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.

In the design of complex facilities and plants, such as those involved for fusion experiments, it is often very challenging to create detailed global structural models in the design phase, able to represent the structure as a whole, and at the same time to take into account the interaction among different components. Indeed, different components are characterized by a different scale and hence require a different level of detail. This work presents an integrated global FE model of the MITICA plant (Padua, IT) for the study of the behaviour and interaction of the structural elements, with particular attention to the final configuration after the installation. The model thus takes into account all the interactions between the various parts, initially designed in a decoupled way, with the aim of verifying, both from a static and dynamic point of view, the main elements and the interface forces between the components. Different constraint configurations have been taken into account according to the possible kinematic behaviour that the plant can assume, and depending also on the use of the facility. A single integrated global model allows also to identify the most critical elements of the structure, and hence to realize detailed sub-models for the local verification, and to eventually design a proper monitoring system.In conclusion, this work demonstrates the importance of having an integrated model, and proposes a strategy specifically tailored for the MITICA plant. This work lays also the foundation to develop Digital Twin system for fusion experiment plant.

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.

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.

Source for production of ion of deuterium extracted from rf plasma (SPIDER) is the 100 keV negative ion source prototype of the ITER neutral beam injector. The cooling plant is one of the SPIDER auxiliary systems where ultrapure water (UPW) is used as the cooling media, thanks to its advantageous properties such as good cooling performance and high resistivity. Water resistivity needs to be maintained above certain values during SPIDER operation to enable the electrical insulation of in-vessel components that operate at different voltages. Nonetheless, degradation of water resistivity was observed in some circuits during operation. Water resistivity degradation had a negative impact on the SPIDER experimental campaign since it limits cooling plant availability. To understand the cause for the observed water degradation, sampling points were installed to sample water during operation. Ad hoc experiments have been performed on the worst affected circuit, and water samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Circulation experiments were conducted at constant temperature, pressure, and flow rate to assess water degradation by monitoring water conductivity increase. ICP-MS analyses on water from the worst affected circuit revealed the presence of Cu in relevant quantities and Zn. Corrosion induced by the presence of a non-compatible component with SPIDER requirements was found. The identified component was made of a galvanized steel plate, carbon steel, and brass. This was found responsible for the contamination of the cooling loop and its water properties' degradation.

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.

The SPIDER in-vessel actively cooled components shall be drained in case of major maintenance to limit, as much as possible, atmospheric corrosion inside the circuits, to prevent water spreading, and finally to allow the execution of vacuum leak tests of the components before re-installation of the SPIDER Beam Source (SBS) inside the Vacuum Vessel. Draining of hydraulic circuits is first foreseen by gravity after opening the end connections of hydraulic circuits and then by blowing out with nitrogen. Injection of pressurized nitrogen is simulated by means of one-dimensional and three-dimensional models to perform transient two-phases analyses in order to calculate the minimum required inlet pressure and estimate the process time.

The voltage holding capability of the MITICA Beam Source at 1 MV is a very challenging issue, which could not be fully addressed so far on the basis of experimental results and of theoretical models available in literature. This paper describes a specific HV test campaign which is planned to be implemented in the MITICA Vacuum Vessel using mock-up electrodes reproducing in detail the geometry of the Beam Source and Accelerator. The test set-up will incorporate all essential features both of the single-gap and of the multi-stage insulation, so as to obtain reliable data on 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 to develop the most effective solutions. The tests will be performed in the MITICA Vessel (already available in the NBTF site) both in vacuum and in low-pressure gas. In the paper, the test strategy and requirements are first introduced. Then the sequence of test configurations is defined. Finally, some design solutions for the electrode realization are described and a preliminary plan together with a list of test equipment is given.

