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A comparative Study on the conversion of an energetic H+ beam inside a cell neutralizer filled with hydrogen gas target is conducted by two different methods to determine the maximum beam neutralization efficiency and corresponding optimal gas thickness in terms of beam energy, ranging from 1 keV to 10 MeV. One method analytically solves the equations of particle flux with a deterministic response. The other uses the random collisional probability based on the Monte Carlo simulation with non-deterministic response, which is implemented by COMSOL software. Effective reactions of charge exchange and reionization are commonly considered in both methods with the energy-depended collisional cross section. The results obtained from the two methods are compared, demonstrating good fitness with a mean difference of ±1.5 %. This acceptive difference arises often due to statistical errors that occur in the Monte Carlo calculations. Additionally, a close agreement is found between calculated and measured efficiencies. [Display omitted] • The Monte Carlo simulation agrees with the analytical solution and experiment. • The hydrogen neutralization is influenced by beam energy and gas target thickness. • Increasing the density increases the rate of neutralization, not its maximum value. • Collisional cross section of hydrogen considerably affects the calculational results. [ABSTRACT FROM AUTHOR]

The paper is devoted to the IPM-2 ion source ion-optical system preparation for the plasma heating experiments with a neutral beam in the Globus-M2 spherical tokamak. Within the framework of this work, a complete renovation of the ion-optical system of the ion source was carried out, including its assembly, adjustment, and testing. To assess the quality of the work performed, measurements were made of the main electrical parameters of the high-energy beam, its energy spectrum and signals from secondary emission probes. Based on the data obtained, the beam power distribution profile and its characteristic dimensions are reconstructed, and the relative concentrations of the energy components of the beam are calculated. [ABSTRACT FROM AUTHOR]

Stable and uniform beams with low divergence are required in particle accelerators; therefore, beyond the accelerated current, measuring the beam current spatial uniformity and stability over time is necessary to assess the beam performance, since these parameters affect the perveance and thus the beam optics. For high-power beams operating with long pulses, it is convenient to directly measure these current parameters with a non-intercepting system due to the heat management requirement. Such a system needs to be capable of operating in a vacuum in the presence of strong electromagnetic fields and overvoltages, due to electrical breakdowns in the accelerator. Finally, the measure of the beam current needs to be efficiently integrated into a pulse file with the other relevant plant parameters to allow the data analyses required for beam optimization. This paper describes the development, design and commissioning of such a non-intercepting system, the so-called beamlet current monitor (BCM), aimed to directly measure the electric current of a particle beam. In particular, the layout of the system was adapted to the SPIDER experiment, the ion source (IS) prototype of the heating neutral beam injectors (HNB) for the ITER fusion reactor. The diagnostic is suitable to provide the electric current of five beamlets from DC up to 10 MHz. [ABSTRACT FROM AUTHOR]

The development of High Voltage Direct Current Gas-Insulated components that will be employed for the power supply of Neutral Beam Injectors, requires specific numerical tools to analyze the electric field distribution, with the purpose of supporting the engineering and operational phases.The tools must comply with the nonlinearity of the material properties of solid insulators and the transport mechanisms in the dielectric gas. Despite its high Global Warming Potential, SF6 will be considered as the baseline dielectric gas for the development of HVDC gas-insulated components to be used for the Acceleration Grid Power Supply of Neutral Beam Injectors. Attention especially to the mechanisms driving to an increase of leakage current flowing in the gas, thus giving rise to possible discharge phenomena is here placed, mainly focusing on radiation and injection regimes. The described methodologies are adopted for the numerical modeling of the 500 kV DC gas-insulated components that will be used for the power supply of the Neutral Beam Injector of the Divertor Tokamak Test facility. In particular, a conceptual design of the SF6 Transmission Line and the SF6-air bushing is conceived.

