ITER Heating Neutral Beam Injector Research Articles

Contributions of the extended ELISE and BATMAN Upgrade test facilities to the roadmap towards ITER NBI

ITER’s NBI systems are a first of its kind system with very challenging targets for the RF-driven ion source and the acceleration stage. In a step ladder approach, the ion source test facilities BATMAN Upgrade (BUG) and ELISE support the activities carried out at the Neutral Beam Test Facility, Padua, which is equipped with the ion source facility SPIDER and with MITICA being equivalent to the ITER Heating Neutral Beam injector (HNB), capable of operating at the full power and pulse length of the ITER HNBs. The contributions of the prototype ion source at BUG (1/8 scale) and the size scaling experiment ELISE (1/2 size ITER source) to the roadmap are manifold: for hydrogen operation the ion source performance is demonstrated in several sequential 1000 s pulses, whereas long pulse deuterium operation is limited by the heat load of the co-extracted electrons on the extraction grid. Measures like special magnetic filter field configurations or biasing of surfaces and improved Cs management are identified. Both facilities have recently been extended to full steady state compatibility and very first insights of the ion source performance with steady state extraction compared to the previously used beam blips (10 s extraction every 150 s) are already gained. A pulse length of 400 s, as required for the first deuterium campaigns at ITER, seems to be feasible soon, whereas the one hour pulse imposes the highest challenge to overcome. Investigations on beam divergence revealed a divergence at the upper limit of the acceptable value for the HNB. Measurements on the beam uniformity on the scale of beamlet groups and grid segments at ELISE demonstrated a uniformity of better than the required 90%. BUG and ELISE gave input to recent implementations at SPIDER; MITICA and ITER’s NBI. Still open points and challenges are addressed, for which a continuation of the step ladder approach is essential.

Design of a Plasma neutraliser for a Fusion reactor or as an upgrade to the ITER heating neutral beam injectors

The global efficiency of neutral beam injectors to be used on a fusion reactor needs to be of the order of, or greater than, 60 %, which is considerably higher than the efficiency of presently operating NBIs. The required efficiency can only be achieved by increasing the neutralisation efficiency from the ≈56 % obtained with the gas neutralisers currently used, which can be done using a plasma or photon neutraliser. This paper examines possible designs of a plasma neutraliser taking into account the many constraints imposed by operation in the environment around a fusion reactor, and a schematic design is proposed that meets those constraints. The design chosen uses a beam driven plasma neutraliser, which requires no additional power systems, diagnostics or interlocks.Using a plasma neutraliser is shown to improve the global efficiency to ≈55 %. Although that is below the desired value, reasons why that might be acceptable are discussed.This paper also discusses the possible use and benefits of such a neutraliser as an upgrade of the ITER heating neutral beam system.

Overview on electrical issues faced during the SPIDER experimental campaigns

SPIDER is the full-scale prototype of the ion source of the ITER Heating Neutral Beam Injector, where negative ions of Hydrogen or Deuterium are produced by a RF generated plasma and accelerated with a set of grids up to ∼100 keV. SPIDER Beam Source design complies with the ITER specific requirements, being fully installed within a vacuum vessel, and with the electrical circuits operating at the residual background pressure. The Power Supply System is composed of high voltage dc power supplies capable of handling frequent grid breakdowns, high current dc generators for the magnetic filter field and RF generators for the plasma generation.During the first 3 years of SPIDER operation different electrical issues were discovered, understood and addressed thanks to deep analyses of the experimental results supported by modeling activities.One of the main issues encountered was the presence of RF discharges on the RF circuit on the backside of the beam source, limiting the operating pressure within the source.The self-excited RF oscillators showed frequency instabilities that were found to be intrinsic limits of the application of this technology to the resonant loads of Neutral Beam Injectors; their understanding allowed assessing the technical basis for the final decision to replace the RF generators in SPIDER and MITICA and change the current ITER baseline to solid-state amplifier technology.Other issues faced regard the effect of the mutual coupling between the RF circuits on board the source, various electromagnetic compatibility problems, the limitation in the operational space to be mitigated improving the SPIDER operating strategies, the revision of the configuration of the magnetic filter field layout to avoid plasma quench.The paper gives an overview on the observed phenomena and relevant analyses to understand them, on the effectiveness of the short-term modifications provided to SPIDER to face the encountered issues and on the design principle of long-term solutions to be introduced during the currently ongoing long shutdown.

