ITER Neutral Beam Injector Research Articles

On the vertical uniformity of an ITER-like large beam

Abstract A beam uniformity higher than 90% is a crucial requirement for the ITER neutral beam injection (NBI) system, together with very low beam core divergence ( The ELISE test facility, equipped with an ITER-like multi-aperture 3-grid system, produces large ITER-like beams, with half the size in vertical direction. Due to the distance of the beam diagnostics from the grids, the overlapping of the beamlets is large, and information on divergence and uniformity can be retrieved in terms of beamlet groups or beam segments. In general, good symmetry in accelerated current density between beam segments is achieved. The vertical beam uniformity is determined by caesium distribution and by plasma parameters at the plasma grid, which are inhomogeneous due to the interplay of filter field and bias potential. In this work, the role of filter field and bias potential on the vertical beam uniformity (in intensity and divergence) between the two beam segments is identified: the filter field mainly affects the extracted current density and its distribution within and between the segments; the bias potential can be used to tune the beam divergence, differently between the beam segments.

Study of high DC voltage breakdown between stainless steel electrodes separated by long vacuum gaps

High voltage (HV) insulation across a single gap in vacuum and low-pressure gas is a critical issue in relation to the development and realization of the electrostatic accelerator for the ITER neutral beam injector (NBI) (Toigo et al 2017 New J. Phys. 19 085004). The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER NBI (De Lorenzi et al 2011 Fusion Eng. Des. 86 742–5). A voltage up to 800kVDC was achieved in the HVPTF during the experimental campaigns with a sphere-plane configurations having variable gap length (from 30 to 150 mm) and pressure ranging from high vacuum (10–7 mbar) to 10–3 mbar in argon. Such an experimental campaign represents one of the few examples where voltages higher than 500–550 kV DC are sustained by a single vacuum gap between electrodes (Rohrbach 1971 CERN Report 71-5) and therefore constitutes additional experience to improve the knowledge of voltage holding across large vacuum gaps. The results in high vacuum indicate that at the beginning of voltage conditioning the breakdown events occur at the same cathodic electric field irrespective of the electrode geometry. However, after sufficient conditioning time, the breakdown voltage distribution seems to depend also on the electric field at the anode and on the total voltage between electrodes. A benchmark between a numerical tool previously developed to predict the voltage holding in high vacuum (voltage holding prediction model, VHPM (Pilan et al 2011 Fusion Eng. Des. 86 742–5)) and the experimental results is also reported and discussed.

RAMI evaluation of the beam source for the DEMO neutral beam injectors

Abstract DEMO is a first-of-a-kind DEMOnstration fusion power plant [1] , [2] and is intended to follow the ITER experimental reactor. The main goal of DEMO will be to demonstrate the possibility to produce electric energy for the grid from the fusion reaction early in the second half of the century. The injection of high energy neutral (1 MeV) particle beams is one of the main tools to heat the plasma up to fusion conditions, control the plasma burn phase and ramp the plasma down. Within the EUROfusion Framework a conceptual design of the Neutral Beam Injector (NBI) for the DEMO fusion reactor is currently being developed. Thereby, Reliability, Availability, Maintainability and Inspectability (RAMI) have to be taken into consideration for the conceptual design of the DEMO NBI, together with the exploitation of the currently available return of experience from the ITER NBIs. Comparing the failure risk of two different source concepts due to the considered failure modes has allowed for further design developments aiming at exploiting the advantages of the modular approach while minimizing its drawbacks.

Reconstruction of the large multi-aperture beam via IR calorimetry technique and beam emission spectroscopy at the ELISE test facility

The ELISE test facility with its half ITER size ion source is an important intermediate step in the European development roadmap toward the ITER neutral beam injection system. Of particular interest is the beam characterization at ELISE in terms of homogeneity and divergence. The two main beam diagnostic systems are beam emission spectroscopy (BES), providing beam intensity and divergence and infrared (IR) calorimetry applied to the surface of a diagnostic calorimeter, which provides a spatially resolved 2D map of the beam power density. Beam parameters such as width, intensity, position and accelerated current are retrieved by a fitting routine which is validated using synthetic beam profiles, thus demonstrating that the beam parameters are correctly reproduced. By comparing results of BES and IR calorimetry for several thousand pulses, it is demonstrated that the beam width is, to a large extent, determined by the beam divergence. BES and IR calorimetry are then combined to reconstruct the beam profile and this is the basis to investigate procedures for improving the beam quality in terms of power density homogeneity.