This paper outlines the main features of a new high efficiency ( η > 62%) high power (∼18 MW D ^0 ) neutral beam (NB) concept based on pragmatic solutions suitable with the reactor requirements. The injector is modular (several beamlines in parallel) with independent ion sources referenced to the ground potential and gas neutralizers held at +1 MV. This topology leads to numerous simplifications; it overcomes the main issues of conventional NB systems, such as the complex 1 MV electrical setup, the difficult ion source remote maintenance, the high caesium consumption. The other key parameter is the gas neutralization concept which minimizes the amount of gas by operating at a low gas target and low neutralizer duct conductance. The implementation of an energy recovery system for the residual 1 MeV D ^− is essential to attain a high wall-plug efficiency. These specific features require thin laminar D ^− beams provided by a pre-acceleration up to 100 keV in slotted grid apertures to form thin blade-like beamlets, followed by the post-acceleration to 1 MeV by merging the beamlets in a single beam in five gaps (+200 kV per gap). All these specific aspects minimize the beams losses and thermal loads along the beamline and enhance the injector reliability and availability.

In the ITER neutral beam injectors (NBI), the presence of an external variable magnetic field generated by the ITER tokamak itself, could deflect the ion beam during acceleration and cause a loss of beam focusing. For this reason, the ion source, the accelerator and the neutralizer will be shielded from external magnetic field by means of a passive magnetic shield and a system of active correction and compensation coils (ACCC). The ACCC will operate in a feedback control loop and thus require the measurement of magnetic field inside the NBI vessel. Magnetic sensors for this application must be capable of measuring DC and slow variable magnetic fields, and be vacuum-compatible, radiation-hard and robust, since they will be subjected to neutron flux produced by fusion reactions in the tokamak and inaccessible for maintenance. This paper describes the realization and tests of fluxgate magnetic sensors prototypes specifically designed for this purpose before the installation in MITICA and ITER. [ABSTRACT FROM AUTHOR]

Stable and uniform beams with low divergence are required in particle accelerators; therefore, beyond the accelerated current, measuring the beam current spatial uniformity and stability over time is necessary to assess the beam performance, since these parameters affect the perveance and thus the beam optics. For high-power beams operating with long pulses, it is convenient to directly measure these current parameters with a non-intercepting system due to the heat management requirement. Such a system needs to be capable of operating in a vacuum in the presence of strong electromagnetic fields and overvoltages, due to electrical breakdowns in the accelerator. Finally, the measure of the beam current needs to be efficiently integrated into a pulse file with the other relevant plant parameters to allow the data analyses required for beam optimization. This paper describes the development, design and commissioning of such a non-intercepting system, the so-called beamlet current monitor (BCM), aimed to directly measure the electric current of a particle beam. In particular, the layout of the system was adapted to the SPIDER experiment, the ion source (IS) prototype of the heating neutral beam injectors (HNB) for the ITER fusion reactor. The diagnostic is suitable to provide the electric current of five beamlets from DC up to 10 MHz.

In the ITER neutral beam injectors (NBI), the presence of an external variable magnetic field generated by the ITER tokamak itself, could deflect the ion beam during acceleration and cause a loss of beam focusing. For this reason, the ion source, the accelerator and the neutralizer will be shielded from external magnetic field by means of a passive magnetic shield and a system of active correction and compensation coils (ACCC). The ACCC will operate in a feedback control loop and thus require the measurement of magnetic field inside the NBI vessel. Magnetic sensors for this application must be capable of measuring DC and slow variable magnetic fields, and be vacuum-compatible, radiation-hard and robust, since they will be subjected to neutron flux produced by fusion reactions in the tokamak and inaccessible for maintenance. This paper describes the realization and tests of fluxgate magnetic sensors prototypes specifically designed for this purpose before the installation in MITICA and ITER.

BackgroundNegative ion source based on neutral beam injector is an important auxiliary heating and current driving system for the China Fusion Experimental Test Reactor (CFETR). Due to the high beam energy and large beam spot of negative ion based neutral beam injection system (NNBI), electrical deflection of residual ions has become the preferred scheme for residual ion dumping.PurposeThis study aims at the conceptual design of NNBI electrostatic residual ion dump system to deflect residual ions for neutralization of the beam.MethodsAccording to the development objectives of NNBI, three polar plates and two channels were adopted for the conceptual design of electrostatic residual ion dump (ERID). A negative voltage was applied to the intermediate plate to form a deflection electric field between the channels. When the mixed beam passed through the deflection channel, the residual ions were deflected to the panel due to the action of electric field force. Through theoretical calculation and beam deposition simulation, the structure of the ERID and more uniform heat load distribution on its surface were preliminarily determined.Results & ConclusionsThe simulation results show that the ERID can deflect the residual ions under the parameter design, providing a reference for future engineering design.

