The Full-Voltage Operation of the Acceleration Grid Power Supply for SPIDER Experiment

The Acceleration Grid Power Supply (AGPS) has been procured by ITER-India for the SPIDER experiment hosted in the Neutral Beam Test Facility (NBTF) in Padua, Italy. SPIDER is the prototype of the negative ion source of the ITER Heating Neutral Beam Injectors. The AGPS is devoted to feed the electrostatic accelerator of SPIDER with a dc voltage down to -96 kV, a maximum current of 75 A and a maximum pulse duration of 1 hour. Realized with Pulse Step Modulation technology, its main components are: three oil-insulated multi-secondary transformers fed by the 22 kV ac distribution network, 150 series-connected water-cooled switched power modules based on IGBTs, an output inductor, and a Control and Protection System. The AGPS has been commissioned at NBTF site on dummy load in 2018, reaching the full voltage. Then, ITER Organization (IO) accepted the system and transferred the Responsibility of Use to Consorzio RFX in 2019, in order to start the SPIDER operation with beam acceleration. However, it was not allowed to exceed the acceleration voltage of 30 kV, corresponding to the voltage ratings of the output cable and load disconnectors. In 2020, these components were replaced by new ones, with full voltage ratings. For space constraints, the number of disconnectors were reduced, accepting some compromise on the flexibility in operation. Then, in 2021 the asseveration for an output voltage down to -96 kV has been achieved, and the first SPIDER experimental campaigns with high acceleration voltage started. While at the beginning the beam current was very limited, it increased during the year with the use of Cesium in the ion source, which caused a substantial improvement of negative ion's production. During the operations, some relevant aspects of the AGPS for which an upgrade could be beneficial have been identified, for example the voltage regulation and ripple in no-load conditions, the voltage regulation range, and some control functions. This paper describes the process and the technical solutions which allowed achieving the asseveration to operate the AGPS at full voltage, reports the first experimental results and discusses the advisable improvements which have been identified.