First operation in SPIDER and the path to complete MITICA.

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

Interpreting the dynamic equilibrium during evaporation in a cesium environment.

The cesium ovens for the prototype source of the ITER neutral beam injectors are currently tested in the CAesium Test Stand (CATS) facility, with a background pressure of 10-6 mbar. Different diagnostics are here installed: two Langmuir-Taylor detectors allow us to determine the Cs vapour evaporation rate from the oven and the Cs density at different positions in the vacuum chamber; and laser absorption spectroscopy is used to measure the density integrated over a line of sight and a quartz crystal microbalance to detect the cesium mass deposited in time over a surface. In this paper, we present a model to describe the dynamic equilibrium in the evaporation chamber of CATS with the first oven tested in order to gain information about the Cs sticking coefficient at the walls. The model hence includes sticking and energy accommodation of the Cs atoms to the walls, calculates the flux density at the surfaces, and provides the Cs atom density at any location in the volume. By this model, we simulate the Cs evaporation and the equilibrium density, comparing the modeled results with the experimental data. As a result, a sticking coefficient of 2% is obtained.

Concept of Plasma Heating and Current Drive Neutral Beam System for Fusion Neutron Source DEMO-FNS

Steady-state operation of a fusion neutron source (FNS) requires plasma heating and current drive by means of additional power delivered by neutral beams. Six neutral beam injectors (NBI) will provide the DEMO-FNS machine with additional heating power up to 30 MW, with neutral particle energy of 500 keV. NBI systems developed for ITER can serve as the prototype for DEMO-FNS, as both systems have similar ion source current, with accelerated beam power in ITER NBI (1MeV) being twice as large as in DEMOFNS. The paper describes the NBI system with account of its integration into DEMO-FNS tokamak complex.

Laser absorption spectroscopy studies to characterize Cs oven performances for the negative ion source SPIDER

The SPIDER H−/D− ion source is currently in operation in the Neutral Beam Test facility (NBTF) at Consorzio RFX (Padova, Italy) to prove the possibility of generating up to 40 A of negative ions, with a maximum extracted current density of 350 A/m2 (H)/285 A/m2 (D) and a fraction of co-extracted electrons not greater than 0.5 (H)/1 (D). These performances are required for the realization of the ITER Neutral Beam Injector (NBI), which should deliver 16.7 MW to the plasma by means of negative ions accelerated up to 1 MeV and neutralized before being injected into the ITER tokamak. In order to obtain such high extracted current densities and low co-extracted electron fractions it is necessary to lower the work function of the surface of the acceleration system grid facing the source; this will be accomplished by coating the surfaces with Cs, routinely evaporated by three ovens. The functionality of the ovens has been tested at the CAesium oven Test Stand (CATS), hosted at NBTF. The test stand is equipped with several diagnostics, among which a Laser Absorption Spectroscopy (LAS) diagnostic. Using a tunable laser diode, the LAS diagnostic gets the high resolution absorption spectrum of the Cs 852 nm D2 line along a line of sight to measure Cs density at ground state. The paper describes the test stand and the LAS diagnostic, together with the characterization of the ovens to be installed in SPIDER. The paper will also study the systematic density underestimation effect caused by Cs ground state depopulation, as a function of laser intensity and of Cs density, in the perspective of correcting the density evaluation.

Simulation and measurement of rarefied gas flow and neutral density profiles through a large multiaperture multigrid negative ion accelerator

In large multi-aperture, multi-grid negative-ion accelerator for fusion application, the background gas density in the electrostatic accelerator causes a loss of negative-ion current and the formation of dangerous stray particles. In addition, to sustain in dynamic equilibrium a sufficient gas pressure in the plasma ion-source, a very large pumping speed is typically necessary. This paper presents and compare simulations and measurements of gas flows and background pressure profiles, through the electrostatic accelerator of the SPIDER beam source, the full-size prototype source of the ITER neutral beam injectors. The gas pressure profiles through the ion accelerator were measured by multiple movable capacitive pressure gauges, mapping the uniformity of the profiles along nine beamlets at three different filling pressures. The gas flow simulations in molecular flow regime were performed with a large-scale view-factor model, and with a single-aperture periodic test particle Monte Carlo model. The models reproduced the measured pressure profiles but additionally, provided also the profiles of local gas density. The gas conductance and the profiles calculated with the full-scale gas flow model correctly reproduce the measured profiles, and the transversal non-uniformities over the 1.6 m × 0.6 m cross section. The calculated gas density profile is also verified by comparing the two different numerical approaches. The gas flow model validated at room temperature can be used to simulate the pressure profile during operation, in presence of gas heating and dissociation by the source plasma.