• Influence of magnetic field in RF plasma source of CRAFT was investigated. • Checkboard and symmetric arrangement of magnets were simulated and tested. • Symmetric arrangement with three rows of magnets gave the best performance in term of plasma parameters. • The results are helpful to optimize the performance of high power ion sources. Radio Frequency (RF) driven negative ion source is the preferred ion source for negative ion based Neutral Beam Injection system (NNBI). The plasma generator, which consists of RF driver(s) and an expansion chamber, is one of the key parts for NNBI. The distribution of cusp magnetic field which is generated by permanent magnets installed at the lateral wall of the expansion chamber is a common issue, due to the great influence on electron density, electron temperature and spatial uniformity of plasma density in the extraction area. Two kinds of configuration of cusp magnetic field (checkerboard type and symmetric type) and three types of arrangement of permanent magnets (with three, four and five rows of magnets) have been carried out with the approach of finite element and the experimental performance. On the RF negative ion source testbed, a moveable Langmuir probe has been adopted to investigate the characteristic of electron density and temperature near the extraction area. Since the symmetric type with three rows of permanent magnets generates the distribution of cusp magnetic field that contribute to extract the plasma from RF driver and improve the probability of particle collision easily, the higher electron density and the lower electron temperature was obtained via simulation. The experimental conclusion is that the symmetric type with three rows of permanent magnets leads to a higher electron density and lower electron temperature, showing a great agreement with simulation results. The results are helpful to optimize the structure and performance of high power ion source, and provide support for the development of large area RF negative ion sources with multi-driver. [ABSTRACT FROM AUTHOR]

A set of novel design solutions for high performance cooling systems have been developed and tested by Consorzio RFX, achieving, with experimental tests, the challenging heat transfer conditions foreseen for Heating and Current Drive Systems of present and future nuclear fusion devices. The project, called Multi-design Innovative Cooling Research & Optimization (MICRO), has the triple objective of: • Verifying the full qualification of the manufacturing process and assess the cooling performance of the present solutions applied on the acceleration grids designed for ITER Heating Neutral Beam Injectors and MITICA experiment, which have similar heat loads as for DEMO1. • Identifying alternative designs realizable with presently qualified manufacturing processes for improving heat transfer mechanisms with limited pressure drops. • Developing further optimized solutions by means of the investigation of the interrelated effects of geometric parameters and thermofluid dynamic properties of the coolant fluid. The main aim of present work is to extend the fatigue life-cycle of high thermal stress components. In the first part of the paper the design description and constraints are presented together with the results obtained from numerical analyses carried out on the full-scaled model of the electrostatic grids; the outcomes of the experimental campaign, together with the characterization of the properties and performance of a set of alternative dielectric fluids, are shown in the second one. [ABSTRACT FROM AUTHOR]

The Acceleration Grid Power Supply Conversion System (AGPS-CS) of the MITICA Neutral Beam Injector has been installed between late 2017 - early 2018 and commissioned in the first half of 2018. This paper is focused on the installation and commissioning activities, with emphasis on the integration issues. The results of the site and acceptance tests are presented, proving the capability of the system to deliver the specified performance. [ABSTRACT FROM AUTHOR]

The use of non-evaporable getter (NEG) pumps is common in many UHV applications, and the properties of the new sintered getters based on the ZAO®alloy make them appealing for applications dealing with large fluxes of hydrogen and its isotopes, as typically occurs in fusion research. In particular, the use of NEG pumps is interesting for the vacuum system of negative ion-based neutral beam injectors, which require huge deuterium gas throughputs for long durations. The key advantages of a NEG pumping solution are the high specific pumping speed and capacity, the ability to withstand a large number of absorption/desorption cycles, the ease of integration. In addition, NEG do not release hydrogen unless power is supplied to heat them, addressing an important safety issue related to uncontrolled release in case of power outage or subsystem failures. This paper reports the experimental characterization of ZAO®sintered getters, in pressure regimes and sorption amounts relevant for the use in the next generation NBI. Single discs and complete pumps were characterized in different conditions of H 2 /D 2 pressure between 10−4 and 10-1Pa and operating temperatures up to 150°C. In particular, the feasibility of getter regeneration within time windows compatible with current NBI operation scenarios was demonstrated. [ABSTRACT FROM AUTHOR]

