Improvements in the SPIDER RF system

SPIDER is the full scale prototype of the ITER Heating Neutral Beam Injector ion source and the most powerful RF driven negative ion source currently in operation. In SPIDER the negative ions are extracted from a plasma generated by a 4×2 array of drivers, relatively small highly engineered antennas, each designed to be supplied with up to 100 kW at the frequency of 1 MHz. The drivers are fed in pairs by 4 independent RF oscillators in the push-pull configuration through 4 RF rigid lines and 4 “normal” L-type matching networks on board of the ion source. The basic concepts, previously developed and tested on smaller scale ion sources, were further improved and combined in a single facility considering all the constraint and restrictions relevant for ITER. The RF generators interact through their loads and the effects observed during the experimentation are frequency and output power modulations, which affect the plasma parameters and the ion beam performances. Furthermore, during the experimental campaign a common mode stray RF current was observed in the power supplies and in the ion source diagnostics system, causing faults and making the measurements unreliable. These issues have different root causes located both on the power supplies side and on the ion source side: the RF stray currents are mainly due to the transducers for the oscillators output voltage measurement, which provides low impedance path for them; the interactions among the oscillators are influenced by the layout of the RF circuit on board on the ion source that introduces large loops, thus magnetic coupling. The paper describes the original RF circuit and system, presents the understanding of the issues identified during the experimental campaigns and discusses the effectiveness of the provisions introduced to improve the performance.