Formation of large negative deuterium ion beams at ELISE.

Negative ion sources for neutral beam injection (NBI) in fusion experiments are based on the surface production of H- or D- on cesiated low work function surfaces. In the recent years, it was demonstrated at the large RF driven ion source of the ELISE (Extraction from a Large Ion Source Experiment) test facility that the requirements for the ITER NBI systems can be fulfilled by hydrogen. This is a big step toward the first operational period of ITER, planned for up to 2035. However, for the following operational period, neutral beam systems working in deuterium are needed. Operation of negative hydrogen ion sources in deuterium is significantly more demanding than in hydrogen: the amount of coextracted electrons is much higher and their increase during pulses is much more pronounced, limiting the achievable performance. This paper presents the results of investigations aimed to improve the insight into the physics related to this isotope effect. Due to the higher atomic mass of deuterium, cesium is removed much more effectively from reservoirs at the walls, resulting in a depletion of these reservoirs and a strongly increased cesium density in the plasma. Additionally, a correlation between the fluxes of charged particles toward the inner ion source surfaces and the coextracted electrons is identified.

SPIDER in the roadmap of the ITER neutral beams

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

The High Voltage Deck 1 and Bushing for the ITER Neutral Beam Injector: Integrated design and installation in MITICA experiment

MITICA is the full scale prototype of the ITER Heating Neutral Beam (HNB) currently under construction at the Neutral Beam Test Facility in Padova (Italy). In the ITER HNB, negative ions (H−/D−) are produced in the Ion Source (IS) polarized to ground at −1012 kV, then extracted and accelerated to ground at 1 MeV energy and finally neutralized. The complex power supply system feeding the IS includes two non-standard equipment, beyond the current industrial standard for insulation voltage level (-1 MVdc) and dimensions: 1) the High Voltage Deck1 (HVD1), a large air insulated Faraday cage (12.5 m (L) x 8.4 m (W) x 9.6 m (H)) hosting the IS power supplies and diagnostics; 2) the High Voltage Bushing Assembly (HVBA), a -1 MVdc air to SF6 Bushing which interfaces the HVD1 with the SF6 insulated Transmission Line (TL) connecting the Acceleration Grid Power Supply system with the IS, carrying inside its High Voltage electrode all IS power and diagnostics conductors. The HVD1 and HVBA are installed inside a large HV hall. The main design choices leading to the final hall layout, integrating the HV experimental equipment and the conventional building plants assuring the necessary clearance required by such very high electric insulation level, are presented. Moreover, the paper reports on the manufacturing design developed by the supplier, the factory type tests to validate the design of the HVD1, the factory tests carried out on the HVBA and the on-site installation, commissioning and testing activities.

Design and mockup tests of the RING photo-neutralizer optical cavity for DEMO NBI

High energy Neutral Beam Injection (NBI) is one of the methods being considered to heat EU DEMO plasma. A major issue of present NBI systems is the limited efficiency of the gas neutralizer (for ITER NBI ˜55%), which impacts on the overall system efficiency. An attractive method, but still undemonstrated at full performance, is the photo-neutralization of the negative D-ion beam, with an expected neutralization efficiency for a full-scale DEMO NBI up to 90%.A possible scheme for photo-neutralization is named RING (Recirculation Injection by Nonlinear Gating) where a laser second harmonic is generated and trapped within a non-resonant optical cavity. A mockup of the optical cavity is being operated in Consorzio RFX with a low repetition rate Nd:YAG laser (f = 10 Hz, λ = 1064 nm) to study the feasibility of the RING concept and its potentiality for a full-scale NBI photo-neutralizer. The 2nd harmonic generation efficiency has been measured using a set of 3 Lithium Triborate (LBO) crystals in a disk configuration, confirming the non-linearity of the process with the total crystal thickness. Losses during the 2nd harmonic recirculation in the cavity have been quantified, and the number of 2nd harmonic roundtrips has been estimated with photodiode measurements in order to evaluate the optical cavity performance. Further improvements are considered to increase the optical cavity photon accumulation.

