Early Operational Experience and Improvements of SPIDER Ion Source Power Supplies at Beam Energies Exceeding 30 keV

At the International Thermonuclear Experimental Reactor (ITER) Neutral Beam Test Facility in Padua (Italy), the ion source full-scale prototype--SPIDER--has been in operation since June 2018 and, as of March 2021, has begun to explore the range of beam energies exceeding 30 keV. This has revealed a number of novel issues with the power supplies of ion source and extractor, mainly associated with grid breakdowns. Those power supplies include a set of high current single quadrant resonant converters, known as ion source support power supplies (ISSSs) and a high voltage pulse step modulator feeding the extraction gap. Grid breakdowns at beam energies exceeding 30 keV have shown the capability to upset the output measurements of the ISSSs, limiting operation and reducing reliability of the whole system. This article describes work undertaken first to identify the issues, found to belong to two distinct categories: general failures of all ISSSs measurements and trips from the output voltage measurements of the extraction gap power supply. Further investigation has associated the failures of the ISSS measurements to transient voltage affecting the electronics during grid breakdowns. A number of modifications to both power circuit and electronics have been undertaken and successfully tested, achieving immunity down to the full SPIDER operating voltage (-96 kV). The extraction gap power supply overvoltage has been quantified and a temporary modification deployed, able to mitigate it significantly. A deeper redesign will be required in this case and is planned during the 2022 shutdown of the experiment.