Early operational experience and improvements of SPIDER ion source power supplies at beam energies exceeding 30 keV

At the ITER Neutral Beam Test Facility in Padua (Italy), the ion source full scale prototype SPIDER has been in operation since June 2018. SPIDER is designed for an acceleration voltage of -100 kV, however early operation has taken place with a limit of -30 kV. As of March 2021, the limitation has been lifted and the experiment 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 to grid breakdowns. Those power supplies include a set of high current single quadrant resonant converters, known as Source Support power supplies and a high voltage pulse step modulator feeding the extraction gap. The Source support power supplies perform a variety of functions, ranging from providing magnetic filter field current to polarising electrodes for minimization of the co-extracted electrons, from heating of the filaments to polarising the magnetic cores for protection from grid breakdowns. The Extraction Grid power supply extracts a negative ion and electron beam from the source, by applying a high voltage down to -12 kV dc to the gap between Plasma Grid and Extraction Grid. Grid breakdowns at beam energies exceeding 30 keV have shown the capability to upset the output measurements of the Source Support power supplies, limiting operation and reducing reliability of the whole system. The paper describes work undertaken firstly to identify the issues, found to belong to two distinct categories, general failures of all Source Support power supplies measurements and trips from the output voltage measurements of a well-defined subset of power supplies. Further investigation has associated the events to specific experimental conditions, pointing out weak points of the design and suggesting remedial actions. A number of modifications could be deployed quickly and tested in June 2021, before the stop to SPIDER operation for a planned major shutdown. The results of those tests are reported, along with the plan of future modifications for achieving immunity of the Source Support power supplies to the adverse effects of grid breakdowns at full acceleration voltage.