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• SPIDER beam non-uniformity and divergence not compliant yet with ITER requirements. • Additional diagnostics, e.g. fixed and insertable langmuir probes, RFEA. • Purpose: assess overall vertical and local beam non uniformities and beam divergence. • Method: improve spatial resolution, with more profiles and other plasma parameters. Each ITER Neutral Beam Injector (NBI) will provide 16.5 MW hydrogen/deuterium particles, electrostatically accelerating negative ions to 1 MeV. The challenging ITER NBI requirements have never been simultaneously attained and are expected to be demonstrated at the ITER Neutral Beam Test Facility (NBTF), at Consorzio RFX (Italy). NBTF includes the MITICA experiment, full-scale NBI prototype with 1 MeV particle energy, and SPIDER, with 100 keV particle energy, devoted to test and optimise the full-scale ion source. The four years of SPIDER operation demonstrated beam acceleration with high current density, but also highlighted that the non-uniformity and divergence of the beam are too large to comply with the ITER requirements, and that the current density is still lower; moreover, these findings can be traced back to non-homogeneity of the source plasma, where negative ions are produced. SPIDER then entered a shutdown dedicated to the improvement of the various plants and of the diagnostic system. The proposed additional diagnostic systems are integrated with the existing ones and mainly aim at increasing the spatial resolution of the plasma measurements, at investigating profiles in other directions and at measuring other parameters. In parallel, most of the already existing diagnostics will be refurbished or improved. [Display omitted] [ABSTRACT FROM AUTHOR]

SPIDER is the full-scale prototype of the plasma source of the negative-ion driven neutral beam injector for the heating and current drive of the ITER plasma. The uniqueness and complexity of the system requested this ad hoc test stand aiming at optimizing the performance of the RF inductively generated plasma, negative ion production and extraction, electron filtering, and robustness and controllability of all systems required to work together. After about three years of operation, presently SPIDER is in a long shutdown, in which the whole plasma source and accelerator were dismounted. In this phase, additional modifications with respect to the original design will be introduced to improve the system performance, driven by the experience acquired in the last years. These include the addition of further sets of permanent magnets in the plasma source expansion chamber and around the RF drivers, with the aim of improving the plasma confinement and consequently its density and possibly its uniformity. The present paper reports the study and the analyses behind this modification, which impacts on the original already complex magnetic configuration, made particularly difficult by the limited space available and the high voltages. The use of ferromagnetic shields, necessary to limit stray fields possibly increasing the breakdown probability, make the design particularly complex because of the greater impact on the previous configuration. An iterative process between analyses to determine the ideal configuration and CAD verifications was required. The analyses had to take into account the new magnetic configuration to be created in the particular area of interest, and the overall configuration in order to not compromise its efficacy. [ABSTRACT FROM AUTHOR]

• SPIDER source was disassembled and inspected after 3.5 years of operation. • Molybdenum coating was found damaged in some components and characterisation was performed. • Caesium deposition was found unevenly distributed, characterisation and cleaning procedures were preliminary tested. • Interaction of high energy particles with surfaces were investigated. SPIDER is the negative ion source testbed for ITER neutral beam injector. It is currently the largest negative ion source ever built, equipped with a 100 keV accelerator, aiming at producing a negative ion beam with an extracted current density in hydrogen of 355 A/m2, beam-on time of one hour. During the first 3.5 years of operation several improvements on the SPIDER source and its operating conditions were implemented. This paper focuses on the description of materials degradation phenomena in SPIDER and on the lessons learnt that were derived from components inspection during the already planned shutdown in 2022. The aim of these inspections was to improve the design of some components and to repair or refurbish damaged components. In particular, this paper describes the characterisation of damaged molybdenum coating, testing of surface cleaning techniques and experimental evidence of caesium vapour uneven distribution inside the plasma chamber. Sputtering due to beam particles was also investigated experimentally to observe its effect on materials used in SPIDER. [ABSTRACT FROM AUTHOR]

We present the development and first use of a set of movable electrostatic probes on the full-scale ITER heating neutral beam prototype negative-ion source SPIDER. The probes access the ion source plasma from the multi-aperture accelerator aiming at the study of the plasma formation and expansion from the RF drivers through the transverse magnetic filter. The magnetic filter separates the relatively high electron-temperature region for plasma formation and hydrogen dissociation form the negative-ion extraction region in which low electron-temperature is required to avoid negative-ion destruction, but also causes non-uniformities and drifts in the large plasma discharge which will be studied by this setup. The set of electrostatic probes encompasses eight RF-compensated Langmuir probes, one double probe, one Mach probe for the assessment of plasma drift velocities, and two gridded retarding field energy analyzers to measure the positive-ion energy distribution function. Electric aspects as well as mechanical constraints given by the large in-vacuum movable structure, and thermal requirements of these relatively heat-flux components made the design challenging. Prototyping and commissioning of the measurement system is discussed offering examples of measured characteristics with the various probes. [ABSTRACT FROM AUTHOR]