Partial discharges detection in 1 MV power supplies in MITICA experiment, the ITER heating neutral beam injector prototype

MITICA (Megavolt ITER Injector & Concept Advancement), the full scale prototype of ITER Heating Neutral Beam, is under realization at the Neutral Beam Test Facility (Padova, Italy). It is designed to deliver 16.5 MW to ITER plasma, obtained by accelerating negative Deuterium ions up to 1 MeV for a total ion current of 40 A and then neutralized.MITICA Acceleration Power Supply is composed of several non-standard equipment, beyond industrial standard for insulation voltage level (-1 MVdc) and dimensions.Voltage withstand tests (up to 1.265 MVdc) have been performed in five subsequent steps (from 2018 to 2019), according to the installation progress, after connecting equipment belonging to different procurements.During integrated commissioning, started in 2021, two breakdowns occurred in a position of the HV plant not still identified, so they could be occurred either in air or in SF6. To identify the locations of possible weak insulation points, the existing diagnostics for partial discharge detection (the precursor of breakdowns) as a first step have been improved on air-insulated parts by consisting in a set of instrumentation, like capacitive probes and off-the-shelf instruments for AC application (acoustic and electromagnetic sensors).The paper deals with the instruments qualification to assess their suitability for DC usage and then with the investigation performed in MITICA, in particular:1) sensitivity assessment campaign, with artificially produced corona effect to identify the minimum threshold of each diagnostics2) voltage application to MITICA plant, moving the instrumentation around equipment and increasing progressively the voltage looking for corona phenomena to identify possible weak insulation points.

The Full-Voltage Operation of the Acceleration Grid Power Supply for SPIDER Experiment

In SPIDER, the full-scale prototype of the negative ion source of the ITER Heating Neutral Beam Injectors, negative ions of hydrogen or deuterium are extracted and accelerated by a set of grids. This article describes the process and the technical solutions that allowed achieving the asseveration to operate the Acceleration Grid Power Supply of SPIDER at full voltage (−96 kV), reports the first experimental results, and discusses some possible enhancements that have been identified.

A Temperature Programmed Desorption Diagnostic for SPIDER Cs Operations

Cesium evaporation, aimed at enhancing the production of negative ions, has recently started during the last experimental campaigns in Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma (SPIDER). In view of the full-scale ITER heating neutral beam injector (HNBI) future operations, Megavolt ITER Injector and Concept Advancement (MITICA), understanding the behavior of cesium during plasma operation is necessary to optimize the generation of negative hydrogen ions. Determining the contribution of the different mechanisms affecting the cesium distribution in the source requires a characterization of the desorption energies of the cesium compounds deposited over the source walls. The temperature programed desorption (TPD) diagnostic allows to estimate these energies using a pair of tungsten filaments, and can also be used as a Langmuir–Taylor detector. Two identical versions of the diagnostic have been assembled, giving the possibility to perform tests and characterizations on one of the copies, while having the second copy installed and operational in SPIDER. This work presents the design and development of the device, as well as experimental results in a testing vacuum chamber and in SPIDER. Finally, first estimations of the desorption energies of cesium compounds are carried out to support the plasma-Cs models for SPIDER.

On the Effectiveness of SPIDER RF System Improvements

Source for the production of ions of deuterium extracted from a radio frequency plasma (SPIDER), the full-scale prototype of the ion source of the ITER heating neutral beam (HNB) injector, started the operation in June 2018. During the first experimental phase, some issues concerned its novel and complex power supply system. Dedicated analyses and studies, carried out to understand and address them, resulted in beam source and plant modifications to be implemented during the SPIDER major shutdown, started in late 2021. To assess the validity of the conceived countermeasures, reducing the risks of introducing additional issues, partial modifications, or mitigating actions were adopted in SPIDER in 2020, as an intermediate step to confirm each intervention with experimental evidence. This article deals mainly with two problems: the stray RF currents, flowing on plant components not intended for that purpose and the mutual interaction of the oscillators supplying the RF driven plasma source. A modification of the output stage of the oscillators allowed the reduction of the stray currents to at least 25% of the original value, while the RF circuits layout improvement resulted in the reduction of the mutual inductance of the nearby circuits to 25% of the original value. The introduced improvements and the obtained experimental results are presented and discussed.

