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NIO1 is a compact and flexible radiofrequency H- ion source, developed by Consorzio RFX and INFN-LNL. Aim of the experimentation on NIO1 is the optimization of both the production of negative ions and their extraction and beam optics. In the initial phase of its commissioning, NIO1 was operated with nitrogen, but now the source is regularly operated also with hydrogen. To evaluate the source performances an optical emission spectroscopy diagnostic was installed. The system includes a low resolution spectrometer in the spectral range of 300-850 nm and a high resolution (50 pm) one, to study respectively the atomic and the molecular emissions in the visible range. The spectroscopic data have been interpreted also by means of a collisional-radiative model developed at IPP Garching. Besides the diagnostic hardware and the data analysis methods, the paper presents the first plasma measurements across a transition to the full H mode, in a hydrogen discharge. The characteristic signatures of this transition in the plasma parameters are described, in particular the sudden increase of the light emitted from the plasma above a certain power threshold., 3 pages, 2 figures. Contributed paper for the ICIS 2015 conference. Accepted manuscript

Counterintuitive experimental evidences have been observed during High Voltage Direct Current (HVDC) tests of two concave, axial-symmetric, electrodes insulated by large vacuum gaps of 3 and 7 cm with voltages from 150 to 370 kVdc. The dissipation of microdischarge power during the conditioning procedure occurs mostly on the anodic side in a region close to the axis of the system where the electric field is at a minimum, far from the positions where the breakdowns have been observed. The analyses of the phenomena are carried out by comparing the temporal evolution of voltages, currents, pressure, measurements of x-ray energy spectra, and images from infrared and visible light cameras. Numerical simulations, based on ray-tracing algorithm, correctly identify the positions where the power dissipation of microdischarges occurs. A mutual exchange of charged particles in the electrostatic field between electrodes seems a reasonable physical mechanism to interpret the observations. These findings suggest a new perspective to review the current literature and interpret new results considering geometric details which were so far omitted: the areas with the most intense electric field, typically located on the surfaces of the electrodes under test, are not necessarily the sole surfaces involved in the HVDC conditioning in high vacuum.

The PRIMA project aims at the construction of two ITER-NBI facilities in Padova (Italy). The first one is called SPIDER which is negative H/D 100 keV RF source, while the second one (MITICA) will be a full scale 1 MeV deuterium beam injector as the one that will be used in ITER. In order to resolve the horizontal beam intensity profile in MITICA and one of the eight beamlets groups in SPIDER, the Close-contact Neutron Emission Surface Mapping (CNESM) system is being developed. The goal of this device is to reconstruct the D− beam evaluating the map of the neutron emission due to interaction of the deuterium beam with the deuterons implanted in the beam dump surface. For this reason, the CNESM diagnostic, which is based on nGEM detectors for fast neutrons, will be placed right behind the SPIDER and MITICA beam dump, i.e. in an UHV (Ultra High Vacuum) environment. Since the nGEM detectors need to operate at atmospheric pressure a vacuum sealed detector box has been designed to be installed inside the vacuum vessel and able to sustain atmospheric pressure inside. This paper describes the status of the CNESM diagnostic and underlines the different phases followed during the realization and installation of the diagnostic on the SPIDER beam dump as well as its imaging performances.

Using radiofrequency (rf) for heating and levitation of alloy samples is greatly simplified by a passive element, the slotted cold crucible (similar to Faraday shield for rf plasma ion sources), which if properly designed can shape and concentrate the axial (non static) magnetic field profile. The effect is better represented and verified in 3D simulations (very time consuming) as shown here, even if some simplified 1D and 2D models may help understanding. Boundary conditions at large radius are discussed. Moreover, the sensitivity to important parameters (levitated weight vs frequency and power, sizes of crucible, sample conductivity, radius of sample to skin depth ratio) are demonstrated in a schematized 3D geometry, easily parametrized. Finally complete 3D simulation of a realistic design (adequate for clean alloy melting) are also reported.

A magnetized plasma strongly interacts with microwaves, when their angular frequency $\omega$ is around the electron cyclotron resonance (ECR) one, that is $\Omega=e\vert \mathrm{B}^{s}\vert /m_{e}$ , with $\mathrm{B}^{s}$ the static flux density. The case of small ion sources with $\mathrm{B}^{s}\cong 0.5$ to 1 T, where wavelength and plasma chamber radius are comparable, is studied with full wave equations, assuming a collision rate $v$ and a plasma of given density. Wave initially launched in the TE mode from a waveguide (TE 11 for a circular waveguide), aligned with the axis $z$ of a cylindrical plasma chamber and of $\mathrm{B}^{s}$ , are partially converted to $E_{z}$ waves; use of azimuthal symmetry to consider only the $m=1$ component is discussed. Large reflections are sometimes observed as a function of ratio of plasma angular frequency $\omega_{p}(z)$ and $\omega$ , possibly due to model simplification (density constant in $x$ and $y$ ). Extension to off-axis waveguides (perhaps rectangular or coaxial) is mentioned.

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 High Voltage Padova Test Facilities (HVPTF) is a R&D project focused on understanding the physical processes behind breakdown and micro-discharges in high-voltage insulation, as well as developing new diagnostics, models, and operational modes to address these challenges and aid in the stable operation of MITICA. The X-rays produced during breakdown have a spectrum that extends from low energy (keV) up to several hundreds of keV and a flux that can reach 106 photons/cm2s. This paper shows the development and preliminary results of a newly designed X-Ray diagnostic based on Gas Electron Multiplier (SXR-GEM). This detector is able to stand very high rate (> MHz) in single photon counting mode and can cover the energy range from 3-50 keV. The XR-GEM detector is equipped with anodic pads (256 pads 6x6 mm2) readout with a new data acquisition system called GEMINI, which gives the possibility to obtain a counting rate of several MHz and sub-ms time resolution together with mm spatial resolution. Preliminary measurement shows that this kind of diagnostic is able to reconstruct the time evolution of the discharge at very high rates and to provide X-rays imaging.

