The ITER experiment, currently under construction in Cadarache in southern France, will require the injection of a considerable amount of additional power from its heating and current drive (HCD) systems in order to sustain long plasma pulses. Among all of the various systems, neutral beam injectors (NBIs) will account for 50 MW of power over three devices, mediated by fast neutrals at 1 MeV in deuterium (or 870 keV in hydrogen). The required extracted negative ion current at the plasma source (PS) can only be reached through efficient negative ion generation and extraction, which in this type of NBIs is achieved by sufficient surface coating of the inner surfaces of the PS with a low work-function metal, such as cesium. This article describes the cesium oven for the ITER Heating NBI (HNBI), which is a device for the controlled evaporation of cesium inside the PS: its general structure is based on the design of the cesium oven for the Source for Production of Ion of Deuterium Extracted from Rf plasma (SPIDER) experiment, however with the necessary modifications to make it compatible with the more stringent ITER guidelines, such as radiation resistance and remote handling compatibility. A prototype has been designed at Consorzio RFX by taking into account this set of requirements, followed by extensive simulation of the thermal distribution.

MITICA (Megavolt ITER injector and concept advancement), the full-scale prototype of ITER heating neutral beam (HNB), is under construction at the Neutral Beam Test Facility (NBTF), Padua, Italy. It is designed to accelerate deuterium negative ion beam with a 40-A current up to 1 MeV. In order to test the capability of the beam source (BS) to withstand a total voltage of 1 MV, it has been decided to conduct electrical tests with a “BS mockup” reproducing the geometry relevant for high-voltage (HV) holding. In addition, these tests will study the influence of an electrostatic shield, referred to as ES06. The ES06 approximately follows the estimated $-$ 600-kV equipotential surfaces inside the vacuum domain, surrounding the parts of the accelerator and ion source at a potential between $-$ 600 kV and $-$ 1 MV to eliminate the largest gaps between the rear side of the BS and the grounded vessel with a voltage difference up to 1 MV. Moreover, the ES06 must be compatible with the real MITICA BS. The ES06 is a complex object with several conflicting constraints (electrostatic and mechanical, together with implications relative to remote handling and integration). It also adds a nonnegligible weight to the accelerator. This article describes the design of the ES06, the numerical models, and the load cases tested and, in particular, presents a mechanical analysis of both the mockup and the real MITICA BS with the ES06 mounted on, including seismic verifications. The design of the ES06 is verified in all the cases tested.

ITER is of key importance in the European fusion roadmap as it aims to prove the scientific and technological feasibility of fusion as a future energy source. The EUROfusion consortium of labs within Europe is contributing to the preparation of ITER scientific exploitation and operation and aspires to exploit ITER outcomes in view of DEMO. The paper provides an overview of the major progress obtained recently, carried out in the frame of the new (initiated in 2021) EUROfusion work-package called 'Preparation of ITER Operation' (PrIO). The overview paper is directly supported by the eleven EUROfusion PrIO contributions given at the 29th Fusion Energy Conference (16-21 October 2023) London, UK [www.iaea.org/events/fec2023]. The paper covers the following topics: (i) development and validation of tools in support to ITER operation (plasma breakdown/burn-through with evolving plasma volume, new infra-red synthetic diagnostic for off-line analysis and wall monitoring using Artificial Intelligence techniques, synthetic diagnostics development, development and exploitation of multi-machine databases); (ii) R&D for the radio-frequency ITER neutral beam sources leading to long duration of negative deuterium/hydrogen ions current extraction at ELISE and participation in the neutral beam test facility with progress on the ITER source SPIDER, and, the commissioning of the 1 MV high voltage accelerator (MITICA) with lessons learned for ITER; (iii) validation of neutronic tools for ITER nuclear operation following the second JET deuterium-tritium experimental campaigns carried out in 2021 and in 2023 (neutron streaming and shutdown dose rate calculation, water activation and activated corrosion products with advanced fluid dynamic simulation; irradiation of several materials under 14.1 MeV neutron flux etc).

