ITER injector and concept advancement (MITICA). The facility investigates the physical phenomena underlying voltage holding in vacuum, in particular the mechanisms causing breakdowns and the electrode conditioning process. At HVPTF, inside a high vacuum chamber, two stainless steel electrodes, separated by a few centimeters gap, can achieve HV values for a maximum potential difference of 800 kV. During the voltage conditioning of the electrodes, current MicroDischarges (MDs) and high-energy X-rays are observed and analyzed. In this work, we present a characterization of the MD dynamics occurring during the conditioning process, with the aim to gain information on the reason of the growing difference between the current contributions measured by the two electrodes. The analysis will be performed due to two new X-rays detectors recently installed: an lutetium yttrium orthosilicate (LYSO) and a LaBr3(Ce) scintillating crystal. The new diagnostics indeed provide the fine dynamics of a single MD, recording energy and time of each single detected X-ray. In particular, we compare the MD dynamics at the beginning and at the end of the conditioning process and try to give a preliminary interpretation of the observations.

The development of MITICA, the prototype for a neutral beam injector for ITER, drives the interest in investigating high HV insulation in vacuum. The High Voltage Padova Test Facility (HVPTF) is an experimental device with the aim of studying the fundamental processes leading to discharges, offering a framework to develop new diagnostics, models, and mode of operations for MITICA. For this purpose, HVPTF features a vacuum chamber containing two electrodes which can achieve an HV difference up to 800 kV. X-ray bremsstrahlung radiation produced by free charges accelerated by the HV was proven to be a promising monitoring mechanism in the past; as such, two scintillating crystals, a LYSO and a LaBr3, coupled with fast electronics were used to conduct hard X-ray spectroscopy. This work describes a newly custom-developed software tool to analyze the spectroscopy from scintillators and integrate it with the HVPTF analog data. The tool was employed to study two experimental sessions, reaching promising results in the characterization of microdischarges, especially in terms of time resolution. Detection limits imposed by pile-up and other processes were identified and addressed, finding the best range of operation of the two scintillators. The performed study opens the way for the analysis of data obtained in all 2020 and 2021 experimental campaigns, thus giving the possibility to implement future improvements in HVPTF X-ray spectroscopy.

High voltage (HV) insulation across a single gap in vacuum and low-pressure gas is a critical issue in relation to the development and realization of the electrostatic accelerator for the ITER neutral beam injector (NBI) (Toigo et al 2017 New J. Phys. 19 085004). The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER NBI (De Lorenzi et al 2011 Fusion Eng. Des. 86 742-5). A voltage up to 800kV(DC) was achieved in the HVPTF during the experimental campaigns with a sphere-plane configurations having variable gap length (from 30 to 150 mm) and pressure ranging from high vacuum (10(-7) mbar) to 10(-3) mbar in argon. Such an experimental campaign represents one of the few examples where voltages higher than 500-550 kV DC are sustained by a single vacuum gap between electrodes (Rohrbach 1971 CERN Report 71-5) and therefore constitutes additional experience to improve the knowledge of voltage holding across large vacuum gaps. The results in high vacuum indicate that at the beginning of voltage conditioning the breakdown events occur at the same cathodic electric field irrespective of the electrode geometry. However, after sufficient conditioning time, the breakdown voltage distribution seems to depend also on the electric field at the anode and on the total voltage between electrodes. A benchmark between a numerical tool previously developed to predict the voltage holding in high vacuum (voltage holding prediction model, VHPM (Pilan et al 2011 Fusion Eng. Des. 86 742-5)) and the experimental results is also reported and discussed.