• Joint experimental campaigns carried out in QST lab in Naka, Ibaraki, Japan. • Full compensation of magnetic criss-cross deflection of beamlets obtained. • Non-uniform extraction assumed for interpreting the experimental results. • Numerical simulations can reproduce the experimental results with good accuracy. In the framework of the collaboration between Consorzio RFX (Padova, Italy) and QST (Naka, Japan) two experimental campaigns have been organized on the Negative Ion Test Stand (NITS) in Naka, employing an ITER-like multi-beamlet configuration. These campaigns were aimed at compensating for the undesired magnetic deflection of the ion beam. The experiments have clarified the influence of operating parameters on the optical properties of the ion beam. The physical phenomena acting on the optics have been investigated, in particular the effect of an asymmetry of the ion current density at meniscus. Comparisons with simulations have been made and the modeling has been improved consequently. During the second experimental campaign, a perfect compensation of the ion deflection has been achieved. [ABSTRACT FROM AUTHOR]

• Spectroscopy is main diagnostic of ITER prototype negative ion beam source SPIDER. • Photodiodes measure visible radiation from source faster than spectrometers. • Custom electronics has low-noise variable gain amplifier, FPGA-based remote control. • Photodiode has14 bit dynamic range, 1.2–135 kHz bandwidth, 100 kΩ to 5 GΩ gain. • First measurements show dependencies with RF power and filter field current. The source prototype SPIDER in the Neutral Beam Test Facility at Consorzio RFX has been developed to demonstrate the target performances of the RF plasma source of ITER Heating Neutral Beam injectors. SPIDER diagnostics characterize the plasma in the source, optimize performances and provide signals for machine protection. Photodiodes measure the time evolution of lines of sight integrated visible radiation emitted by the plasma in the source. Some are equipped with interference filters to select specific spectral lines, e.g. those of cesium to control its evaporation, which assists the H¯ production, and of metal impurities, which might indicate wall damages. Other lines of sight are fed to grating spectrometers to measure the spectrum at a slower rate, either at low resolution for spectral survey of hydrogen Balmer lines and impurities or at high resolution for molecular Fulcher bands. The overall layout of the photodiode diagnostic and preliminary results during the first SPIDER operation are presented. The custom photodiode electronics is described in detail, including a low-noise variable gain amplifier, FPGA-based remote control and suitably shaped digital output for feedback control. Main characteristics are: linear response up to 10 V, 14 bit dynamic range and 1.2–135 kHz bandwidth depending on gain in range 100 kΩ to 5 GΩ. Measured light through the drivers has allowed to recognise plasma features and dependencies with RF power and filter field current, as well as dominant frequencies in signal fluctuations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Neutral Beam Injectors make use of large vacuum pumps to pump hydrogenic species. • Non-Evaporable Getters materials remove hydrogen in passive way. • Strategy to choose the design of the mock-up of a large getter pump. Non-evaporable getter (NEG) technology is a potential candidate as pumping system for future neutral beam injector applications. The strategy to validate the use of getter pump technology is based on the realization of a relatively large pump mock-up that has to be tested in fusion-relevant conditions. The objectives of this mock-up are to demonstrate that a pump of large dimensions and capacity is usable. This paper deals with the design of the mock-up, based on conceptual studies which involve at first 3D gas flow simulations considering different modular mock-up pumps based on NEG sintered disks. In addition transient thermal simulations with FE method have been performed with the aim to analyze the thermal response of the mock-up. The conceptual design has been carried out in order to define the best configuration to obtain high pumping speed with low spatial gradient of gas concentration inside the getter material. The suggested solution will exhibit a modular structure of getter disks, which on one hand simplifies the mechanical assembling, and on the other hand allows interpretative modelling at different scales. It is foreseen to test the pump mockup in the TIMO facility at KIT Karlsruhe. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

