Status of the SPIDER source after 3.5 years operation

A combination of additional heating and current drive systems is necessary for ITER to reach fusion conditions and to control plasma configuration. Among them, two Neutral Beam Injectors (NBI) will provide 33MW hydrogen/deuterium particles, electrostatically accelerated to 1 MeV. To reach efficient neutralisation at such beam energy, the use of negative ions is required by conversion of atoms and positive ions thanks to the interaction with caesiated surfaces. NBI requirements have never been simultaneously attained; hence, the ITER Neutral Beam Test Facility (NBTF), at Consorzio RFX (Italy), is tasked to prove that these requirements can be obtained in the MITICA experiment, the full-scale NBI prototype with 1MeV particle energy. SPIDER, 100keV particle energy source, is devoted to testing and optimising the fullscale ion source requirements reaching the goals of: extracted beam uniformity, negative ion current density (355/330 A/m² in hydrogen/deuterium for 1000/3600 s) and beam optics (beam divergence < 7 mrad; beam aiming direction within 2 mrad). SPIDER started operation in June 2018. The experimental programme involved some years without caesium, to assess the capabilities of the various plants. In 2021, caesium was injected for the first time and higher current density beams were accelerated. Soon after, a shutdown was started dedicated to the improvement of the beam source operation; particularly, the beam source was taken out of the vacuum vessel, dismantled and characterised. This contribution describes the status of the beam source after three and a half years of operation. The source exhibits evidence of: non-uniform distribution of caesium across the plasma electrode; back-streaming positive ions impinging on the rear side of the plasma box; electrical discharges outside and inside the source; interaction of the plasma with the source surfaces. The relationship between the evidences on the source and the operational conditions observed during the experimental campaign will be investigated. This work has been carried out within the framework of the ITER-RFX Neutral Beam Testing Facility (NBTF) Agreement and has received funding from the ITER Organization. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.