To reach fusion conditions and control the plasma configuration in ITER, the next step in tokamak fusion research, two neutral beam injectors (NBIs) will supply 16.5 MW each, by neutralizing accelerated negative hydrogen or deuterium ions. The requirements of ITER NBIs (40A/1 MeV D-ions for <1 h, 46A/870 keV H-ions for <1000 s) have never been simultaneously attained. So in the Neutral Beam Test Facility (NBTF, Consorzio RFX, Italy) the operation of the full-scale ITER NBI prototype (MITICA) will be tested and optimised up to full performances, focussing on accelerator (including voltage holding), beam optics, neutralisation, residual ion removal. The NBTF includes also the full-scale prototype of the ITER NBI source with 100 keV particle energy (SPIDER), for early investigation of: negative ion production and extraction, source uniformity, negative ion current density and beam optics. This paper will describe the main results of the first two years of SPIDER operation, devoted to characterizing plasma and beam parameters, including investigation of RF-plasma coupling efficiency and magnetic filter field effectiveness in reducing co-extracted electrons. SPIDER is progressing towards the first caesium injection, which aims at increasing the negative ion density. A major shutdown, planned for 2021, to solve the issues identified during the operation and to carry out programmed modifications, will be outlined. The installation of each MITICA power supply and auxiliary system is completed; in-vessel mechanical components are under procurement by Fusion for Energy (F4E). Integration, commissioning and test of the power supplies, procured by F4E and QST, as the Japanese Domestic Agency (JADA), will be presented. In particular, 1.0MV insulating tests were carried out step-by-step and successfully completed. In 2020 integrated tests of the power supplies on the accelerator dummy load started, including the assessment of their resilience to accelerator grid breakdowns using a short-circuit device located in vacuum. The aggressive programme, to validate the NBI design at NBTF and to meet ITER schedule (requiring NBIs in operation in 2032), will be outlined. Unfortunately, in 2020 the coronavirus disease infection affected the NBTF activities. A solution to proceed with integrated power tests despite the coronavirus is presented.

The SPIDER experiment is aiming to test and optimize the full scale prototype of negative ion source for the ITER Heating Neutral Beam Injector, required to heat up ITER plasma with 16.5 MW injected power. The experiments started in May 2018 and are presently running, aiming to achieve the target parameters of 355A/m2 H2 and 285A/m2 D2 ion current density accelerated up to 110 kV for a pulse duration up to 1 h. The in-vessel actively cooled components of SPIDER experiment shall be drained and dried in case of major maintenance to limit as much as possible atmospheric corrosion inside the hydraulic circuits and components, to prevent water spreading, and to allow the execution of vacuum leak tests of the hydraulic circuits before re-installation inside the vacuum vessel. In particular drying of SPIDER Beam Source is needed for the beginning of SPIDER shutdown phase, scheduled in summer 2021. This paper presents the conceptual design of a dedicated drying system, tailored on the SPIDER Beam Source needs and requirements.

The ITER Neutral Beam Test Facility, in an advanced stage of construction in Padova, includes the installation, tests, and optimization of the full prototype of the ITER Heating Neutral Beams injector (HNBs), named MITICA. The MITICA Neutral Beam Injector will host its main components in a SS304L vacuum vessel composed of two modules, connected between them on site: the Beam Source Vessel (cubic shape of 5 m side and 67 tons weight) containing the Beam Source and the Beam Line Vessel (section 4.5 m x 4.5 m, length 11 m and 76 tons weight) containing the Beam Line Components and the Cryopumps. The manufacturing is described, going through the FE analyses performed to assess the structural integrity, the materials selection, the welding qualifications, the implementation of the double barrier sealings and the control of the deformations. The Factory Acceptance Tests of the individual vessels are presented, including their Helium Leak Tests. The two vessels have been assembled on-site inside the MITICA bio-shield. The main outcomes of on-site final assembly and Site Acceptance Tests are described. Both the vessels have been detail designed, manufactured, installed and tested by De Pretto Industried, from Schio, (VI) Italy, supporting fusion technology’s applications from more than 50 years. The design and the technical support were provided by Consorzio RFXa, while the procurement was managed by Fusion For Energye.

