Analysis of micro-discharges fine dynamics via x-ray detection on the high voltage Padova test facility experiment

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 International Thermonuclear Experimental Reactor (ITER). One of the purposes 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 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 total voltage differences up to 800 kVDC with adjustable gap widths up to 250 mm. Among the diagnostics, a gas electron multiplier (GEM) detector is installed for acquisition of x-ray emission on a radial line of sight of the vessel. This paper presents a study of the experimental sessions featuring stainless-steel needle-plane electrodes. The analysis is based on the GEM data, in relation to the information on current and voltage of the two power supplies. The events are characterised in terms of both temporal and spatial evolution, providing sequential emission profiles with spatial resolution of tens of millimetres on timescales of the order of hundreds of nanoseconds.