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.

A two- and three-dimensional (2D and 3D) Cartesian, three-velocities (3 V), Particle-in-Cell Monte Carlo collisions (PIC-MCC) model of a tandem-type Inductively Coupled Plasma (ICP) discharge is presented. The conditions are similar to those of negative ion sources for fusion applications, i.e., a high absorbed power (on the order of 100 kW), a high-density plasma (typically 5 × 1017 m-3), and a low neutral gas pressure (0.3 Pa) in a large volume vessel with a magnetic field barrier. We show that the plasma transport properties may be calculated with sufficient accuracy by implementing a larger than the real value of the vacuum permittivity in Poisson's equation. This approach is appropriate for nonturbulent plasmas, provided that the sheath length is small with respect to the quasi-neutral plasma dimensions. Furthermore, the calculation of the radio-frequency (RF) power coupling with the plasma (which is provided by an external antenna) is simplified by assuming that the electrons in the discharge interact with a uniform RF power profile and that their energy distribution function is Maxwellian in that region. Such approximation is relevant when the electron collision mean-free path is larger than the discharge dimensions and electrons are nonmagnetized. The simulation results are used to describe the plasma transport across the magnetic filter including the role of the Hall current (E × B and diamagnetic drifts), the dynamics of neutrals (notably the question of neutral depletion and physical chemistry), and lastly, the physical mechanisms involved with the extraction of negatively charged particles from the ion source, namely negative ions and electrons.

The negative ion source SPIDER at Consorzio RFX (Italy) is a crucial experimental step towards the development of the Heating Neutral Beam (HNB) injectors for ITER. The goal of SPIDER is to operate and optimize a full-scale negative ion source in hydrogen and deuterium, with particles accelerated up to 100 keV. One of the processes having a considerable effect on the global efficiency of HNB systems is the partial neutralization of negative ions by gas collisions during their acceleration. This process is commonly referred to stripping losses and it can account up to 29% of the extracted current in a full-scale accelerator. The electrons generated by the neutralization of partially-accelerated negative ions can also contribute significantly to the total amount of heat deposited onto the acceleration grids. The investigation of stripping losses in SPIDER can provide useful methods to extrapolate and estimate also for the full scale HNB injector, called MITICA, currently under manufacturing. The purpose of this work is to develop a tool to estimate stripping losses in the accelerator of full scale HNB injectors based on beam emission spectra. The Beam Emission Spectroscopy (BES) technique has been employed to investigate stripping losses during the first SPIDER experimental campaign with caesium evaporation, characterized by larger extracted current densities compared to precaesium operations. Experimental data interpretation required synergy with beam models, necessary to account for the beam-gas interaction processes in the accelerator. In this work we describe the method of analysis and provide first estimates of stripping losses in SPIDER from data obtained during the first caesium operation campaign.

In the road-map for a fully operational ITER Neutral Beam Injector (NBI), SPIDER represents a necessary step to test its negative ion source design since the required performances have never been achieved simultaneously by any previous experiment. The smaller scale of SPIDER to the full ITER NBI prototype (named MITICA, which will operate in the same test facility in Padova) allows more flexibility in the experiments and, taking advantage of its larger set of diagnostic, will provide insightful results for the entire project development. This paper is the result of the experience gained within the first two years of SPIDER operation, with the focus on the Ion Source and Extraction Power Supply (ISEPS) system. A review of the system development, of the main outcomes of the first experimental phase and the description of the still open issues, is provided.

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 ITER Heating Neutral Beam (HNB) injector is required to deliver 16.7 MW power into the plasma from a neutralised beam of H-/D- negative ions, produced by an RF source and accelerated up to 1 MeV. To enhance the H-/D- production, the surface of the acceleration system grid facing the source (the plasma grid) will be coated with Cs because of its low work function. Cs will be routinely evaporated in the source by means of specific ovens. Monitoring the evaporation rate and the distribution of Cs inside the source is fundamental to get the desired performances on the ITER HNB. In order to proper design the source of the ITER HNB and to identify the best operation practices for it, the prototype RF negative ion source SPIDER has been developed and built in the Neutral Beam Test Facility at Consorzio RFX. A Laser Absorption Spectroscopy diagnostic will be installed in SPIDER for a quantitative estimation of Cs density. By using a wavelength tunable laser, the diagnostic will measure the absorption spectrum of the 852 nm line along 4 lines of sight, parallel to the plasma grid surface and close to it. From the absorption spectra the line-integrated density of Cs at ground state will be measured. The design of this diagnostic for SPIDER is presented, with details of the layout and of the key components. A preliminary installation of the diagnostic on the test stand for Cs ovens is also described, together with its first experimental results; the effect of ground state depopulation on collected measurements is discussed and partially corrected.

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 halfsize 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 ion source NIO1 (Negative Ion Optimization 1) is a versatile multiaperture H - source capable of continuous regime operation, with the plasma generated by a 2 MHz/2.5 kW radiofrequency (rf) power supply and extraction of nine beamlets. It aims to partly reproduce the conditions of much larger ion sources, built or in construction for the neutral beam injectors of fusion devices, in a compact and modular ion source, where effects of individual source components can be rapidly verified and compared to simulation code results. Several modifications of the magnetic configuration (both inside the ion source and the embedded magnets inside the accelerator grids) were investigated. Saturation of beneficial effect of filter field at large strength (>0.01 T) leads us to use softer magnetic filters, and the advantages of a crossed deflection field are noted. The rf system takes full advantage of the generator bandwidth (0/ + 20 kHz used). The result database and the integration of major diagnostic systems with the control system are also summarized, with some details on voltage holding and beam uniformity.