SPIDER gas injection and vacuum system: From design to commissioning

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

A new tungsten wire calorimeter for the negative ion source testbed BATMAN Upgrade

Within the framework of the ion source development for the ITER and DEMO Neutral Beam Injection (NBI) systems, IPP Garching has recently upgraded the radiofrequency-driven negative ion source testbed BATMAN. One of the requirements for the ITER NBI system is to produce a beam power density homogeneity above 90% over its large beam size extracted from an ion source of 1 × 2 m2. The beam uniformity is going to be investigated, on a smaller scale, in BATMAN Upgrade with the new ITER-like beam optics. The testbed is equipped with several beam diagnostic tools to measure beam power, uniformity and divergence. A new tungsten wire calorimeter (TWC) has recently been developed to characterize the beam quantitatively and with an improved spatial and temporal resolution. The TWC consists of an array of thin tungsten wires with a diameter of 0.3 mm, placed in the beam path. The wires are heated by the beam up to 3000 °C and emit visible light, which is observed by an optical camera. While the existing TWC provides a qualitative impression of beam power characteristics and is placed at a distance of almost 2 m from the beam extraction system, the new TWC is positioned only 19 cm downstream from the extraction system and allows observation of the single beamlets for the first time in this testbed. In order to determine the correlation between the pixel intensity measured by the camera and the power density impinging on the wires, one wire is equipped with an ohmic heating system which allows heating up the wire with a known power, thus enabling calibration of the diagnostic tool.In this paper the design, including FEM simulations of the wire thermal behavior, manufacturing and installation of the new TWC inside the BATMAN Upgrade testbed are described and the first, very promising, observations of beamlet intensities are presented.

Achievements of the ELISE test facility in view of the ITER NBI

Neutral Beam Injection (NBI) for ITER shall deliver in total 33 MW heating power to the plasma with two injectors at a beam energy of 1 MV. Taking neutralization efficiency and all losses along the beam path into account a negative ion current density of 329 A/m2 (H− for 1000s) and 286 A/m2 (D− for 3600 s) has to be extracted from each ion source (size 1 × 2 m2) with a beam uniformity >90%. For the optimization of the radio frequency driven (RF) negative ion source and its extraction system the ELISE test facility has been set up at IPP as a size scaling experiment with a source area of 0.9 × 1 m2 and an extraction area of 0.1 m2.Since its experimental start in 2013 continuous progress was achieved improving the system technically for long pulse operation at high RF power (total 300 kW for 4 drivers) as well as approaching ITER´s physics requirements for the ion beam. For hydrogen ˜90% of the required ion current density can be extracted for long pulses (1000s); the accelerated current density even exceeded the ITER target. For deuterium operation the main challenge is the amount, temporal in-stability and spatial asymmetry of co-extracted electron current which limit the applicable RF power and the source performance.Shaping the magnetic field topology and controlling the electric fields inside the source together with an improved caesium conditioning procedure have been identified as possible tools to improve the source performance.

NBImag: A Useful Tool in the Design of Magnetic Systems for the ITER Neutral Beam Injectors

NBImag is a code suitable for the design and optimization of complex magnetic field configurations, such as that of a multiaperture, multistage negative ion source and accelerator. The NBImag code has been developed for the design of the ITER neutral beam injector, whose full-size prototype, MITICA, is presently under construction in Padua, Italy. The ITER injector shall produce a focused beam of neutral particles (H or D) having an energy of about 1 MeV and a total power of 16.5 MW for 3600-s continuous operation. The NBImag code is based on an integral formulation and allows an efficient calculation of any static magnetic field configuration on the basis of the geometry of the magnetic sources, with linear material and permanent magnets. NBImag also includes magnetic force and inductance calculations, based on the same formulation. Thanks to the capability of efficiently describing a large number of permanent magnets with limited computational effort, NBImag has also been integrated with different automatic optimization procedures for the solution of inverse magnetic problems. This paper describes the formulation of the code and optimization algorithms, the validation against analytical models and experimental measurements, and the application to the design of MITICA.

Review of particle-in-cell modeling for the extraction region of large negative hydrogen ion sources for fusion.