In nuclear physics experiments, high voltage platforms are fundamental both for initial acceleration of ions and for mass spectrometers of exotic nuclei. These devices require a well defined beam energy, that poses challenging requests to high voltage design, rising the question of which electrode design rules are necessary for a stable voltage operation, since tolerable energy rms relative fluctuation are in the 10–5 order. Dependence of mass resolution from several platform component perfomance is discussed, including the ion source specifications, the ion beam emittance and the platform voltage stability, related to ripple from power supplies, and to microdischarges, whose possibility is to be suppressed with accurate electrode and acceleration tube design. In the example of the High Resolution Mass Spectrometer under construction in Legnaro, a comparatively moderate voltage V p ≅ 120 kV together with a reduced energy spread (and emittance) in the injection line gives the desired mass resolution of 1 : 20000, as shown by theory and simulation; some platform features are also described.

The reliability of HV insulation across a single gap, in vacuum and low-pressure gas, is known to be a critical issue to be addressed in the development and realization of the electrostatic accelerator for the ITER Neutral Beam Injector (NBI). This paper describes recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF) with the aim of developing an adequate high-voltage insulation system for the MITICA accelerator, the prototype for the ITER NBI. The experiments were carried out on a double-polarity configuration, using sphere-to-sphere electrodes, separated by 30 mm gap. The results showed that the onset of microdischarges is not solely dependent on the cathodic electric field, but also depends on the applied voltage, which determines the energy of the emitted electrons hitting the anode. Furthermore, this phenomenon seems to be dependent on a mutual exchange of charged particles between specific accumulation points on both electrode surfaces. The outcome of these experiments is reported and analyzed with both analytical and numerical models.

In the vast literature on waves and plasma interaction, the skin depth, the stochastic heating and the radiofrequency (rf) magnetic field effects emerge as important factors in simulations, still relying on effective collision rate models: a closed form and universal graph are given for stochastic heating. A simulation code based on iteration between plasma and rf calculations is recalled, and typical results are shown, for a simple induction ion source design.

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.

Heating neutral beam (HNB) injectors, necessary to achieve burning conditions and to control plasma instabilities in ITER, are characterized by such demanding parameters that a neutral beam test facility (NBTF) dedicated to their development and optimization is being realized in Padua (Italy) with direct contributions from the Italian government (through Consorzio RFX as the host entity) and the ITER international organization (with kind contributions from the ITER domestic agencies of Europe, Japan and India) and technical and scientific support from various European laboratories and universities. The NBTF hosts two experiments: SPIDER, devoted to ion source optimization for the required source performance, and MITICA, the full-size prototype of the ITER HNB, with an ion source identical to SPIDER. This paper gives an overview of the progress towards NBTF realization, with particular emphasis on issues discovered during this phase of activity and on solutions adopted to minimize the impact on the schedule and maintain the goals of the facilities. The realization of MITICA is well advanced; operation is expected to start in 2023 due to the long procurement time of the in-vessel mechanical components. The beam source power supplies, operating at 1 MV, are in an advanced phase of realization; all high-voltage components have been installed and the complex insulation test phase began in 2018. At the same time, construction and installation of SPIDER plant systems was successfully completed with their integration into the facility. The mechanical components of the SPIDER ion source were installed inside the vessel and connected to the plants. Integrated commissioning with the control, protection and safety systems ended positively and the first experimental phase has begun. The first results of the SPIDER experiment, with data from operational diagnostics, and the plans for the 1 MV insulation tests on the MITICA high-voltage components are presented.

Nell’agosto del 1961, a Zagabria per la mostra nove tendencije, si riunì un gruppo di artisti, provenienti da diverse aree culturali: italiana, francese e tedesca. Le neoavanguardie così costituitesi erano maturate nello stesso ambiente culturale, quello della rivista “Čovjek i Prostor” (Uomo e spazio), che era stato un laboratorio anche per l’attività critica di Vera Horvat Pintarić (1926) e principalmente di Matko Meštrović (1933) e Radoslav Putar (1929-1994). Osservando i nuovi fermenti artistici di ambito italiano, francese e tedesco, questi ultimi due studiosi contribuirono, ognuno con un peso diverso, a elaborare l’ideologia originaria della manifestazione internazionale di nove tendencije.

The ITER project requires additional heating by two neutral beam injectors, each accelerating up to 1 MV a 40 A beam of negative deuterium ions for one hour. Such requirements have never been reached, so it was decided to build in Padova a facility (PRIMA) that hosts two experimental devices: SPIDER, a 100 kV negative H/D RF beam source, and MITICA, a full-scale injector for the ITER NBI. SPIDER has begun operation in 2018, while MITICA is expected to start after 2020. In both devices the accelerated deuterium beam impinges on an actively cooled beam dump used to stop the deuterons. Detection of fusion neutrons produced between beam–deuterons and dump-embedded deuterons will be used as a means to resolve the horizontal beam intensity profile. A neutron detection system called Close-contact Neutron Emission Surface Mapping (CNESM) is installed right behind the SPIDER beam dump, with the aim to provide the neutron emission map of the beam dump surface. The core of this diagnostic system is an nGEM (neutron-Gas Electron Multiplier) detector which will be able to reconstruct the fast neutron beam profile with an efficiency of about 10−4. A crucial point in order to correctly reconstruct the profile of the deposited D − power is the directionality discrimination capability of the detector. This paper reports on the results of the characterization of the nGEM directionality capabilities, performed at the Frascati Neutron Generator (FNG) using 2.5 MeV neutrons, before installation of the detector inside the SPIDER vacuum vessel.

