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Description
The High Voltage Padova Test Facility (HVPTF) is an experiment set in Padova, Italy, operating in the framework of the Neutral Beam Test Facility project of the ITER tokamak. The purpose of HVPTF is to study the phenomenology of discharge events occurring between electrodes at high voltage differences over long vacuum gaps, which is crucial in the development of the Neutral Beam Injector (NBI) foreseen for ITER. The facility hosts a cylindrical vacuum vessel with stable pressure control, where two electrodes of different possible geometries can be mounted. Two independent power supplies allow for a total voltage difference up to 800 kVDC, with adjustable gap widths up to 250 mm. A monitoring system records current and voltage of the two power supplies, together with pressure and composition of the gas extracted from the chamber, all sampled at a rate of 100 Hz. Additional sets of diagnostics are installed to observe the dynamics of the emissions during the discharges, both in the visible/IR/UV (by fast cameras at different angles) and in the X-rays spectra (by scintillators and Gas Electron Multiplier(GEM)-based detectors).
During the last year, several experimental campaigns have taken place, with different electrodes configurations. This paper presents a study of the stainless-steel needle-plane configuration. The analysis is based on the data collected by the GEM detector, together with the information on current and voltage of the two power supplies. The observed events are characterized in terms of both temporal and spatial evolution, providing sequential emission profiles with millimetric resolution on timescales of the order of hundreds of nanoseconds. Additionally, preliminary electron transport simulations performed with a custom-developed software are presented, in support of a tentative explanation of the observed phenomena.