The development of the kinetic particle model for the extraction region in negative hydrogen ion sources is indispensable and helpful to clarify the H-beam extraction physics. Recently, various 3D kinetic particle codes have been developed to study the extraction mechanism. Direct comparison between each other has not yet been done. Therefore, we have carried out a code-to-code benchmark activity to validate our codes. In the present study, the progress in this benchmark activity is summarized. At present, the reasonable agreement with the result by each code have been obtained using realistic plasma parameters at least for the following items; (1) Potential profile in the case of the vacuum condition (2) Temporal evolution of extracted current densities and profiles of electric potential in the case of the plasma consisting of only electrons and positive ions.

The NIO1 (Negative Ion Optimization, phase 1) experiment is a versatile test bench operated by Consorzio RFX and INFN in the framework of the activities aimed at the enhancement of negative ion sources for production of large ion beams for plasma heating in nuclear fusion devices and accelerator applications. The nominal beam current of 135mA at -60kV is divided into 9 beamlets, with multi-aperture extraction electrodes. The plasma is continuously sustained by a 2MHz radiofrequency power supply, with a standard matching box. A High Voltage Deck (HVD) is placed inside the lead shielding surrounding NIO1 and is fed by an insulation transformer installed in a nearby box. It contains the radiofrequency generator, the gas control, electronics and power supplies for the ion source. A closed-circuit water cooling system was installed for the whole system, with a branch towards the HVD. The present contribution gives a detailed description of the electrical plants and the fast protection systems for NIO1. High voltage and low voltage circuits are present in NIO1. The description of the former include the extraction and accelerator power supplies and the passive devices devoted to protecting the power supplies themselves against possible breakdowns in the accelerator. Low voltage circuits are dedicated to biasing the plasma grid and the bias plate as well as to producing a static magnetic field (magnetic filter field) and are protected against the effect of accelerator breakdowns. A specific system was developed to coordinate the fast switch-off of the power supplies and to trigger the main control unit in case a breakdown is detected in either high-voltage power supplies. Details on the fast protection circuitry and examples of its operation are presented. This work was performed with partial financial support by EUROfusion.

This document reports on failure modes and effects analysis document (FMEA) developed for NIO1 experiment.

NIO1 is a compact radio frequency (RF) ion source designed to generate a 60 kV-135 mA hydrogen negative ion beam and it aims at continuous operation, which implies a detailed thermo-mechanical analysis of the beam-facing components, in particular the accelerator grids. A 3D analysis of the entire NIO1 beam has been performed for the first time with a fully 3D version of EAMCC, a relativistic particle tracking code based on the Monte-Carlo method for describing the transport of particles inside the accelerator and the main secondary particle formation processes responsible of relevant heat loads on the accelerator grids. The main results are presented in this report, after a brief description of the device.

NIO1 (Negative Ion Optimization 1) is a versatile ion source designed to study the physics of production and acceleration of H- beams up to 60 keV. In ion sources, the gas is steadily injected in the plasma source to sustain the discharge, while high vacuum is maintained by a dedicated pumping system located in the vessel. This report presents experimental tests and calculations carried out to characterize the gas flow in NIO1. The three dimensional gas flow in NIO1 is studied in the molecular flow regime by the AVOCADO code. The analysis of the gas density profile along the accelerator considers the influence of effective gas temperature in the source, of the gas temperature accommodation by collisions at walls, and of the gas particle mass. The calculated source and vessel pressures are compared with experimental measurements in NIO1 during steady gas injection. This work was also presented at ICIS conference.

The present report, summarizes the work carried out in 2015 on the NIO1 experiment at Consorzio RFX in Padova, Italy. Experimental campaigns will be presented together with the modelling activities and the activities carried out on various plants.

The paper reports on the results of the activity aimed at the experimental characterisation of the NIO1 RF driver electrical impedance at different frequencies.

The report deals with the breakdown tests to analyse the performance of both the EGPS HV protection circuit and the Fast Protection Unit developed for the NIO1 facility.

The report deals with the design of a HV protection system for the NIO1 facility against arcs in the acceleration and extraction gaps. The system comprises both a HV protection circuit to protect the EGPS during these occurrences and a Fast Protection Unit to coordinate the operation of the NIO1 high voltage power supplies in the case an arc has been detected.

The work reports on the tests performed to identify the current and voltage waveforms of the NIO1 circuit components during induced arc in different test circuit set up. Furthermore a prototype of an arc current sensor circuit to be installed in a fast protection unit (to coordinate the high voltage off of the high voltage power supplies in case of a breakdown) was tested.

The radiofrequency ion source NIO1 has an RF line composed by two strips of copper, air insulated, to connect the matching network to the RF load. In this document the results of a specific campaign aimed to characterize the electrical behaviour of this component are presented. In particular the results of measurement campaign with an impedance analyser are reported to evaluate the inductance and capacitance per meter of the NIO1 RF line. Furthermore results are compared with those obtained following the current literature and by a Finite Element approach.

Negative ion sources are key components of the neutral beam injectors for thermonuclear fusion experiments. The NIO1 experiment is a radio frequency ion source generating a 60 kV-135 mA hydrogen negative ion beam. The beam is composed of nine beamlets over an area of about 40 × 40 mm². This experiment is jointly developed by Consorzio RFX and INFN-LNL, with the purpose of providing and optimizing a test ion source, capable of working in continuous mode and in conditions similar to those foreseen for the larger ion sources of the ITER neutral beam injectors. At present research and development activities on these ion sources still address several important issues related to beam extraction and optics optimization, to which the NIO1 test facility can contribute thanks to its modular design, which allows for quick replacement and upgrading of components. This contribution presents the installation phases, the status of the test facility and the results of the first experiments, which have demonstrated that the source can operate in continuous mode.