The ITER neutral beam test facility under construction in Padova will host two experimental devices: SPIDER, a 100 kV negative H/D RF beam source, and MITICA, a full scale, 1 MeV deuterium beam injector . SPIDER will start operations in 2016 while MITICA is expected to start during 2019. Both devices feature a beam dump used to stop the produced deuteron beam. Detection of fusion neutrons produced between beam-deuterons and dump-implanted deuterons will be used as a means to resolve the horizontal beam intensity profile. The neutron detection system will be placed right behind the beam dump, as close to the neutron emitting surface as possible thus providing the map of the neutron emission on the beam dump surface. The system uses nGEM neutron detectors . These are Gas Electron Multiplier detectors equipped with a cathode that also serves as neutron–proton converter foil. The cathode is designed to ensure that most of the detected neutrons at a point of the nGEM surface are emitted from the corresponding beamlet footprint (with dimensions of about 40×22 mm2) on the dump front surface. The size of the nGEM detector for SPIDER is 352 mm×200 mm. Several smaller size prototypes have been successfully made in the last years and the experience gained on these detectors has led to the production of the full size detector for SPIDER during 2014. This nGEM has a read-out board made of 256 pads (arranged in a 16×16 matrix) each with a dimension of 22 mm×13 mm. This paper describes the production of this detector and its tests (in terms of beam profile reconstruction capability, uniformity over the active area, gamma rejection capability and time stability) performed on the ROTAX beam-line at the ISIS spallation source (Didcot-UK).

Neutral beam injection (NBI) for plasma heating and current drive is necessary for International Thermonuclear Experimental reactor (ITER) tokamak. Due to its various advantages, a radio frequency (RF) driven plasma source type was selected as a reference ion source for the ITER heating NBI. The ITER relevant RF negative ion sources are inductively coupled (IC) devices whose operational working frequency has been chosen to be 1 MHz and are characterized by high RF power density (~9.4 W cm−3) and low operational pressure (around 0.3 Pa). The RF field is produced by a coil in a cylindrical chamber leading to a plasma generation followed by its expansion inside the chamber. This paper recalls different concepts based on which a methodology is developed to evaluate the efficiency of the RF power transfer to hydrogen plasma. This efficiency is then analyzed as a function of the working frequency and in dependence of other operating source and plasma parameters. The study is applied to a high power IC RF hydrogen ion source which is similar to one simplified driver of the ELISE source (half the size of the ITER NBI source).

The realization of the ITER Neutral Beam Test Facility (NBTF) and the start the experimental phase are important tasks of the fusion roadmap, since the target requirements of injecting to the plasma a beam of Deuterium atoms with a power up to 16.5 MW, at 1MeV of energy and with a pulse length up to 3600s have never been reached together before. The ITER NBTF, called PRIMA (Padova Research on ITER Megavolt Accelerator), is hosted in Padova, Italy; it includes two experiments: MITICA, the full-scale prototype of the ITER injector and SPIDER, the full-size negative ion source. The realization promoted by the ITER organization is carried out with the contribution of the European Union, channeled through the Joint Undertaking for ITER (F4E), of the Consorzio RFX which hosts the Test Facility, the Japanese and the Indian ITER Domestic Agencies (JADA and INDA) and several European laboratories, such as IPP-Garching, KIT-Karlsruhe, CCFECulham, CEA-Cadarache. The early start of operation of PRIMA experiments is urgent because sufficient experimental time is necessary to face and solve the issues related to the achievement of the desired performance in time for the ITER operation, requiring NBI since the beginning. Substantial progresses have been recently achieved: the buildings construction, begun in October 2012, has been completed by the end of 2015 and the installation of some components has been started since the end of 2014. The SPIDER realization is well advanced: the installation phase is proceeding in good agreement with the general plan; it is expected to be almost completed by the end of 2016. In parallel, the commissioning of the SPIDER power supply (PS) and auxiliary plant systems is being proceeding. Tests at full power and remote control are planned, including also those addressed to reproduce the grid breakdowns and to test the relevant protections. The design of the MITICA injector components was completed in 2015their procurement is being made through a number of tenders, some of them already launched. The HV Power Supply system of the MITICA 1MV accelerator, provided by JADA, was delivered on site in December 2015. The challenging installation of these components, including the step-up transformers and the SF6 gas insulated transmission line, started soon after and will go on throughout 2016. The present phase, with the PRIMA buildings continuously filled with new components, with the installation activities progressing and with also the commissioning and testing phase starting represents a substantial step forward toward the main target. The paper will describe the main challenges the Project Team has dealt during this phase and the important feedback derived for the ITER HNB systems both from the technical and the organizational standpoints.

SPIDER is one of two projects of the ITER Neutral Beam Test Facility under construction in Padova, Italy, at the Consorzio RFX premises. It will have a 100 keV beam source with a full-size prototype of the radiofrequency ion source for the ITER neutral beam injector (NBI) and also, similar to the ITER diagnostic neutral beam, it is designed to operate with a pulse length of up to 3600 s, featuring an ITER-like magnetic filter field configuration (for high extraction of negative ions) and caesium oven (for high production of negative ions) layout as well as a wide set of diagnostics. These features will allow a reproduction of the ion source operation in ITER, which cannot be done in any other existing test facility. SPIDER realization is well advanced and the first operation is expected at the beginning of 2018, with the mission of achieving the ITER heating and diagnostic NBI ion source requirements and of improving its performance in terms of reliability and availability. This paper mainly focuses on the preparation of the first SPIDER operations - integration and testing of SPIDER components, completion and implementation of diagnostics and control and formulation of operation and research plan, based on a staged strategy

In spite of the wide use of RF sources, a complete understanding of the mechanisms regulating the RF-coupling of the plasma is still lacking so self-consistent simulations of the involved physics are highly desirable. For this reason we are developing a 2.5D fully kinetic Particle-In-Cell Monte-Carlo-Collision (PIC-MCC) model of a cylindrical ICP-RF source, keeping the time step of the simulation small enough to resolve the plasma frequency scale. The grid cell dimension is now about seven times larger than the average Debye length, because of the large computational demand of the code. It will be scaled down in the next phase of the development of the code. The filling gas is Xenon, in order to minimize the time lost by the MCC collision module in the first stage of development of the code. The results presented here are preliminary, with the code already showing a good robustness. The final goal will be the modeling of the NIO1 (Negative Ion Optimization phase 1) source, operating in Padua at Consorzio RFX.