• A -800 kV transmission line for neutral beam injector was design in this study, mainly about the insulating gap. • The ITER-like transmission line structure and coaxial structure both has been considered. • The simulation results shows that the coaxial has more advantages in terms of volume and electric field uniformity. • The design basis in accordance with specifications has been presented in the manuscript. The high-voltage transmission line carries electricity, water, and gas for the high-potential components in the NNBI system. High voltage transmission lines used in -800 kV acceleration grade NNBI systems require the transmission of multiple high voltages of different voltage levels. Its insulation design not only guarantees the reliability of insulation but also requires a small occupation volume. The FEA method is used for insulation design of multi-conductor transmission lines. In the design of -800 kV transmission line, coaxial and non-coaxial structures are mainly considered. In the case of SF6 filling space, conductor radius and conductor position are the main factors affecting insulation strength. Both structures can make its insulation strength meet the requirements. However, it is found that the coaxial transmission line is more compact and concise when it meets the insulation requirements, but it is difficult to install and maintain. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Heating Neutral Beam Injectors (HNBs) for ITER will have to deliver 16.7 MW beams of H/D particles at 1 MeV energy. The beams will be produced from H −/ D − ions, generated by a radiofrequency plasma source coupled to an ion acceleration system. A prototype of the ITER HNB ion source is being tested in the SPIDER experiment, part of the ITER Neutral Beam Test Facility at Consorzio RFX. Reaching the design targets for beam current density and fraction of coextracted electrons is only possible by evaporating cesium in the source, in particular on the plasma facing grid (PG) of the acceleration system. In this way the work function of the surfaces decreases, significantly increasing the amount of surface reactions that convert neutrals and positive ions into H −/ D −. It is then of paramount importance to monitor the density of negative ions and the density of Cs in the proximity of the PG. Monitoring the Cs spatial distribution along the PG is also essential to guarantee the uniformity of the beam current. In SPIDER, this is possible thanks to the Cavity Ringdown Spectroscopy (CRDS) and the Laser absorption Spectroscopy diagnostics (LAS), which provide line-integrated measurements of negative ion density and neutral, ground state Cs density, respectively. The paper discusses the CRDS and LAS measurements as a function of input power and of the magnetic and electric fields used to reduce the coextraction of electrons. Negative ion density data are in qualitative agreement with the analogous measurements in Cs-free conditions. In agreement with simulations, Cs density is peaked in the center of the source; a top/bottom non uniformity is also present. Several effects of plasma on Cs deposition and negative ion production are presented. [ABSTRACT FROM AUTHOR]

• Feasibility analysis of SEE diagnostic system is given based on FEM. • A SEE diagnostic system for CRAFT NBI is designed. • Preliminary experiments of SEE diagnostic system is obtained on test facility. Neutral beam injection is one of the main auxiliary heating methods in Tokamak nuclear fusion device. The calorimeter which will be applied in Neutral beam injection system at Comprehensive Research Facility for Fusion Technology (CRAFT) is design for beam cutoff measurement, but limited by the design of double-layer water-cooling copper tubes paralleled to each other, the diagnostic system based on thermocouples cannot give the horizontal beam profile. To satisfy the requirement of diagnostic system, a beam diagnostic system based on Secondary Electron Emission (SEE) is designed. In order to achieve this goal, the rules of secondary electron emission are analyzed based on finite element methods (FEM) and the primary experiment has been carried out. Firstly, the laws of secondary electron emission during the beam bombarding on the calorimeter are analyzed; secondly, a set of signal acquisition system is set up, including sampling and filtering; finally, a preliminary experiment is carried out and the relationships between the signal of secondary electrons and the parameters, such as beam energy, beam current and bias voltage, are obtained. These results lay a foundation for the subsequent construction of the beam diagnostic system for CRAFT neutral beam injector (NBI). [ABSTRACT FROM AUTHOR]

