RF in vacuum boundaries: issues and test bed/experimental results

ITER is the world's largest experiment designed to prove the feasibility of nuclear fusion reaction of deuterium and tritium nuclei as energy source with the aim of producing 500 MW of thermal power for up to 1 hour. The reaction occurs in a magnetic confined plasma heated mainly by means of two Heatin g Neutral Beam Injectors (NBI) each designed to deliver up 17 MW, by accelerating 40 A of deuterium negative ions with a voltage up to - 1 MV and then neutralizing them to allow their penetration into the plasma core. The performances requested to ITER NBI have never been achieved before at the same time in a single device, which pushed the development of two separates experiments SPIDER and MTICA being built in Padova, Italy, in a large experimental facility called PRIMA (Padova Research on ITER Megavolt A ccelerator). SPIDER is the - full size negative ion source of the NBI with an acceleration of - 100 kV, while MITICA is the full scale prototype of the heating NBI with the acceleration of - 1 MV. The negative ions are extracted from a cold plasma within 8 cyl indrical chambers filled with deuterium at a pressure of about 0.3 Pa, inductively coupled with coils wound around each chamber and each supplied with up to 100 kW at 1 MHz. The 8 assemblies composed of the chamber, the radiofrequency coil and all the related passive structures, called drivers, are placed in the backside of the ion source and work in a deuterium atmosphere with very low pressure and with applied voltages up to 17 kV rms. High electric fields are foreseen locally in the driver region, si nce small gaps between the RF coil and relevant feeders and grounded components have been designed to limit the overall dimensions. To study the voltage holding capability of RF drivers an experimental program was started at Consorzio RFX with the design and construction of the High Voltage Radio Frequency Test Facility (HVRFTF). The paper will present the specific issues related to the RF drivers to be investigated during the first experimental campaign with the HVRFTF.