The deployment of neutral beam injectors in future fusion plants is beset by the particularly poor efficiency of the neutralization process. Beam-generated plasma neutralizers were proposed as a passive and intrinsically safe scheme of efficient plasma neutralizers. The concept is based on the natural ionization of the gas target by the beam, and on a suitable confinement of the secondary plasma. The technological challenge of such a concept is the magnetic confinement of the secondary plasma: a proof-of-principle for the concept is needed. The possibility to test of such a system in the small negative ion beam system NIO1 is discussed in this paper. The constraints given by the facility are first discussed. A model of beam-gas interaction is developed to provide the charge-state of beam particles along the neutralizer, and to provide the source terms of plasma generation. By using a cylindrical model of plasma diffusion in magnetic fields, the ionization degree of the target is estimated. In the absence of magnetic fields the diffusion model is validated against experimental measurements of the space-charge compensation plasma in the drift region of NIO1. Finally, the feasibility study for a beam-generated plasma neutralizer in NIO is presented. The neutralizer length, required gas target thickness, and a very simple magnetic setup were considered, taking into account the integration in NIO1. For the basic design a low ionization degree (1%) is obtained, however a promising plasma density up to hundred times the beam density was calculated. The proposed test in NIO1 can be the starting point for studying advanced schemes of magnetic confinement aiming at ionization degrees in the order of 10%.The deployment of neutral beam injectors in future fusion plants is beset by the particularly poor efficiency of the neutralization process. Beam-generated plasma neutralizers were proposed as a passive and intrinsically safe scheme of efficient plasma neutralizers. The concept is based on the natural ionization of the gas target by the beam, and on a suitable confinement of the secondary plasma. The technological challenge of such a concept is the magnetic confinement of the secondary plasma: a proof-of-principle for the concept is needed. The possibility to test of such a system in the small negative ion beam system NIO1 is discussed in this paper. The constraints given by the facility are first discussed. A model of beam-gas interaction is developed to provide the charge-state of beam particles along the neutralizer, and to provide the source terms of plasma generation. By using a cylindrical model of plasma diffusion in magnetic fields, the ionization degree of the target is estimated. In the absence ...

The ITER Neutral Beam Test Facility (NBTF), called PRIMA (Padova Research on ITER Megavolt Accelerator), is hosted in Padova, Italy and includes two experiments: MITICA, the full-scale prototype of the ITER heating neutral beam injector, and SPIDER, the full-size radio frequency negative-ions source. The NBTF realization and the exploitation of SPIDER and MITICA have been recognized as necessary to make the future operation of the ITER heating neutral beam injectors efficient and reliable, fundamental to the achievement of thermonuclear-relevant plasma parameters in ITER. This paper reports on design and R&D carried out to construct PRIMA, SPIDER and MITICA, and highlights the huge progress made in just a few years, from the signature of the agreement for the NBTF realization in 2011, up to now - when the buildings and relevant infrastructures have been completed, SPIDER is entering the integrated commissioning phase and the procurements of several MITICA components are at a well advanced stage

An axisymmetric system to recover beam energy from partially neutralized Hbeams was recently proposed, for a given beam acceleration voltage Vs . In the case of ion source NIO1 Vs may range from 20 to 60 kV. A realistic beam with 3 mrad divergence, and a composition of 25 : 50 : 25 of H- , H0 and H+ has been considered. The collector works by decelerating the Hions (into a system similar to a Faraday cup provided with an exit hole electrode), so that they are radially deflected by space charge and anode lens effects, and collected to a low kinetic energy Kc (less than 1 keV), while neutral and H+ ions can pass through the exit hole electrode. A following collector can recover H+ energy. Since the space charge calculations are challenging for highly nonlinear problem and for a possible (numerically unstable) virtual cathode phenomena different computation tools were compared for simulations. Stabilization techniques are compared. Limits for local perveance are discussed. Also mesh asymmetry effects and the related transverse oscillations of H+ beam may be observed. Efficiency over 90 % can be reached in typical conditions. The secondary yield (which is low thanks to low impact energy Kc and Faraday cup concept) is estimated.

Neutral beam injection is one of the most important methods of plasma heating in thermonuclear fusion experiments, allowing the attainment of fusion conditions as well as driving the plasma current. Neutral beams are generally produced by electrostatically accelerating ions, which are neutralised before injection into the magnetised plasma. At the particle energy required for the most advanced thermonuclear devices and particularly for ITER, neutralisation of positive ions is very inefficient so that negative ions are used. The present paper is devoted to the description of the phenomena occurring when a high-power multi-ampere negative ion beam travels from the beam source towards the plasma. Simulation of the trajectory of the beam and of its features requires various numerical codes, which must take into account all relevant phenomena. The leitmotiv is represented by the interaction of the beam with the background gas. The main outcome is the partial neutralisation of the beam particles, but ionisation of the background gas also occurs, with several physical and technological consequences. Diagnostic methods capable of investigating the beam properties and of assessing the relevance of the various phenomena will be discussed. Examples will be given regarding the measurements collected in the small flexible NIO1 source and regarding the expected results of the prototype of the neutral beam injectors for ITER. The tight connection between measurements and simulations in view of the operation of the beam is highlighted.

A neutron diagnostic based on Gas Electron Multiplier is proposed for the MITICA beam injector test facility. The detection system is called Close-contact Neutron Emission Surface Mapping (CNESM) and aims at providing the beam intensity profile on the horizontal direction by measuring the neutron emission from the beam dump surface by placing a detector right behind the dump. CNESM uses Gas Electron Multiplier detectors equipped with a cathode that also serves as neutron-proton converter foil, named nGEM. The cathode, made of a thin polythene film and an aluminium film, is designed for detection of neutrons emitted with and angle between 30 and 70 degrees with respect to the deuterium beam axis. Neutron scattering in the dump and neutron detection with the nGEM were simulated with the MCNP6.1.1 code.

