Study of protection strategies against breakdown effects in the SPIDER experiment

The prototype of the ITER HNB Ion Source will be tested in the facility called SPIDER (Source for the Production of Ions of Deuterium Extracted from RadioFrequency plasma), under construction at Consorzio RFX site in Padua, Italy. During the operation, several breakdowns are expected to occur across the -96 kV Accelerator gap, causing transient overvoltages between parts normally at low voltage and large overcurrents flowing from the Power Supplies to the source. The paper firstly describes the development of the fast transient electrical model of the overall SPIDER electric system and then the results of the analyses performed with the reference design configuration, showing severe overvoltages in particular on the Ion Source and on the HV Bushing feedthroughs. The installation of some countermeasures, as local capacitors or a damping resistor was finally simulated, showing an effective limitation of these electrical stresses.