Toward the 1 MV power supply system for the ITER neutral beam injector consisting of various components with oil insulation, gas insulation and vacuum insulation, design and manufacturing of dc ultra-high-voltage component have been progressed. Thermal design of long transmission line up to 100 m was established. In order to design the high-temperature water channel for the plasma grid at 1 MV potential, the resistivity of high-purity water at 180 °C was measured for the first time. With the obtained resistivity, realistic high-temperature water circulating system was designed. A high-voltage bushing to transmit multiple voltages simultaneously has been also developed through a careful experimental study on vacuum insulation characteristics with multi-gaps coaxial electrodes. With the empirical scaling the electrostatic screen was designed taking into account effect of multi-gap and expected deformation such as thermal elongation during operation. After the assembly, stable voltage holding of 240 kV at each stage was confirmed in the manufactured HV bushing. [ABSTRACT FROM AUTHOR]

ITER Neutral Beam Injectors (NBIs) need to be shielded from the relatively strong stray magnetic field generated by the Poloidal Field Coils of the Tokamak. For this reason both the Heating Neutral Beams (HNB) and the Diagnostic Neutral Beam (DNB) will be provided with a Passive Magnetic Shield and with a system of Active Correction and Compensation Coils (ACCC). The ACCC will operate in feedback control and thus require the measurement of magnetic field inside the NBI vessel, i.e. in an environment subjected to the neutron flux coming from the Tokamak. To this purpose, magnetic sensors which are robust, radiation hard, drift-immune and remote-handling compatible are required. Flux-gate magnetic sensors are a good candidate for this task, but commercial sensors of this kind have typically a limited measured range (below 0.1 mT). The feasibility of a flux-gate sensor for the ITER NBI has been studied by developing a numerical model which includes magnetic core hysteresis, and which demonstrated that, by suitable choice of the core magnetic properties and geometry, it is possible to increase the measurement range by at least 2 orders of magnitude. On this basis, a flux-gate sensor prototype has been realized at Consorzio RFX. Experimental tests carried out so far have demonstrated that the measurement range can be increased to ∼10 mT with acceptable accuracy and frequency bandwidth. [ABSTRACT FROM AUTHOR]

A common problem of the multi-beamlet, negative-ion based Heating Neutral Beam (HNB) Injectors presently used in fusion research, is the undesired deflection of negative ions due the permanent magnets embedded in the Extraction Grid (EG) for the suppression of the unwanted co-extracted electrons. A new solution has been recently developed at Consorzio RFX, which makes use of an additional set of permanent magnets, also embedded in the EG, with a series of advantages in terms of performances and versatility with respect to the traditional electrostatic compensation. This solution is now the new standard for ITER. After a thorough validation of the design by different numerical models, it was decided to experimentally test this solution for the first time on the Negative Ion Test Stand (NITS) at National Institute for Quantum and Radiological science and technology (QST), in Japan, within the framework of a scientific cooperation agreement between QST and Consorzio RFX. To this purpose, an EG having an ITER-like profile and compatible with NITS accelerator was designed and constructed at Consorzio RFX. This paper describes the design solutions adopted for this grid and its related permanent magnets, the construction phase, and a brief report on the first experimental results. [ABSTRACT FROM AUTHOR]

A long pulse acceleration of high-power-density hydrogen negative ion beams of 184 MW/m 2 (0.97 MeV, 190 A/m 2 ) has been successfully achieved up to a facility limit of 60 s by using a 5 stage multi-aperture multi-grid accelerator, which is the first demonstration of the ITER relevant beam with a pulse length over the time constant of cooling capability of the water-cooled acceleration grids. In order to extend the pulse length of such high power density beams, the extraction and acceleration geometries were tuned to match the beam optics to high power density by increasing the transmission at the extractor and the acceleration electric field at the first acceleration gap. After the modifications, the available current density at the same acceleration voltage was increased by 36%, and grid heat loads at each acceleration grid were reduced to less than 3% of the electric input power, which satisfied an allowable level for long pulse acceleration. Furthermore, since the conditioning procedure has been established for long pulse acceleration, a rapid conditioning of total 16 operational days up to 1MeV, 60 s class has been achieved, which was about half of previous results. Because JT-60SA and ITER NBIs are designed by the same concept, these results are applicable to the modifications and operations toward these NBIs directly. [ABSTRACT FROM AUTHOR]