The ion sources play an important role in the framework of the nuclear fusion, as a fundamental part of the neutral beam injection systems needed for the auxiliary heating of the plasma magnetically confined in fusion devices. In the case of large experiments, such as ITER and beyond, the use of negative ions was found necessary to increase the neutralization efficiency of high-energy beams, calling for an additional effort in the research on the physical mechanisms regulating their formation and extraction as a beam from the source. This paper summarizes the analyses investigating the physical phenomena occurring in the source and their effect on the optics of the extracted beam. The results, carried out by means of advanced numerical tools, are particularly useful to yield a better evaluation of the real performances of the system, and particularly of the accelerator, whose modeling usually neglect some characteristics of the source. These analyses were performed for the specific case of the SPIDER experiment (Source for Production of Ion of Deuterium Extracted from an RF plasma), representing the major test bed for negative ion source worldwide. The choice of a specific device for the source modeling was found useful to support its operational phase and to help the experimental data interpretation and the identification of dangerous operating conditions. Nonetheless, the validity of the results here presented still holds in more general cases.

In the framework of the strategy for the development and the procurement of the NB systems for ITER, it has been decided to build in Padova a test facility, including two experimental devices: a full size plasma source with low voltage extraction and a full size NB injector at full beam power (1 MV). These two different devices will separately address the main scientific and technological issues of the 17 MW NB injector for ITER. In particular the full size plasma source of negative ions will address the ITER performance requirements in terms of current density and uniformity, limitation of the electron/ion ratio and stationary operation at full current with high reliability and constant performances for the whole operating time up to 1 h. The required negative ion current density to be extracted from the plasma source ranges from 290 A/m2 in D2 (D−) and 350 A/m2 in H2 (H−) and these values should be obtained at the lowest admissible neutral pressure in the plasma source volume, nominally at 0.3 Pa. The electron to ion ratio should be limited to less than 1 and the admissible ion inhomogeneity extracted from the grids should be better than 10% on the whole plasma cross-section having a surface exposed to the extraction grid of the order of 1 m2. The main design choices will be presented in the paper as well as an overview of the design of the main components and systems.

The geometry of a single aperture in the extraction grid plays a relevant role for the optimization of negative ion transport and extraction probability in a hybrid negative ion source. For this reason, a three-dimensional particle-in-cell/Monte Carlo collision model of the extraction region around the single aperture including part of the source and part of the acceleration (up to the extraction grid (EG) middle) regions has been developed for the new aperture design prepared for negative ion optimization 1 source. Results have shown that the dimension of the flat and chamfered parts and the slope of the latter in front of the source region maximize the product of production rate and extraction probability (allowing the best EG field penetration) of surface-produced negative ions. The negative ion density in the plane yz has been reported.

NIO1 is a compact radio frequency (RF) ion source based on inductively coupled plasma (ICP), jointly developed by Consorzio RFX and INFN-LNL [1], installed at RFX and currently in its initial operation phase. It is designed to generate a 60 kV-135 mA hydrogen negative ion beam, composed of 3 x 3 beamlets over an area of about 40 x 40 mm2. There are three acceleration grids named plasma grid (PG) at -60 kV, extraction grid (EG) at -52 kV and post acceleration grid (PA) at the ground voltage, followed by a repeller electrode (REP) for a better control of the space charge compensation of the extracted beam [1]. A major difference with other H- ICP sources is that NIO1 aims at continuous operation (in conditions similar to those foreseen for the larger ion sources of the Neutral Beam Injectors for ITER, exploiting its flexibility to address the several still open important issues related to beam extraction, optics, and performance optimization) which implies a detailed thermo-mechanical analysis of the beam-facing components, in particular the accelerator grids. Together with the first operation of NIO1, the construction of a new ion extraction system was started for optimizing the beam optics and exploring alternative electrostatic and magnetic configurations [2]. In particular, the accelerator will be modified by completely replacing the extraction grid: the new electrode will feature larger apertures with an increased chamfer at the hole exit and the realization of other slots in between apertures, to place additional magnets, useful to optimize the electron filtering and residual ion deflection [3]. A fully 3D analysis of the entire NIO1 beam considering the new extraction grid has been performed for the first time by a fully 3D version of EAMCC [4,5], a relativistic particle tracking code based on the Monte-Carlo method for describing the transport of particles under prescribed electric and magnetic fields and the main secondary particle formation processes responsible of non-negligible heat loads on the accelerator grids. The H- beam, the beam halo fraction and the co-extracted electrons have been simulated for determining the heat loads on grids and the power transmitted out of the accelerator. The main results are presented in this paper, after a brief description of the device, the proposed upgrade and the reference conditions for the simulations.

Powerful radiofrequency (rf) and microwave generator are of widespread in particle accelerators and sources, for the lower voltages required by rf accelerators as compared to electrostatic ones (which have a better power efficiency). Large accelerator projects request efficiency optimization, especially promising in the case of klystrons, where efficiency is determined by the intermediate cavity tunings, and for large tube ratings (multiMW), by the self electrostatic potential of the electron beam itself. Tubes with many electron beamlets are an option to mitigate this effect. Also modern developments in computational physics offer the possibility to study and improve the klystron bunching mechanism more directly, and general features and trends are discussed. The gyrotron and applications to ion sources are also summarized.