In a nuclear fusion reactor, some components of the system are subjected to huge thermal loads. These components, such as grids, neutralizers, calorimeters, ducts, divertors, blankets etc. have to be actively cooled in order to avoid damage, functionality problems and specially to ensure their reliability. The design process of such components requires taking into account several physical phenomena, with the goal of not only satisfying the functionality conditions, but also with a focus on the structural verifications and component lifetime. For assessment of the structural design for components in ITER, the ITER Structural Design Criteria for In-vessel Components (SDC-IC) represent the reference verification criteria. The aim of this work is to investigate the relationship between the cooling system performance and the fatigue life. In this work a generalized framework for the evaluation of cooling system components is presented, taking into account the necessary input parameters, performing numerical analyses, division of component into several interest regions and assessing the fatigue life. The performed numerical analyses are a set of coupled CFD and thermo-structural analyses. A case study is presented for a critical component, the Grounded Grid (GG), inside the SPIDER beam source experiment at Consorzio RFX, Padova, Italy. The presented test case includes cooling system effectiveness analysis, assessment of the temperature distribution on housing during operations, mapping the pressure distribution inside the cooling system and finally assessment of the maximum number of allowed cycles.

ITER envisages the use of two heating neutral beam injectors plus an optional one as part of the auxiliary heating and current drive system. The 16.5 MW expected neutral beam power per injector is several notches higher than worldwide existing facilities. A Neutral Beam Test Facility (NBTF) was established at Consorzio RFX, exploiting the synergy of two test beds, SPIDER and MITICA. SPIDER is dedicated to developing and characterizing large efficient negative ion sources at relevant parameters in ITER-like conditions: source and accelerator located in the same vacuum where the beam propagates, immunity to electromagnetic interferences of multiple radio-frequency (RF) antennas, avoidance of RF-induced discharges on the outside of the source. Three years of experiments on SPIDER have addressed to the necessary design modifications to enable full performances. The source is presently under a long shut-down phase to incorporate learnings from the experimental campaign. Parallelly, developments on MITICA, the full-scale prototype of the ITER NBI featuring a 1 MV accelerator and ion neutralization, are underway including manufacturing of in-vessel components, while power supplies and auxiliary plants are already under final testing and commissioning. Integration, commissioning and tests of the 1MV power supplies are essential for this first-of-kind system, unparalleled both in research and industry field. The integrated test to confirm 1MV output by combining invertor systems, DC generators and transmission lines extracted errors/accidents in some components. To realize a concrete system for ITER, solutions for the repair and the improvement of the system were developed. Hence, NBTF is emerging as a necessary facility, due to the large gap with existing injectors, effectively dedicated to identify issues and find solutions to enable successful ITER NBI operations in a time bound fashion.

The MITICA (Megavolt ITER Injector and Concept Advancement) experiment, currently in the installation and commissioning phase at the Neutral Beam Test Facility in Padua-Italy, will be equipped with a Beam Source (BS) expected to generate a 40A negative ion beam to the energy of 1MeV (values for D). It is composed of an ion source to generate a H or D plasma and of a multiple-Aperture multiple-grid (MAMuG) Accelerator [1], where ions extracted are accelerated in five consecutive stages of 200 kV each. The engineering and fabrication of the beam source for MITICA BS started in October 2018, when a contract between Fusion for Energy (F4E) and Alsymex (France) was signed. Presently, all the components and sub-assemblies have been manufactured, except for a few non-critical items. Assembly of the accelerator is well advanced and ion source is on-going. A review of the fabrication of the main components and aspects of their assembly is reported in this paper, with a focus on the main achievements.

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. 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 modelling activities. 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.