The ITER Neutral Beam Test Facility includes development, testing, and optimization of the full prototype of the ITER Heating Neutral Beam injectors (HNBs), named MITICA. A 40 MW precursor D-/H- beam will be produced and then neutralised and filtered along the beamline, aiming to obtain 18 MW D-0/H-0 beam at the calorimeter and 16.5 MW during operation into the plasma. A gas neutraliser and an electrostatic residual ion dump will process the beam together with a calorimeter, necessary for commissioning and conditioning in HNBs, and for beam dumping in MITICA. The three beamline components are formed by in-vacuum actively cooled panels made of seamless CuCrZr drilled bars and swirl tubes without armour directly exposed to beam particles. Heat fluxes up to 20 MW/m(2) will be exhausted through water-cooled channels operating up to 1 h in subcooled boiling. Critical heat flux and fully developed boiling were verified using semi-empirical correlations validated for divertor fingers and limiter plates. These correlations were implemented in 1D-3D customised codes simulating local nucleate boiling heat transfer conditions. Moreover, the panels were verified against creep-fatigue by simulating plastic strains under the expected cyclic thermal loads. During beam operation, the beamline components performance will be monitored using 700 embedded thermo-mechanical sensors for protection and for ITER HNBs requirement verification. The design solutions were developed consistently with intermediate inspections during assembly to limit repairs, modification of compensation members for tolerance control in dimensional chains, remote handling operations, and nuclear radiation level at ITER. The design of coolant-vacuum barriers was adapted to implement corrosion, welding, and inspection requirements. Prototypes implementing different manufacturing solutions were produced by three potential suppliers of MITICA beamline components. This strategy led to an early assessment of production reliability and cost-effective manufacturing techniques based on proven industrial expertise. Then, the supplier selected for the procurement of the beamline components will be supported with addressed performances during the follow up.

MITICA, the full scale prototype of ITER Heating Neutral Beam Injector required to heat up ITER plasma with 16.5 MW injected power, is under realization at the Neutral Beam Test Facility (NBTF) in Padova (Italy) with the contributions of JApanese and EUropean Domestic Agencies (DADA and EUDA, respectively). The objective of MITICA is to produce a 16.5 MW neutral beam, obtained by accelerating negative Deuterium ions up to 1 MeV for a total ion current of 40 A and then neutralized. MITICA Power Supply (PS), installed from 2016 to 2019, includes several non-standard equipment, with ratings well beyond the present industrial standard for insulation voltage level (-1 MVdc) and dimensions: the Acceleration Grid Power Supply (AGPS), composed of five DC Generators (DCG) rated for -200 kVdc each, connected in series to produce -1 MVdc acceleration voltage; the Ion Source and Extraction Power Supply system (ISEPS); the large air insulated Faraday cage (High Voltage Deckl, HVD1) hosting ISEPS and connected to the Transmission Line (TL) through an air-to-SF6 Bushing (High Voltage Bushing Assembly, HVBA); a 100 m gas (SF6) insulated TL, connecting AGPS and ISEPS to the beam source installed inside the vacuum vessel through the SF6-to-vacuum HV Bushing (HVB). The definition of the interfaces both between components supplied by the different DA's and towards the buildings has been studied and finalized as far as possible during the design phase. Nevertheless, during the installation phase some issues emerged and had to be solved, minimizing modifications of the components already manufactured. The paper deals with the experience gained during the installation activities, focusing on solutions to interface the aforementioned equipment with NBTF buildings according to the stringent dimensional requirements and to the electrical insulation issues of the TL from the buildings. In particular, the solutions adopted to realize the electrical and mechanical interfaces between the TL and the HVBA are described in detail.

The requirements of ITER neutral beam injectors (1 MeV, 40 A negative deuterium ion current for 1 h) have never been simultaneously attained; therefore, a dedicated Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Padova, Italy). The NBTF includes two experiments: SPIDER (Source for the Production of Ions of Deuterium Extracted from Rf plasma), the full-scale prototype of the source of ITER injectors, with a 100 keV accelerator, to investigate and optimize the properties of the ion source; and MITICA, the full-scale prototype of the entire injector, devoted to the issues related to the accelerator, including voltage holding at low gas pressure. The present paper gives an account of the status of the procurements, of the timeline, and of the voltage holding tests and experiments for MITICA. As for SPIDER, the first year of operation is described, regarding the solution of some issues connected with the radiofrequency power, the source operation, and the characterization of the first negative ion beam.

