Studies on the requirements and design of the High Voltage Radio Frequency Test Facility

The two projects of the ITER Neutral Beam Test Facility in Padova are MITICA, the full scale prototype of the heating Neutral Beam Injector (NBI) and SPIDER, the full-size negative ion source of the NBI. Both include a Radio Frequency (RF) Ion Source where plasma is produced by the inductive coupling with coils wound around vacuum chambers called driver cases. Each coil is fed at 1 MHz up to a power of 100 kW, which corresponds to a voltage of about 12 kV rms, with nominal plasma parameters. According to the ITER requirements, the ion source is in vacuum to avoid radiation induced conductivity; concern on the voltage hold off of the source components exists, in particular related to the contact zone between the coil and the driver case characterized by intensification of the E-field. The development of a test bed called High Voltage Radio Frequency Test Facility (HVRFTF) has been launched to study the issues related to the voltage hold off in this operational environment and to verify the electrical design of the drivers adopted in the ion sources of SPIDER and MITICA. The paper presents the studies addressed to identify the HVRFTF requirements, to conceive and design suitable driver mock-up to reproduce operative conditions relevant to the RF drivers of SPIDER and MITICA for the characterization of their dielectric strength, to work out a suitable RF circuit for the HVRFTF to generate the required voltage and to design a flexible test arrangement allowing the variation of the quantities which influence the voltage hold off, such as the pressure, geometry and materials of the devices under test, in order to perform parametric analyses.