• Upgrade of RFX-mod experiment proposed for studies of RFP and Tokamak physics. • High-performance polymers and surface coatings for in-vessel components insulation. • In-vessel components made from Metal Additive Manufacturing . • Effective collaboration between research institutions and industries. • Support of local authorities to scientific research and technological innovation. A substantial modification of the toroidal complex of the RFX experiment, named RFX-mod2, is currently under completion, involving the whole core system of the machine and in particular the vacuum vessel, the entire plasma facing components and a wide set of in-vessel diagnostic systems . The combination of challenging requirements, in terms of electrical insulation and vacuum compatibility of in-vessel components, and stringent geometrical constraints to comply with interfaces to existing machine components, in particular external coils and diagnostic systems, called for the adoption of novel technological solutions developed in collaboration with local industries in the framework of an industrial innovation project co-funded by Regione Veneto. The paper presents an overview of the technological solutions implemented and tested during the manufacturing phase of the new components of the RFX-mod2 machine complex.

An improvement for the vacuum system of the multidriver radio frequency (RF) prototype negative ion source SPIDER has been developed, to allow operating at high RF power, while minimizing the breakdown probability on the outside of the plasma source. A first-of-its-kind large nonevaporable getter (NEG) pump, based on a modular design of 384 cartridges totaling about 350 kg of ZAO® alloy (composed by Zr-Ti-V-Al) with an installed pumping speed at a room temperature of 330 m3/s for hydrogen, will complement the existing pumping system, based on eight cryogenic pumps and four turbomolecular pumps totaling about 90 m3/s in hydrogen. The vessel pressure during absorption is expected to be between 20 and 40 mPa, while during the getter regeneration, the peak operating pressure will be around 2 Pa. The NEG pump will use an additional vessel module, with integrated thermal shields to protect the in-vessel components during activation and regeneration of the pump, to be carried out at temperatures in the range of 550–600 °C. Integrated thermal analyses were carried out to verify the low heat load on pre-existing in-vessel components with a low limit of acceptable temperature, and to provide boundary conditions for the mechanical verifications of the pump structure. The scenario of cyclic hydrogen load was calculated considering the SPIDER operation modes, the expected gas throughput, and the cumulated load to the pump, to determine the regeneration temperature and auxiliary pumping necessary to make the regeneration duration compatible with the high availability of the system. The upgrade of the auxiliary pumping system is also described, as well as the mitigation of air or water exposure risk during regeneration of the NEG pump.

Two heating Neutral Beam Injectors (NBIs), required for plasma heating and current drive, are foreseen for ITER operation. Each beam will be generated by a 40A current of Deuterium negative ions, accelerated up to the specific energy of 1MeV and then neutralized, delivering to the plasma a power up to 16.5 MW each. The beam source (BS) will be constituted by an RF-driven negative ion source at –1 MV potential and by an electrostatic accelerator (consisting of 5 stages at intermediate potentials). All components will be installed in a vacuum vessel, together with a high-capacity cryo-pumping system that controls the background gas pressure.In order to validate the ITER NBI design and address all the outstanding issues related to these demanding requirements, a full-scale prototype called MITICA (Megavolt ITER Injector & Concept Advancement) is under construction in Padova at Consorzio RFX. Voltage insulation in vacuum and/or very low-pressure gas on a single gap is indeed one of the expected issues that MITICA will have to deal with. An effective solution for increasing the voltage holding capability of the system consists in the use of an additional intermediate electrostatic shield, biased at an intermediate potential, placed between the ion source and the vacuum vessel. In this paper, the electrostatic design of the shield is presented, by considering the voltage holding capability. A novel 3D version of a numerical tool, called Voltage Holding Prediction Model (VHPM), is applied to the additional intermediate shield design to assess the expected voltage holding capability of the experiment in high vacuum.