The Acceleration Grid Power Supply (AGPS) is a system devoted to supply the acceleration grids of the MITICA experiment, the full scale prototype of the ITER Neutral Beam Injector (NBI), being built in Padova (Italy) to test the NBI in advance of the operation in ITER. The procurement of the AGPS is split in two parts: the low voltage conversion system, namely the AGPS-Conversion System (AGPS-CS,) being procured by the European domestic agency, and the high voltage DC Generators (AGPS-DCG), being procured by the Japanese domestic agency. After the contract award, in late 2015, the executive design phase of the AGPS-CS has been finalized. The paper will present the final design of the AGPS-CS and will discuss the main choices in particular concerning the protection from internal faults of the inverters. [ABSTRACT FROM AUTHOR]

Several novel design solutions for high performance cooling systems have been developed by Consorzio RFX, permitting to simulate the challenging heat transfer conditions foreseen in the future fusion devices. The project, called Multi-design Innovative Cooling Research & Optimization (MICRO), aims, on one hand, to verify the present solution applied inside the NBI accelerator grids (i.e. MITICA experiment) and, on the other, to perform further improvements in the heat transfer process with an acceptable pressure drop and reliable manufacturing process. The main advantages rely on the possibility to extend the fatigue life-cycle of different high thermal stress components and to investigate the possibility to employ alternative dielectric fluids instead of water. Such design solutions would in fact allow the exploitation of less performing fluids in terms of cooling capability. This is particularly relevant in view of DEMO and future power plants characterized by higher efficiency and reliability. The paper gives a description of the Computation Fluid Dynamics (CFD) analysis and the samples manufacturing process. [ABSTRACT FROM AUTHOR]

A radio frequency (RF) ion source was designed and developed for neutral beam injector in Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). Recently, the RF ion source was upgraded with new RF power generator (50 kW with frequency of 1 MHz), a new matching unit and the faraday shield with water cooling. Two new methods were used for the ion source tests on the test bed. The high pressure was used for the ignition of plasma and change to low pressure of 0.3 Pa to maintain the plasma. The RF plasma can be generated without the start filament successfully. In order to avoid the plasma oscillation with high power, the RF power was set to increase with two stages without change the matching unit. High power of 46 kW with pulse length of 22 s was achieved on the test bed and the plasma duration can be extended too. [ABSTRACT FROM AUTHOR]

ITER envisages the use of two heating neutral beam injectors plus an optional one as part of the auxiliary heating and current drive system, to reach the desired performances during its various phases of operation. The 16.5 MW expected neutral beam power per injector is several notches higher than worldwide existing facilities. In order to enable such development, a Neutral Beam Test Facility (NBTF) was established at Consorzio RFX, exploiting the synergy of two test beds, called SPIDER and MITICA. SPIDER is dedicated 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, in particular events/issues occurred during operation, which led to the identification of improvement opportunities/necessities (e.g. RF discharges, local burns, water leaks, other damages, configuration/design upgrades to maximize chances/margin to quest target parameters). Parallelly, developments on MITICA, the full-scale prototype of the ITER Neutral Beam Injector (NBI) featuring a 1 MV accelerator and ion neutralization, are underway including manufacturing of the beam source, accelerator and the beam line components, while power supplies and auxiliary plants, already installed, are under final testing and commissioning. Integration, commissioning and tests of the 1 MV power supplies are essential for this first-of-kind system, unparalleled both in research and industry field. 1.2 MV dc insulating tests of high voltage components were successfully completed. The integrated test to confirm 1 MV output by combining invertor systems, DC generators and transmission lines extracted errors/accidents in some components. To realize a concrete system for ITER, said events have been addressed and solutions for the repair and the improvement of the system were developed. Hence, NBTF is emerging as a necessary facility, due to the large gap with existing injectors, effectively dedicated to identify issues and find solutions to enable successful ITER NBI operations in a time bound fashion. The lessons learned during the implementation on NBTF and future perspectives are here discussed. [ABSTRACT FROM AUTHOR]

