RF Stray Currents in SPIDER Power Circuits: Model Assessment and Experimental Results

The ITER Neutral Beam Test Facility (NBTF), Padua, Italy, hosts two different experiments: Source for Production of Ion of Deuterium Extracted from Rf plasma (SPIDER), the prototype of the ion source (IS) of ITER neutral beam injector (NBI), and Megavolt ITER Injector and Concept Advancement (MITICA), the prototype of the ITER NBI. The ISs of SPIDER and MITICA are driven by radio frequency (RF) power, for a total of 800 kW at 1 MHz. The RF power is delivered to the inductively coupled plasma drivers of the IS by four tetrode oscillators. Operation of SPIDER at high power pointed out the presence of RF stray currents circulating in the electric system. These currents hinder the correct operation of the system, causing damage to its components. To improve the 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 circuit. The aim of the work presented in this article is to extend the model to four RF circuits, to study the impact of their mutual coupling and of the common potential references on RF stray currents magnitude, with a view to improve the comprehension of the issue and the effect of the provisions to mitigate it. The results obtained with this model are compared to SPIDER experimental measurements for validation.