Design and procurement of the Drying System for SPIDER beam source

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

The beamline for the ITER heating neutral beam injectors: A case study for development and procurement of high heat flux components

The ITER Neutral Beam Test Facility includes development, testing, and optimization of the full prototype of the ITER Heating Neutral Beam injectors (HNBs), named MITICA. A 40 MW precursor D−/H− beam will be produced and then neutralised and filtered along the beamline, aiming to obtain 18 MW D0/H0 beam at the calorimeter and 16.5 MW during operation into the plasma. A gas neutraliser and an electrostatic residual ion dump will process the beam together with a calorimeter, necessary for commissioning and conditioning in HNBs, and for beam dumping in MITICA.The three beamline components are formed by in-vacuum actively cooled panels made of seamless CuCrZr drilled bars and swirl tubes without armour directly exposed to beam particles. Heat fluxes up to 20 MW/m2 will be exhausted through water-cooled channels operating up to 1 h in subcooled boiling. Critical heat flux and fully developed boiling were verified using semi-empirical correlations validated for divertor fingers and limiter plates. These correlations were implemented in 1D–3D customised codes simulating local nucleate boiling heat transfer conditions. Moreover, the panels were verified against creep-fatigue by simulating plastic strains under the expected cyclic thermal loads. During beam operation, the beamline components performance will be monitored using 700 embedded thermo-mechanical sensors for protection and for ITER HNBs requirement verification.The design solutions were developed consistently with intermediate inspections during assembly to limit repairs, modification of compensation members for tolerance control in dimensional chains, remote handling operations, and nuclear radiation level at ITER. The design of coolant-vacuum barriers was adapted to implement corrosion, welding, and inspection requirements.Prototypes implementing different manufacturing solutions were produced by three potential suppliers of MITICA beamline components. This strategy led to an early assessment of production reliability and cost-effective manufacturing techniques based on proven industrial expertise. Then, the supplier selected for the procurement of the beamline components will be supported with addressed performances during the follow up.

1 MV power supplies integration issues in MITICA experiment, the ITER Heating Neutral Beam Injector prototype

Abstract MITICA, the full scale prototype of ITER Heating Neutral Beam Injector required to heat up ITER plasma with 16.5 MW injected power, is under realization at the Neutral Beam Test Facility (NBTF) in Padova (Italy) with the contributions of JApanese and EUropean Domestic Agencies (JADA and EUDA, respectively). The objective of MITICA is to produce a 16.5 MW neutral beam, obtained by accelerating negative Deuterium ions up to 1 MeV for a total ion current of 40 A and then neutralized. MITICA Power Supply (PS), installed from 2016 to 2019, includes several non-standard equipment, with ratings well beyond the present industrial standard for insulation voltage level (-1 MVdc) and dimensions: • the Acceleration Grid Power Supply (AGPS), composed of five DC Generators (DCG) rated for -200 kVdc each, connected in series to produce -1 MVdc acceleration voltage; • the Ion Source and Extraction Power Supply system (ISEPS); • the large air insulated Faraday cage (High Voltage Deck1, HVD1) hosting ISEPS and connected to the Transmission Line (TL) through an air-to-SF6 Bushing (High Voltage Bushing Assembly, HVBA); • a 100 m gas (SF6) insulated TL, connecting AGPS and ISEPS to the beam source installed inside the vacuum vessel through • the SF6-to-vacuum HV Bushing (HVB). The definition of the interfaces both between components supplied by the different DA’s and towards the buildings has been studied and finalized as far as possible during the design phase. Nevertheless, during the installation phase some issues emerged and had to be solved, minimizing modifications of the components already manufactured. The paper deals with the experience gained during the installation activities, focusing on solutions to interface the aforementioned equipment with NBTF buildings according to the stringent dimensional requirements and to the electrical insulation issues of the TL from the buildings. In particular, the solutions adopted to realize the electrical and mechanical interfaces between the TL and the HVBA are described in detail.

Voltage hold off test of the insulating supports for the plasma grid mask of SPIDER

Abstract A temporary mask with a limited number of apertures for the Plasma Grid (PG) of SPIDER, the full scale prototype of the Ion Source of the ITER Heating Neutral Beam Injector, was conceived to explore SPIDER operation with a range of vessel pressure larger than the original requirements. The voltage hold off of PEEK insulating supports, called “pushers”, which held in position the PG mask was experimentally verified on a testbed capable of reproducing the working conditions of the pushers in SPIDER (electric field, magnetic field, vessel pressure and PG temperature). This paper deals with the design of the testbed and reports on the results of the relevant experimental campaign.