Highlights • The details of design of a large area RF plasma generator for the EAST neutral beam injector was presented. • The performance of large area RF plasma generator was reported for the first time. • The parameters of large area plasma generator were measured and presented. Abstract A large area radio frequency (RF) ion source was developed for neutral beam injector in Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). An experiment of RF source was installed in the test facility. The dimension of large area RF source is 862 mm (L) × 472 mm (W) × 420 mm (D). Six turns of coil is used to transfer RF power to the plasma. The plasma density was measured by the movable Langmuir probe in front of the plasma grid. The electron density of 6 × 1017 m−3 was achieved with RF power of 30 kW and source pressure of 0.5 Pa. The density uniformity was estimated as 95%. The heat loading on the Faraday shield and coil were also measured and acceptable for the long pulse operation with high power in the future. The details of the first tests of large area RF plasma source are presented in this paper. [ABSTRACT FROM AUTHOR]

The Acceleration Grid Power Supply of the MITICA test facility in Padova (Italy) is currently under commissioning. The power conversion system, the DC generator, and the High Voltage equipment have been individually commissioned, whereas the integration tests are ongoing. It is a challenging process due to the unconventional application, to the variety of different electrical technologies involved and to the complexity of the interfaces. During the integrated tests of the power supplies the achievement of 700 kV stable operation has been demonstrated for the first time in a Neutral Beam Injector, but an unexpected event occurred, most likely a breakdown in the HV part, which resulted in a fault of the DC generator. A subsequent test using an auxiliary power supply was performed to check the voltage withstanding capability of the HV plant, but another breakdown occurred at around 1MV. This paper describes the activity performed to identify the location of the breakdowns affecting the integrated tests. A test campaign has been devised with increased diagnostic capabilities and specific strategy conceived to trigger intentional breakdowns in specific locations and collect measurement patterns for different cases. The results of the campaign will be presented and the current understanding of the issue will be described, with a view on future tests and further improvements of diagnostics. [ABSTRACT FROM AUTHOR]

The Divertor Tokamak Test facility (DTT) is the new tokamak under construction in Frascati (Italy), with the main aim of testing different divertor magnetic configurations and technologies in view of DEMO. The Heating and Current Drive function will be provided also by one Neutral Beam Injector (NBI), accelerating negative Deuterium ions with an energy of 510 keV, giving an output power of 10 MW maximum. The Acceleration Grid Power Supply (AGPS), with 3 stages rated 167 kV each, will feed the NBI accelerator. For optical reasons, the insulating gap among the acceleration grids is at the limit of the voltage holding in vacuum, therefore frequent arc breakdowns (BD) among the grids will occur, requiring the quick switch-off of the AGPS to limit the arc energy. For the AGPS, beside the ITER-like scheme including gas-insulated HV components, an innovative approach based on the Modular Multilevel Converter (MMC) technology is being considered. This would include air-insulated HV converters, requiring a huge building volume, which at present is not available in close proximity to the tokamak building. The alternative of installing the MMC converters in a new building to be erected in an available area located far from the tokamak, and connecting the AGPS to the load with HV coaxial cables, is under study. This paper presents the studies carried out to verify the impact of the cables length on the arc energy, in case of breakdown among the grids, and to estimate the magnitude of the voltage oscillations due to resonance effects. These oscillations can occur not only between each hot pole and the return conductor, endangering the cable insulation, but also between the return conductor and local ground, with potential consequences on the laying method, the human safety and the electromagnetic compatibility with sensitive apparatus nearby, which have all been evaluated. [ABSTRACT FROM AUTHOR]

High Voltage Bushing (HVB) is the key component of Diagnostic Neutral Beam (DNB) system of ITER as it provides access to high voltage electrical, hydraulic, gas and diagnostic feedlines to the beam source with isolation from grounded vessel. HVB also provides primary vacuum confinement for the DNB system. Being Safety Important Class (SIC) component of ITER, it involves several configurational, technological and operational challenges. To ensure its operational performance & reliability, particularly electrostatic behavior, half scale down Prototype High Voltage Bushing (PHVB) is designed considering same design criteria of DNB HVB. Design optimization has been carried out followed by finite element (FE) analysis to obtain DNB HVB equivalent electric stress on different parts of PHVB, taking into account all design, manufacturing & space constraints. PHVB was tested up to 60 kV without breakdown, which validates its design for the envisaged operation of 50 kV DC. This paper presents the design of PHVB, FEA validation, manufacturing constraints, experimental layout with interfacing auxiliary systems and operational results related to functional performance. [ABSTRACT FROM AUTHOR]