Particle-in-cell (PIC) codes are used since the early 1960s for calculating self-consistently the motion of charged particles in plasmas, taking into account external electric and magnetic fields as well as the fields created by the particles itself. Due to the used very small time steps (in the order of the inverse plasma frequency) and mesh size, the computational requirements can be very high and they drastically increase with increasing plasma density and size of the calculation domain. Thus, usually small computational domains and/or reduced dimensionality are used. In the last years, the available central processing unit (CPU) power strongly increased. Together with a massive parallelization of the codes, it is now possible to describe in 3D the extraction of charged particles from a plasma, using calculation domains with an edge length of several centimeters, consisting of one extraction aperture, the plasma in direct vicinity of the aperture, and a part of the extraction system. Large negative hydrogen or deuterium ion sources are essential parts of the neutral beam injection (NBI) system in future fusion devices like the international fusion experiment ITER and the demonstration reactor (DEMO). For ITER NBI RF driven sources with a source area of 0.9 × 1.9 m2 and 1280 extraction apertures will be used. The extraction of negative ions is accompanied by the co-extraction of electrons which are deflected onto an electron dump. Typically, the maximum negative extracted ion current is limited by the amount and the temporal instability of the co-extracted electrons, especially for operation in deuterium. Different PIC codes are available for the extraction region of large driven negative ion sources for fusion. Additionally, some effort is ongoing in developing codes that describe in a simplified manner (coarser mesh or reduced dimensionality) the plasma of the whole ion source. The presentation first gives a brief overview of the current status of the ion source development for ITER NBI and of the PIC method. Different PIC codes for the extraction region are introduced as well as the coupling to codes describing the whole source (PIC codes or fluid codes). Presented and discussed are different physical and numerical aspects of applying PIC codes to negative hydrogen ion sources for fusion as well as selected code results. The main focus of future calculations will be the meniscus formation and identifying measures for reducing the co-extracted electrons, in particular for deuterium operation. The recent results of the 3D PIC code ONIX (calculation domain: one extraction aperture and its vicinity) for the ITER prototype source (1/8 size of the ITER NBI source) are presented.

Design of remote handling, nuclear and vacuum compatible connectors for mineral insulated thermocouples of ITER neutral beam injectors

Mineral insulated (MI) thermocouples will be connected to MI extension cables through custom connectors for sensors positioned on in-vessel components of the ITER neutral beam injectors.The connectors can house up to 64 metal-ceramic terminations of MI thermocouples locked in position by insulating plates and bushes; an external shielding shell will be mounted as protection. Coupling between shields and insulating plates have been developed with shadow edges to avoid deposition of conductive coatings of sputtered materials, so preserving the electrical insulation.Electrical design has been developed to shield signals from electromagnetic interference and to isolate the sensor shield from the vessel grounding, so avoiding ground loops.Remote handling design incorporates a long ball-ended dowel that locates in a circular hole and short parallel dowels that locate in a short slot to provide high level of position control within a compact arrangement. The plug has been provided with two grips for the manipulator interface.Mechanical design has been accomplished by selecting materials and solutions compatible with vacuum and radiation resistance in the ITER environment, considering the manufacturing feasibility especially for machinable glass-ceramic used for plates and bushes, verifying the mating alignment of standard contact pins also at the signal feedthrough interface.

Technology developments for a beam source of an NNBI system for DEMO

Based on the requirements for heating and current drive of a DEMO (tokamak) device a neutral beam injection (NBI) system faces several challenges, from which the achievement of high wall-plug efficiency is the driving parameter for the beam line design. For boosting the wall-plug efficiency of the ITER NBI system from about 0.25 to target values around 0.4 (pulsed DEMO) or 0.5–0.6 (steady state DEMO), the standard gas neutralizer concept has to be modified or replaced by the innovative laser neutralizer concept for which the modular concept of the ion source allows for adaption. The technology developments of ITER’s high power RF-driven ion source are continued by optimization of the RF-driver geometry and the resulting overlap of the driver plasmas in a large expansion chamber. Insights in the caesium evaporation and distribution in the ion source could allow for a reduction of the caesium consumption. On the other hand, the caesium dynamics influences the temporal source performance and the beam uniformity such that investigations on alternatives to caesium became an R&D task. Finally, the status of proof-of-principle experiments on the laser neutralization technique and its challenges for up-scaling from a lab scale experiment to a beam line are discussed.