SPIDER plasma grid masking for reducing gas conductance and pressure in the vacuum vessel

• SPIDER Beam started operation in spring 2018. • SPIDER is the first ITER relevant beam source operating completely in vacuum. • Discharges on back of the source occurred due to high H2 pressure in the vessel. • A “mask” installed onto Plasma Grid, reduces nr. of accelerator apertures to 80. • Thanks to this mask, it was possible to operate SPIDER strongly reducing discharges. SPIDER experiment is operating at the PRIMA site in Padova (I) since June 2018, with the aim of testing and optimizing the negative ion source prototype for ITER Heating Neutral Beam Injectors. In the first operational phase it was discovered that, as the in-vessel hydrogen pressure exceeds the design requirements, discharges occur on the back of the radio frequency source. A specific operational campaign allowed defining a threshold below which the discharge probability is strongly reduced. In order to extend the operational range of the source pressure above the nominal value, while a significant upgrade of the vacuum pumping system is designed and realized, it was decided to proceed with the SPIDER operations by applying a temporary solution. A mask was installed on the beam source plasma grid, closing most of its apertures, in order to reduce the gas conductance between the inside of the radio frequency source and the surrounding volume. At first only 80 over 1280 apertures are left open, with a specific layout properly arranged so as to guarantee the possibility to diagnose the beam characteristics and to evaluate its uniformity. In the paper the plasma grid masking system is described. Finally, an overview of the behavior of plasma grid mask during SPIDER operations is given.

CRISP: A compact RF ion source prototype for emittance scanner testing.

A movable Allison type emittance scanner is being developed to characterize the phase-space distribution of the beamlets of spectral phase interferometry for direct electric-field reconstruction, the prototype RF negative ion source of the ITER heating neutral beam injector. To test the electronics and verify the capability of the device to resolve nearby beamlets, a compact RF ion source prototype has been set up, capable of accelerating 1 mA of helium ions up to a voltage of 2 kV. A commercial 100 W RF generator creates a plasma inside a Pyrex tube, with a density between 1015 and 1016 m-3 and an electron temperature up to 15 eV. Three multi-aperture grids in accel-decel configuration extract and accelerate the ions, which are measured with a Faraday cup. We present in this paper the characterization of the ion source and its first operation, showing that it is suitable for the commissioning of the Allison scanner.

Manufacturing, Installation, and Commissioning of the Residual Ion Dump Power Supply for MITICA Experiment

The Residual Ion Dump Power Supply (RIDPS) is devoted to feed the Electrostatic Residual Ion Dump, a device which shall collect the residual ions at the output of the gas Neutralizer in the ITER heating neutral beam injectors (HNBIs) and Megavolt ITER Injector and Concept Advancement (MITICA), the full-scale prototype located in Padua (Italy), as part of the Neutral Beam Test Facility. This power supply has to provide an average voltage of up to 25 kV, plus an ac low-frequency voltage component with a maximum amplitude of 5 kV, with sinusoidal or trapezoidal waveform. The nominal output current is 60 A, and the maximum pulse duration is 1 h. The RIDPS for both MITICA and ITER is being provided by OCEM Energy Technology s.r.l. The manufacturing of the RIDPS for MITICA has been concluded in summer 2018 and the Site Tests in Padua have been successfully completed in May 2019. In this article, the most significant issues faced during the design, integration and tests, and the solutions identified are described. The most interesting results of the Factory and Site Tests are presented, with particular emphasis on those proving the compliance of the RIDPS with the most critical requirements.

Design and development of an Allison type emittance scanner for the SPIDER ion source.

Low divergence negative ion beams are crucial for the development of ITER-like fusion reactors. SPIDER is the prototype beam source of the ITER heating neutral beam injector, and it recently started beam acceleration, up to a voltage of 30 kV. The main diagnostics used to measure beamlet divergence are a movable diagnostic calorimeter (STRIKE), which gives the thermal footprint of the beamlets; beam emission spectroscopy; and visible imaging. These systems do not allow a direct measurement of single beamlet phase-space distribution, which is useful for comparison with numerical simulations and to estimate accelerator performances. To this purpose, a movable Allison type emittance scanner for the SPIDER negative ion beam was developed and proposed for the installation on the STRIKE supporting structure. This paper describes the numerical analyses performed to dimension the mechanical and electrical components, such as the Faraday cup and the slits. An analytical approach based on the integration of an arbitrary phase-space distribution was adopted in order to simulate the device performances. The constraints due to the operation in a high heat load environment are discussed.