Three years of experiments on SPIDER allowed characterization of the main features of the source plasma and of the negative ion beam, in the original design configuration. For the large dimensions of the source chamber, and of the extraction area, the investigation of the single-beamlet currents and of the source plasma uniformity had to be carried out to extend the knowledge gained in smaller prototype sources. The configuration of the multiple RF drivers and filter field topologies were found to cause a peculiar behavior in the plasma confinement in the drivers, creating left-right asymmetries which were also visible in the extracted negative ion currents, even after the early implementation of a new scheme of plasma-grid current send and return busbars that greatly improved performance at high filter fields. The plasma properties in the driver and expansion region as well as the positive ion energy at the extraction region were studied in different experimental conditions, and interpreted also with the support of numerical models, suggesting that an improved plasma confinement could contribute to the increase of the plasma density, and to a certain extent to a lowering of the plasma potential profile; both effects shall contribute to increase the presence of cold negative ions for the formation of low-divergence beamlets. Early results related to unwanted RF discharges on the back of the plasma source and the gas conductance of the beam source suggested the reduction of the vessel pressure as mitigation, leading to the definition of a new pumping system. The difficulties related to the simultaneous operation, stable control and high-power operation of multiple RF self-oscillating vacuum tube based RF generators were an unambiguous obstruction to the experimentation, calling for the implementation of RF solid-state amplifiers. The initial tests related to caesium management, the non-uniform plasma properties at different locations across the plasma grid, and the challenges in the measurement of the current and divergence of the accelerated beamlet, unambiguously resulted in the need of new diagnostic systems to investigate with better resolution the spatial uniformities. This contribution summarises how the main experimental findings in the previous experimental campaigns are driving modifications to the SPIDER experiment, during the present shut down, in view of future operations.

An overview of the recent operations and the main results of cesium injection in the Source for the Production of Ions of Deuterium Extracted from Rf plasma (SPIDER) negative ion source are described in this contribution. In experiments without cesium injection, all SPIDER plants were tested to verify the basic expectations on the operational parameters (e.g., electron cooling effectiveness of magnetic filter field) and to determine its operational region. For beam properties, it was shown that the current density varies across the beam in the vertical direction. In preliminary cesium experiments, the expected increase of negative ion current and simultaneous decrease of co-extracted electrons were found, along with the influence of the control parameters (polarization of the plasma electrodes, magnetic filter field) on the SPIDER beam uniformity in the horizontal and vertical directions. It was shown that non-Gaussian tails can be identified in the angular distribution on the plane perpendicular to the beam propagation direction. Stray particles, nonhomogeneous beam and large divergence might result in unexpected heat and particle loads over ITER neutral beam injector (NBI) accelerator grids; it is the goal of SPIDER to assess and possibly to identify suitable methods for controlling these beam features. A major shutdown, planned for late 2021, to solve the issues identified during the operation and to carry out scheduled modifications, is outlined. Such improvements are expected to allow SPIDER to pursue the ITER requirements in terms of negative ion current, electron-to-ion ratio, and beam duration.

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.

The electrical insulation of the Megavolt ITER Injector and Concept Advancement (MITICA) beam source (BS) at 1 MV in vacuum is a challenging issue, which could not be fully addressed so far on the basis of experimental results and of theoretical models available in literature. A specific high-voltage (HV) test campaign is being prepared to validate and optimize the voltage holding capability of the BS insulation under realistic conditions, using full-size mockup electrodes reproducing in detail the geometry of the BS and accelerator. The proposed test strategy will address both the single-gap and the multistage insulation, so as to obtain a verification of voltage holding at 1 MV before the installation of the real components. This approach is intended to reduce the risk related to the HV insulation at 1 MV and, if necessary, to allow the development of effective corrections. In this article, the test motivations and requirements are defined, and the electrode implementation and diagnostic setup are described. Finally, the test configurations and the experimental procedure are discussed.

The negative-ion based neutral beam injector for heating and current drive of the ITER plasma (ITER HNB) is under development, at present focusing on the optimization of the full-scale plasma source in the SPIDER test stand. The production of H − or D − ions in the ion source is based on the low work function surfaces obtained by caesium evaporation. This paper describes the caesium conditioning procedure and the corresponding beam performances during the first operation of SPIDER with caesium. Technical solutions to overcome present limitations of the test stand are described. The influence of source parameters on the caesium effectiveness was investigated in short beam pulse operation; with total radio-frequency (RF) power of 400 kW and filling pressure below 0.4 Pa, and a limited number of extraction apertures, a negative ion current density of about 200 A m −2 was extracted in hydrogen, with beam energy lower than 60 keV. Beam optics and beam uniformity were assessed thanks to the acceleration of isolated ion beamlets. A possible procedure to accelerate a uniform beam was demonstrated at low RF power. The results obtained in this first investigation provided key indications on the operation of one of the largest existing sources of accelerated negative hydrogen-like ions.