Thanks to the presence of a transverse magnetic flux density ($B_{x}$ and $B_{y}$ where $z$ is the extraction axis), the undesired extraction of electrons from a negative ion source is reduced and it is due to collisions. The electron transport is studied with a kinetic model, including Vlasov–Poisson effects and atomic collisions. The integrodifferential equations (IDE) resulting from a reduction to a one-dimensional problem (1-D) by integration on characteristic orbits are strongly affected by the trapped orbits, as here evaluated; a kernel calculation with a partial wave approximation is introduced. Dependencies from the local drift velocity $v_{d}$ and effective Larmor radius $L_{e}$ are found. Solutions are investigated in simple cases with a constant electron current (no additional electron production). Equilibrium solution and electron conductivity are analytically obtained. Presheath solutions are discussed; the approximated conversion to differential equations that are adequate for presheath only (with moderated electric field gradient $E_{z,z}>-eB_{x}^{2}/m$) and their numeric solutions coupled to Poisson equation are reported, and compared to iterative IDE solutions. Examples with different values of $L_{e}$ and mean free path (mfp) ratio are described.

A 34 mm diameter radio frequency (rf) oven system previously developed on bench was inserted and tested into the Electron Cyclotron Resonance (ECR) Ion Source Alice, producing beams from natural copper and silver samples; charge range was typically i=10–13 for copper and i=10–19 for silver, which well compares to previous source yield for xenon (charges 11–20). Some issues of oven design, including wire section effects, and circuit matching, are discussed; taps on the coupling transformer improved the flexibility of rf matching to different crucible materials (tantalum or steel). Details of operating experience (cleaning the oven and replacing sample) are reported; sample duration was more than 100 h and temperature Ts in excess of 1750 K were demonstrated. The ion source operation depends on both the oven distance Loe from ECR plasma and the bias voltage Vb of the sample. Best conditions were found for close (Loe≅70 mm) or preferably intermediate positions (Loe≅106 mm) and for sample negative respect to ...

Fast neutron detectors with a sub-centimetric space resolution are required in order to qualify neutron beams in applications related to magnetically-controlled nuclear fusion plasmas and to spallation sources. Based on the results obtained with small area prototypes, the first medium-size (20 x 35.2 cm2 active area) nGEM detector has been realized for both the CNESM diagnostic system of the SPIDER NBI prototype for ITER and as a beam monitor for fast neutrons beam lines at spallation sources, too. The nGEM is a Triple GEM gaseous detector equipped with polyethylene layers used to convert fast neutrons into recoil protons through the elastic scattering process. This paper describes the performance of the medium-size nGEM detector tested at the VESUVIO beam line of the ISIS spallation source. Being this detector the actual largest area fast neutron detector based on the GEM technology, particular attention was paid in the study of detector response in different points over the active area. Measurements of GEM counting rate (both as a function of VGEM and of time) and of the capability of the detector to reconstruct the beam in different positions are presented. This detector serves as a basis for the realization of an even larger area detector that will be used in the MITICA NBI prototype for ITER that represents the evolution of SPIDER.

The physics design of the accelerator for the heating neutral beamline on ITER is now finished and this paper describes the considerations and choices which constitute the basis of this design. Equal acceleration gaps of 88 mm have been chosen to improve the voltage holding capability while keeping the beam divergence low. Kerbs (metallic plates around groups of apertures, attached to the downstream surface of the grids) are used to compensate for the beamlet–beamlet interaction and to point the beamlets in the right direction. A novel magnetic configuration is employed to compensate for the beamlet deflection caused by the electron suppression magnets in the extraction grid. A combination of long-range and short-range magnetic fields is used to reduce electron leakage between the grids and limit the transmitted electron power to below 800 kW.

The ITER neutral beam test facility under construction in Padova will host two experimental devices: SPIDER, a 100 kV negative H/D RF source, and MITICA, a full scale, 1MeV deuterium beam injector. A detection system called Close - contact Neutron Emission Surface Mapping (CNESM) is under development with the aim to resolve the horizontal beam intensity profile in MITICA and one of the eight beamlet groups in S PIDER, with a spatial resolution of 3 and 5 cm2 respectively. This is achieved by the evaluation of the map of the neutron emission due to interaction of the deuterium beam with the deuterons implanted in the beam dump surface. CNESM uses nGEM detectors, i .e. GEM detectors equipped with a cathode that also serves as neutron - proton converter foil. The diagnostic will be placed right behind the SPIDER and MITICA beam dump, i.e. in an UHV environment, but the nGEM detectors need to operate at atmospheric press ure, so to contain the detector a vacuum sealed box has been designed to be installed inside the vacuum vessel and at atmospheric pressure inside. The box design was driven by the need to minimize the neutron attenuation and the distance between the beam d ump surface and the detector active area. This paper presents the status of the CNSESM diagnostics and describes the design of the detectors and of the sealed box (in particular the analysis carried out to define its parameters, the necessary pumping and l eak test procedures to ensure the compatibility of the box with the UHV environment and the proposed installation/removal procedure ). Also the general layout of the diagnostic as part of the SPIDER experiment will be discussed. Finally the preliminary des ign of MITICA CNESM diagnostic will be introduced. This work was set up in collaboration and financial support of Fusion for Energy.

The ITER project requires additional heating by two neutral beam injectors, each accelerating to 1 MV a 40 A beam of negative deuterium ions, to deliver to the plasma a power of about 17 MW for one hour. As these requirements have never been experimentally met, it was recognized as necessary to setup a test facility, PRIMA (Padova Research on ITER Megavolt Accelerator), in Italy, including a full-size negative ion source, SPIDER, and a prototype of the whole ITER injector, MITICA, aiming to develop the heating injectors to be installed in ITER. 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 which hosts the Test Facility. The Japanese and the Indian ITER Domestic Agencies (JADA and INDA) participate in the PRIMA enterprise; European laboratories, such as IPP-Garching, KIT-Karlsruhe, CCFE-Culham, CEA-Cadarache and others are also cooperating. Presently, the assembly of SPIDER is on-going and the MITICA design is being completed. The paper gives a general overview of the test facility and of the status of development of the MITICA and SPIDER main components at this important stage of the overall development; then it focuses on the latest and most critical issues, regarding both physics and technology, describing the identified solutions.