Numerical analyses and tests for optimized and enhanced heat transfer solutions in DEMO

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.

Design and preliminary operation of a laser absorption diagnostic for the SPIDER RF source

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

Progress in the design of MITICA control and interlock systems

The construction of MITICA, the full-size prototype of the ITER Heating Neutral Beam Injectors (HNBs) is in progress at the Neutral Beam Test Facility (NBTF) located in Padova, Italy. The design of the central control (CODAS) and interlock (CIS) systems is progressing taking into account the requirements coming from the MITICA plant units, in terms of number and type of interface signals, data sampling frequency values, operating states, protection actions and required reaction times and reliability. As the HNBs will be ITER components, additional requirements derive from the ITER plant control design handbook that specifies the design criteria and standard technologies to be applied to ITER plant systems.A distinction must be made in MITICA between plant units that will be part of the HNB, such as the power supply systems, and plant units that are auxiliaries, such as the gas and vacuum system. In the NBTF these components shall interface with MITICA CODAS and CIS, whereas in ITER they will be autonomous plant systems providing services to the HNB. The distinction reported above suggests defining the architecture of MITICA Instrumentation and Control (I&C) as split into the following functional blocks: i) HNB plant system including components that will be part of the HNB; ii) Service plant system including all operational plant units that need control in the NBTF but are implemented outside the ITER HNB; iii) Diagnostics plant system; iv) Central I&C (CODAS and CIS).The paper presents the requirements for MITICA CODAS and CIS and derives from these the system architecture. The design of CODAS and CIS is discussed with reference to the lesson learned in the implementation of SPIDER CODAS and CIS (the NBTF ion source test bed) and to the technologies suitable for the implementation.

Conceptual design of Integrated Bivalent Auxiliary System (SABI) supporting the MITICA cooling plant and cryogenic plant

The first ever built full-scale prototype of the ITER heating neutral beam injector is the MITICA experiment at PRIMA Neutral Beam Test Facility, under realization in Padova, Italy.This experiment consists of several in-vessel components: most of them are actively cooled by a large Cooling Plant, still under construction. Coolant is deionized water produced by a Chemical Control System (CCS).In the PRIMA facility, a cryogenic plant (currently in manufacturing phase) is also foreseen to serve the cryopumps to be installed in the MITICA Beam Line Vessel.During the installation of cooling plant and the design phase of cryogenic plant, two different issues came into evidence: waste water (produced by the CCS during operations) could not be delivered directly into the public network and the warm compressor of the cryogenic plant, originally air-cooled, changed to a water-cooled system.For the first issue the solution was found in a Water Treatment Unit (WTU) that requires the supply of hot water to distil the wastewater and to concentrate it.The second issue and the need to balance the heat loads and the cost and space constraints lead to an innovative system: it is essentially based on a high temperature water-water heat pump, by means of which the heat rejected by the warm compressor is used to heat up the WTU.Moreover this will reduce the operational costs by introducing a significant energy saving.

Simulation and verification of air cooling system for -1MVdc MITICA High Voltage hall in Padova

The first ever built full-scale prototype of the ITER heating neutral beam injector is the MITICA experiment at PRIMA-NBTF (Neutral Beam Test Facility), under realization in Padua, Italy.The MITICA experiment includes many auxiliary plants; this paper is focused on the simulation and verification of the air cooling system of the High Voltage Power Supply (−1 MVdc), hosted in a Faraday cage (HVD1, High Voltage Deck1) inside a dedicated Building (High Voltage Hall, HVH) at PRIMA-NBTF.The HVD1 hosts the MITICA ISEPS (Ion Source Power Supply) in a two-floor cage, air insulated to ground for -1MVdc, with huge dimensions 12.5 m (L) × 8.4 m (W) × 9.6 m (H).Severe cleanliness constraints are required in the HVH hosting the HVD1, due to the high voltage operative conditions (the −1 MVdc Faraday cage is air insulated).The thermal behavior and air distribution in HVH are investigated as described in this paper.