RF Stray Currents in SPIDER and MITICA power circuits: model assessment and experimental results

The ITER Neutral Beam Test Facility (NBTF) in Padova (Italy) hosts two different experiments: SPIDER, the prototype of the ion source of ITER Neutral Beam Injector (NBI) and MITICA, the prototype of the ITER NBI. The ion sources of SPIDER and MITICA are driven by Radio-Frequency (RF) power, for a total of 800kW at 1MHz. The RF power is delivered to the Inductively Coupled Plasma (ICP) drivers of the ion source by four tetrode oscillators through four RF circuits, each one consisting of a coaxial transmission line and a capacitive matching network. During SPIDER experimental campaign, the circulation of RF Stray Currents in the electric system has been pointed out. These currents hinder the correct operation of the system, thus preventing to achieve the full performances due to the excessive overheating of its components. To have a better comprehension of the issue, after an overall circuital investigation and the identification of a possible reclosing path for the RF Stray Currents, a simplified model of SPIDER electric system was developed, initially focusing on a single RF generator. The aim of the work presented in this paper is to upgrade this model, through a step-by-step procedure, by adding the other three RF oscillators, and their related RF circuits, in order to model a more complete version of the electric system of SPIDER. This upgraded model will allow increasing the knowledge of the issue and, in particular, understanding the impact of the four RF oscillators on the RF Stray Currents, also thanks to the support of the SPIDER's experimental results. After that, the task is trying to apply this model also to the MITICA ion source RF circuit in order to try predicting and, if possible, limiting the circulation of the RF Stray Currents during future MITICA operations.