RF solid-state generators for the high-power ion sources of NBTF experiments and ITER HNB

NBTF hosts two experiments, MITICA and SPIDER, full-scale prototypes of ITER HNB and its RF-driven ion source, respectively. SPIDER and MITICA plasma sources, used to generate negative ions that are subsequently extracted and accelerated to nominal energy, are similar in design and consist of 8 inductively coupled RF drivers, powered by 4 generators at 800 kW and 1 MHz. SPIDER operation, started in 2018, pointed out severe issues caused by the technology employed in RF generators, based on tetrode free-running oscillators. One of these limits, namely the onset of frequency instabilities, prevented operation at the full rated power of 200 kW. In addition, tetrodes require high voltage to operate, which translates to risk of flashovers and the necessity to perform conditioning procedures, limiting the overall reliability. These disadvantages, combined with the positive experience gained in the meanwhile on smaller facilities with solid state amplifiers, led to the proposal of a complete re-design of the radiofrequency power supplies. This contribution describes the specifications and design criteria of the solid-state amplifiers for SPIDER and MITICA, driven by the necessity to achieve nominal power, mitigate the risk of obsolescence, and improve the reliability through modularity. We detail the topology of the generators, consisting of class D amplifier modules combined to achieve the required 200 kW. Due to the non-standard application, we gave particular focus to the integration of generators in the RF systems of SPIDER and MITICA. Analyses were performed to verify the impact of harmonic distortion on transmission line and RF load components, to address the effect of mutual coupling between RF circuits on the generator output modulation, and to assess the magnitude of common mode currents in the electric system. These studies, as well as the experience gained from SPIDER operation, helped to define dedicated circuital design provisions and control strategies.