RF induced discharges in vacuum in the SPIDER experiment

SPIDER is the full-size prototype of ITER Neutral Beam Injector ion source, in operation since June 2018 as part of the ITER Neutral Beam Test Facility located in Padova, Italy. The ion source of SPIDER is composed of eight Radio Frequency drivers operated in low hydrogen gas pressure, 0.3 Pa, driven at 1 MHz with a total nominal power of 800 kW, an extraction and acceleration system is used to produce a negative ion beam with an energy up to 100 keV. RF breakdowns outside the ion source beset the SPIDER experimentation since the beginning of the operations. Such breakdowns could be sustained by the power supplies but in many cases cause the plasma pulses to stop and might cause source damages. The scope of this work is to summarize the experimental pieces of evidence, classify the type of breakdowns and describe the electrical model for these events. Three classes of diagnostics have been used to investigate the RF breakdowns: electrical measure-ments from the Ion Source and Extraction Power Supply, visible cameras (both fast and slow) and optical fibers which collect lights from the rear of the SPIDER experiment. Each of these signals is somehow affected by the RF breakdowns; their comparative analysis allows improving the un-derstanding of this issue and helped the identifications of different types of breakdowns that can occur outside the SPIDER Ion Source. A discussion of the reasons behind the occurrence of the discharges, induced by the Radio Frequency, concludes the work.