Neutral Beam Injectors (NBI) [typical injector MITICA (Megavolt ITer Injector Concept Advancement) specification are 1 MV 1280 beamlets total MV, beamlets, 55 Aof D- beam] , which need to be strongly optimized in the perspective of DEMO reactor, request a thorough understanding of the negative ion source used and of the multi-beamlet optics. A relatively compact RF ion source, named NIO1 (Negative Ion Optimization 1), with 9 beam apertures for a total H- current of 130 mA, 60kV acceleration voltage, was installed at Consorzio RFX, including a high voltage deck and a X-ray shield, to provide a test bench for source optimizations in the framework of the accompanying activities in support to the ITER NBI test facility. NIO1 operation has started in July 2014, at zero extraction voltage. Plasma is heated with a tunable 2 MHz (rf) radiofrequency. NIO1 status and plasma experiments both with air and with hydrogen as filling gas are described, up to a 1.7 kW rf power. Transition to inductively coupled plasma is reported in the case of air and briefly summarized for hydrogen.

Two Neutral Beam Injectors (NBI) will provide a substantial fraction of the heating power necessary to ignite thermonuclear fusion reactions in ITER. The development of the NBI system at unprecedented parameters (40 A of negative ion current accelerated up to 1 MV) requires a strong demonstration activity, which was endorsed by ITER to optimise the crucial components and systems. A test facility, PRIMA (Padova Research on ITER Megavolt Accelerator), is presently in the final phase of construction at Consorzio RFX (Padova, Italy) in the CNR research area and will house two experiments, named SPIDER and MITICA. A full-size negative ion source, SPIDER (Source for the Production of Ions of Deuterium Extracted from Rf plasma), will be operated in the facility to demonstrate the creation and extraction of a D-/H- current up to 50/60A on a wide surface (more than 1m2) with uniformity within 10%. The second experimental device is the prototype of the whole ITER injector, MITICA (Megavolt ITer Injector and Concept Advancement), aiming to develop the knowledge and the technologies to guarantee the successful operation of the two injectors to be installed in ITER, including the capability of 1MV voltage holding at low pressure. The beam source is the key component of the system, whose design results from a tradeoff between requirements of the optics and real grids with finite thickness and thermo-mechanical constraints due to the cooling needs and the presence of permanent magnets. The experimental effort is supplemented by numerical simulations devoted to the optimisation of the accelerator optics and to the estimation of heat loads and currents on the various surfaces. In the paper the main requirements will be discussed and the design and the status of the main components and systems will be described. Particularly a review of the accelerator physics and a comparison between the designs of the SPIDER and MITICA accelerators are presented.

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.

Neutral beam injection is the main method for plasma heating in magnetic confinement fusion devices. In high energy injector (E>100 keV/amu), neutrals are obtained with reasonable efficiency by conversion of negative ions (H-or D-) via electron detachment reactions. In the case of ITER injectors, which shall operate at 1 MeV, a total ion current of similar to 40 A is required to satisfy the heating power demand. Gridded electrodes are therefore used in the accelerator, so that 1280 negative ion beamlets are accelerated together. A carefully designed aiming system is required to control the beamlet trajectories, and to deliver their power on a focal point located several meters away from the beam source. In nowadays injectors, the aiming is typically obtained by aperture offset technique or by grid shaping. This paper discuss an alternative concept of beamlets aiming, based on an electrostatic "steerer" to be placed at the end of the accelerator. A feasibility study of this component is also presented, and its main advantages and drawbacks with respect to other methods are discussed.

Neutron GEM-based detectors represent a new frontier of devices in neutron physics applications where a very high neutron flux must be measured such as future fusion experiments (e.g. ITER Neutral beam Injector) and spallation sources (e.g. the European Spallation source). This kind of detectors can be properly adapted to be used both as beam monitors but also as neutron diffraction detectors that could represent a valid alternative for the He-3 detectors replacement. Fast neutron GEM detectors (nGEM) feature a cathode composed by one layer of polyethylene and one of aluminium (neutron scattering on hydrogen generates protons that are detected in the gas) while thermal neutron GEM detectors (bGEM) are equipped with a borated aluminium cathode (charged particles are generated through the B-10(n,alpha)Li-7 reaction). GEM detectors can be realized in large area (1 m(2)) and their readout can be pixelated. Three different prototypes of nGEM and one prototype of bGEM detectors of different areas and equipped with different types of readout have been built and tested. All the detectors have been used to measure the fast and thermal neutron 2D beam image at the ISIS-VESUVIO beamline. The different kinds of readout patterns (different areas of the pixels) have been compared in similar conditions. All the detectors measured a width of the beam profile consitent with the expected one. The imaging property of each detector was then tested by inserting samples of different material and shape in the beam. All the samples were correctly reconstructed and the definition of the reconstruction depends on the type of readout anode. The fast neutron beam profile reconstruction was then compared to the one obtained by diamond detectors positioned on the same beamline while the thermal neutron one was compared to the imaged obtained by cadmium-coupled x-rays films. Also efficiency and the gamma background rejection have been determined. These prototypes represent the first step towards the realization of new neutron beam monitors for fusion experiments and spallation sources.

Two Neutral Beam Injectors (NBI) are foreseen to provide a substantial fraction of the heating power necessary to ignite thermonuclear fusion reactions in ITER. The development of the NBI system at unprecedented parameters (40 A of negative ion current accelerated up to 1 MV) requires the realization of a full scale prototype, to be tested and optimized at the Test Facility under construction in Padova (Italy). The beam source is the key component of the system and the design of the multi-grid accelerator is the goal of a multi-national collaborative effort. In particular, beam steering is a challenging aspect, being a tradeoff between requirements of the optics and real grids with finite thickness and thermo-mechanical constraints due to the cooling needs and the presence of permanent magnets. In the paper, a review of the accelerator physics and an overview of the whole R&D physics program aimed to the development of the injector source are presented.

