Overview on electrical issues faced during the SPIDER experimental campaigns

SPIDER is the full-scale prototype of the ion source of the ITER Heating Neutral Beam Injector, where negative ions of Hydrogen or Deuterium are produced by a RF generated plasma and accelerated with a set of grids up to ~100 keV. SPIDER Beam Source design complies with the ITER specific requirements, being fully installed within a vacuum vessel, and with the electrical circuits operating at the residual background pressure. The Power Supply System is composed of high voltage dc power supplies capable of handling frequent grid breakdowns, high current dc generators for the magnetic filter field and RF generators for the plasma generation. During the first 3 years of SPIDER operation different electrical issues were discovered, understood and addressed thanks to deep analyses of the experimental results supported by modeling activities. One of the main issues encountered was the presence of RF discharges on the RF circuit on the backside of the beam source, limiting the operating pressure within the source. The self-excited RF oscillators showed frequency instabilities that were found to be intrinsic limits of the application of this technology to the resonant loads of Neutral Beam Injectors; their understanding allowed 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 regard the effect of the mutual coupling between the RF circuits on board the source, various electromagnetic compatibility problems, the limitation in the operational space to be mitigated improving the SPIDER operating strategies, the revision of the configuration of the magnetic filter field layout to avoid plasma quench. The paper gives an overview on the observed phenomena and relevant analyses to understand them, on the effectiveness of the short-term modifications provided to SPIDER to face the encountered issues and on the design principle of long-term solutions to be introduced during the currently ongoing long shutdown.