The article will present and discuss the issues found in Source for Production of Ion of Deuterium Extracted from RF plasma (SPIDER) experiment with the operation of the radio frequency (RF) generators based on self-excited tetrode oscillators and the advantages expected from their replacement with RF solid-state amplifiers. The assessment of the solution based on new solid-state amplifiers for both SPIDER and Megavolt ITER Injector and Concept Advancement (MITICA) experiments operating in Padua, Italy, in the Neutral Beam Test Facility, will be described, outlining the main design challenges and open points.

Source for the production of ions of deuterium extracted from a radio frequency plasma (SPIDER) is a full-size negative ion source and a multibeamlet 100-kV accelerator, operational since 2018. It serves as the test bed for the full development of the heating neutral beam system for megavolt ITER injector and concept advancement (ITER). In 2020, the third grounded grid segment broke at the electrodeposited copper layer near one of the four feeders causing leakage. This article reports the redesign development of the SPIDER GG segment. In the first part of the article, several design configurations are proposed and compared to each other. The comparison serves to identify a suitable redesign configuration. A set of coupled analyses, including computational fluid dynamic (CFD) and finite-element method (FEM) analyses, is then performed to finally evaluate the redesigned component in accordance with the ITER structural design criteria for in-vessel components (ITER SDC-IC) criteria.

SPIDER is the 100 keV full-size Negative Ion Source prototype of the ITER Neutral Beam Injector, operating at Consorzio RFX in Padova, Italy. The largest Negative Ion Source in the world, SPIDER generates an RF driven plasma from which Deuterium or Hydrogen negative ions are produced and extracted. At the end of 2021, a scheduled long-term shutdown started to introduce major modifications and improvements aiming to solve issues and drawbacks identified during the first three years of SPIDER operations. The first action of the shutdown period was the disassembly and characterization of the SPIDER beam source after removal from the vacuum vessel and its placement inside the clean room. Each component was carefully assessed and catalogued, following a documented procedure. Some source components, i.e., the Plasma Grid, Extraction Grid and Bias Plate, revealed the presence of different and non-uniform red, white and green coatings that might be correlated to back-streaming positive ions impinging on grid surfaces, electrical discharges and caesium evaporation. Thus, several analyses have been carried out to understand the nature of such coatings, with the study still ongoing. The evidence of caesium evaporation and deposition on molybdenum-coated SPIDER components, such as the formation of oxides and hydroxides, is demonstrated through surface characterization analyses with the use of the Scanning Electron Microscope (SEM), X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS).

The SPIDER experiment is operating at the ITER Neutral Beam Test Facility PRIMA since June 2018 to study the operation of a full ITER-size negative ion source. In order to increase the negative ion extracted current and reduce the co-extracted electron current, caesium is evaporated inside the source. In fact, the adsorption of Cs onto the plasma grid, i.e. the first grid of the extraction system, lowers the surface work function, enhancing the probability of surface conversion of impinging hydrogen particles into negative ions. At the SPIDER test facility, Cs will be evaporated by heating a liquid Cs reservoir in three Cs ovens located at the back-plate of the ion source. The ovens must assure a controllable and reproducible evaporation. A SPIDER Cs oven prototype was procured and tested in the CAesium Test Stand (CATS) at PRIMA site to validate the design. Following the experimental tests, few improvements were introduced in the final design of the three Cs ovens for the SPIDER source. Then, the ovens were procured in 2019 and commissioned before their installation in SPIDER in 2020. This paper reports the functional tests of the SPIDER Cs ovens and of their embedded diagnostics. These tests were carried out at CATS, with the aim of characterizing the three ovens, in order to validate them for their use in SPIDER in the experimental campaign with caesium evaporation planned for the end of 2020. The ovens were successfully tested and characterized in terms of evaporation rate for different oven temperatures. Moreover, the three ovens show a good reproducibility and reliability.