Overview on electrical issues faced during the SPIDER experimental campaigns

In SPIDER the beam source is fully installed within a vacuum vessel and operated in the residual gas pressure, including 8 radiofrequency (RF) Inductively Coupled Plasma drivers each fed with a power up to 100 kW, their matching networks, and the set of grids to extract and accelerate the negative ions at 100 keV. SPIDER power supply system and the beam source were designed taking into account normal and anomalous operation, with the support of simulation tools and benefiting from the experimental results obtained with the most significant devices. Nevertheless, the need to satisfy the specific ITER requirements and the relevant complexity of the circuits led to explore novel conditions and encounter electrical issues during the SPIDER experimental campaigns, not easily predictable. One of the main issues encountered was the presence of RF discharges in the vacuum outside the ion source, limiting the operating pressure range and causing pulse interruption and source damages. The self-excited RF oscillators showed frequency instabilities intrinsic limits of the application of this technology to the resonant loads of Neutral Beam Injectors, assessing the technical basis for the final decision to replace the RF generators in SPIDER and MITICA and change the current ITER baseline to solid-state amplifier technology. Other issues faced were regarding the limitation in the operational space to be mitigated, the revision of the configuration of the magnetic filter field layout to avoid plasma quench, the effect of the mutual coupling between the RF circuits on board the source, various electromagnetic compatibility problems requiring significant effort to be well understood. The paper will describe the studies leading to the comprehension of the problems, the identification of solutions to mitigate them, and will discuss the effectiveness of provisions already applied and the actions in progress for those to be implemented during the current shutdown.