Consorzio RFX and INFN-LNL are building a flexible small ion source (Negative Ion Optimization 1, NIO1) capable of producing about 130 mA of H- ions accelerated at 60 KeV. Aim of the experiment is to test and develop the instrumentation for SPIDER and MITICA, the prototypes, respectively, of the negative ion sources and of the whole neutral beam injectors which will operate in the ITER experiment. As SPIDER and MITICA, NIO1 will be monitored with beam emission spectroscopy (BES), a non-invasive diagnostic based on the analysis of the spectrum of the H-alpha emission produced by the interaction of the energetic ions with the background gas. Aim of BES is to monitor direction, divergence, and uniformity of the ion beam. The precision of these measurements depends on a number of factors related to the physics of production and acceleration of the negative ions, to the geometry of the beam, and to the collection optics. These elements were considered in a set of codes developed to identify the configuration of the diagnostic which minimizes the measurement errors. The model was already used to design the BES diagnostic for SPIDER and MITICA. The paper presents the model and describes its application to design the BES diagnostic in NIO1.

Fusion reactors will need high flux neutron detectors to diagnose the deuterium-deuterium and deuterium-tritium. A candidate detection technique is the Gas Electron Multiplier (GEM). New GEM based detectors are being developed for application to a neutral deuterium beam test facility. The proposed detection system is called Close-contact Neutron Emission Surface Mapping (CNESM). The diagnostic aims at providing the map of the neutron emission due to interaction of the deuterium beam with the deuterons implanted in the beam dump surface. This is done by placing a detector in close contact, right behind the dump. CNESM uses nGEM detectors, i.e. GEM detectors equipped with a cathode that also serves as neutron-proton converter foil. After the realization and test of several small area prototypes, a full size prototype has been realized and tested with laboratory sources. Test on neutron beams are foreseen for the next months. © 2014 American Institute of Physics.

Neutral beam injectors are the main source of plasma heating in advanced fusion devices. To support the realization and optimize the performances of the injectors of ITER tokamak, a dedicated test bed is under construction at Consorzio RFX, Padova. This facility will include a full scale prototype of the injector, named megavolt ITER injector concept advancement. Its aim is to demonstrate and optimize a 16-MW neutral beam power, obtained by neutralization of a 56-A deuterium negative ion beam D- accelerated to an energy of 1 MeV. Since the expected current density available at the source is ~300 A/m2, a large extraction area is needed in order to obtain the required current, so that the beam is formed by the superposition of 1280 beamlets extracted from the source by means of a set of grids biased at increasing potentials. This paper presents the studies carried out to optimize the beam optics, in order to satisfy the ITER requirements on beam aiming and focusing over long distances. This includes the compensation of the electrostatic interaction among beamlets and the effect of supporting structures. The most promising methods to steer the beamlets are studied and a comparison among different design configurations is made.

Fast neutron detectors with a sub-millimetric space resolution are required in order to qualify neutron beams in applications related to magnetically-controlled nuclear fusion plasmas and to spallation sources. A nGEM detector has been developed for the CNESM diagnostic system of the SPIDER NBI prototype for ITER and as beam monitor for fast neutrons lines at spallation sources. The nGEM is a triple GEM gaseous detector equipped with polypropylene and polyethylene layers used to convert fast neutrons into recoil protons through the elastic scattering process. This paper describes the results obtained by testing a nGEM detector at the ISIS spallation source on the VESUVIO beam line. Beam profiles (?x=14.35 mm, ?y=15. 75 mm), nGEM counting efficiency (around 10-4 for 3 MeV

Negative ions are fundamental to produce intense and high energy neutral beams used to heat the plasma in fusion devices. The propagation of such beams is subject to strong interaction with the background gas, whose presence along the beamline is due to leaks from the plasma source and from the gas neutralizer. The secondary particles created in this interaction have different effects depending on the local intensity of the external electric field: where E is high they are accelerated and dumped on electrodes or transmitted in the drift region with poor optics, while in low field region some of them are accumulated in the potential well induced by the beam space charge, influencing the local space charge distribution and the optics. The modeling of these effects is usually addressed bysplitting the domain into two regions, using specific codes for each of them and assuming special boundary conditions to approximate the presence of the neighbor region. This paper describes a study carried out on these phenomena exploiting a 2D-axialsimmetric PIC code whose domain can extend from the beam extraction region all the way to the drift region.This feature allows to simulate at the same time both aforementioned aspects, highlighting possible interactions among them. The results are discussed and compared with the existing numerical codes, as SLACCAD and BYPO.

The extraction of a beam from ion sources is dominated by the strong space charge of the beam, due to the initial low speed of the particles. Several mathematical and computational issues are discussed, with reference to a diode design based on thin anode lens effect, yielding a parallel beam at its exit, which is the first block of many high current electrostatic accelerators. Perturbation to uniform current density are analyzed. Effect of a thick anode lens is also treated.

A neutron diagnostic for high current deuterium beams is proposed for installation on the spectral shear interferometry for direct electric field reconstruction (SPIDER, Source for Production of Ion of Deuterium Extracted from RF plasma) test beam facility. The proposed detection system is called Close-contact Neutron Emission Surface Mapping (CNESM). The diagnostic aims at providing the map of the neutron emission on the beam dump surface by placing a detector in close contact, right behind the dump. CNESM uses gas electron multiplier detectors equipped with a cathode that also serves as neutron-proton converter foil. The cathode is made of a thin polythene film and an aluminium film; it is designed for detection of neutrons of energy >2.2 MeV with an incidence angle < 45°. CNESM was designed on the basis of simulations of the different steps from the deuteron beam interaction with the beam dump to the neutron detection in the nGEM. Neutron scattering was simulated with the MCNPX code. CNESM on SPIDER is a first step towards the application of this diagnostic technique to the MITICA beam test facility, where it will be used to resolve the horizontal profile of the beam intensity.