The ITER Heating Neutral Beam injector (HNB) will be equipped with a beam source that will provide a negative beam of 40A (H or D). The R&D activities undertaken in Europe to pursue this challenging goal comprise three experiments: -ELISE - the half size ion source experiment operating in IPP Garching -SPIDER - the full size ion source experiment at Neutral Beam Test Facility (NBTF) site in Padua -MITICA - the full size full energy ITER injector prototype being also established at the NBTF The procurement of the beam source for SPIDER started in October 2012, when a contract between Fusion for Energy and Thales, Microwave & Imaging Sub-Systems (as group leader of a Consortium created with CECOM Srl, Galvano-T GmbH and Zanon SpA) was signed. The scope of the contract has been completed in October 2017 with the delivery of the SPIDER beam source at the NBTF site in Padova Italy. A review of the fabrication of the main parts and of the aspects of their assembly is reported in the paper, with a focus on some of the major issue encountered and the achievement of tight alignment tolerances of the accelerator.

To reach fusion conditions and control plasma configuration in ITER, a suitable combination of additional heating and current drive systems is necessary. Among them, two Neutral Beam Injectors (NBI) will provide 33 MW hydrogen/deuterium particles electrostatically accelerated to 1 MeV; efficient gas-cell neutralisation at such beam energy requires negative ions, obtained by caesium-catalysed surface conversion of atoms inside the ion source. As ITER NBI requirements have never been simultaneously attained, a Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Italy), including two experiments. MITICA is the full-scale NBI prototype with 1 MeV particle energy. SPIDER, with 100 keV particle energy, aims at testing and optimising the full-scale ion source: extracted beam uniformity, negative ion current density (for one hour) and beam optics (beam divergence <7 mrad; beam aiming direction within 2 mrad). This paper outlines the worldwide effort towards the ITER NBI realisation: the main results of the ELISE facility (IPP-Garching, Germany), equipped with a half-size source, are described along with the status of MITICA; specific issues are investigated by small specific facilities and by joint experiments at QST and NIFS (Japan). The SPIDER experiment, just come into operation, will profit from strong modelling activities, to simulate and interpret experimental scenarios, and from advanced diagnostic instruments, providing thorough plasma and beam characterisation. Finally, the results of the first experiments in SPIDER are presented, aimed at a preliminary source plasma characterisation by plasma light detectors and plasma spectroscopy.

The SPIDER Beam Source (BS), the first prototype of a full scale ion source for the ITER Heating Neutral Beam injector, was delivered to the Neutral Beam Test Facility (NBTF) site in Padova (Italy) after about five years procurement phase. A huge effort was devoted during the procurement for quality controls and testing at the supplier's workshops. Several activities were also carried out on NBTF site for verification/adjustment of interfaces, solution of still open issues, as well as final tests before and after installation inside the vacuum vessel. The NBTF Team undertook the BS site acceptance tests including: pressure and leak tests of the hydraulic circuits; electrical tests; measurement of magnetic field profiles; functionality tests of diagnostics installed on the BS; checks of grids alignment by means of laser tracker. Accurate positioning of the BS inside the vacuum vessel was performed and various service lines were connected in a tight space. Several improvements were undertaken in order to guarantee reliability and reduce the risks during the commissioning and experimental campaign in 2018. After installation, the integrated commissioning phase was initiated, powering the RF and high voltage circuits, followed by the first operation in vacuum.

The ITER project requires at least two Heating Neutral Beam Injectors (NBIs), each accelerating up to 1 MV a 40 A beam of negative H-/D-.ions, to deliver to the plasma a total power of about 33 MW for one hour. Since these requirements have never been experimentally met, it was recognized necessary to build-up a test facility, named PRIMA including both a full-size negative ion source (SPIDER - Source for Production of Ion of Deuterium Extracted from Rf plasma) and a full prototype of the ITER injector (MITICA - Megavolt ITER Injector & Concept Advancement). This realization is made with the main contribution of the European Union, through the Joint Undertaking for ITER (F4E), the ITER Organization and Consorzio RFX (CRFX) that hosts the Test Facility in Padova, Italy. SPIDER is a Radio Frequency ion source that has the same characteristics foreseen for the ITER NBI but with beam energy limited to 100 keV. The mission of SPIDER is to increase the understanding of the source operation and to optimize the source performance in terms of extracted current density, uniformity and pulse duration. The paper describes the Gas injection and Vacuum System (GVS), from the analysis of requirements to the system commissioning. In particular, it presents the rationale behind the main design choices and specific manufacturing details of the gas injection plant feeding the RF source. Furthermore, the paper describes the interfaces with the SPIDER central interlock and safety systems. Finally, the main results of the GVS commissioning with the control and interlock systems are presented.