ITER Neutral Beam Test Facility Research Articles

Corrosion phenomena and deposits in ITER Neutral Beam Test Facility primary cooling circuits

The ITER Neutral Beam Test Facility (NBTF) is hosted in Padua and includes two experiments: MITICA, the 1 MeV full-scale prototype of the ITER HNB injector, and SPIDER, the 100 keV full-size ITER Radio Frequency (RF) negative ion source. SPIDER and MITICA experiments are actively cooled by Ultrapure Water (UPW) to electrically insulate in-vessel components that are biased to high voltage levels. Water conductivity is an important monitored parameter to ensure components' insulation by limiting the leakage current of active cooled equipment. A very low conductivity water is especially important in MITICA, where components need to operate up to 1 MVdc, an insulation level beyond the actual industrial standard. Careful selection of suitable materials for any in-vessel (vacuum insulation) and out-vessel component (air insulation) is of utmost importance, as their interaction with water and different environment may affect their chemical and mechanical properties. Additionally, materials selection can severely influence water chemical characteristics: cooling circuits are made of metals (copper, aluminium, steel) and insulating materials (plastic, rubber) when connecting parts at different electric potentials.During the first years of cooling plant exploitation, it was shown that water degrades more quickly than estimated by design. The Primary Circuits (PCs) that showed the most severe water degradation during operation are SPIDER and MITICA power supply ones, respectively called PC01 and PC08. This paper describes the results of specific experimental tests performed on MITICA PC08 to evaluate possible causes of water degradation and detect sources of contaminants that might compromise future experimental campaigns. The cooling circuit was subdivided in different sections and water circulation tests were performed at constant temperature and flowrate. Water samples were collected and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analyses were performed to quantify the type and amount of metals released. Moreover, components samples collected along their cooling circuits were characterized by Scanning Electron Microscope (SEM) technique to detect undesired contaminants and immersed in UPW to study their corrosion behaviour using metal release tests.

A Versatile Board for Event-Driven Data Acquisition.

Event-driven data acquisition is used to capture information from fast transient phenomena typically requiring a high sampling speed. This is an important requirement in the ITER Neutral Beam Test Facility for the development of one of the heating systems of the ITER nuclear fusion experiment. The Red Pitaya board has been chosen for this project because of its versatility and low cost. Versatility is provided by the hosted Zynq System on Chip (SoC), which allows full configuration of the module architecture and the OpenSource architecture of Red Pitaya. Price is an important factor, because the boards are installed in a hostile environment where devices can be damaged by EMI and radiation. A flexible solution for event-driven data acquisition has been developed in the Zynq SoC and interfaced to the Linux-based embedded ARM processor. It has been successfully adopted in a variety of data acquisition applications in the test facility.

Characterization of cesium and H−/D− density in the negative ion source SPIDER

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.

Assessment of IEEE 1588-Based Timing System of the ITER Neutral Beam Test Facility

MITICA is one of the two ongoing experiments at the ITER Neutral Beam Test Facility (NBTF) located in Padova (Italy). MITICA aims to develop the full-size neutral beam injector of ITER and, as such, its Control and Data Acquisition System will adhere to ITER CODAC directives. In particular, its timing system will be based on the IEEE1588 PTPv2 protocol and it will use the ITER Time Communication Network (TCN) on a dedicated Ethernet LAN. Following the ITER device catalog, the National Instruments PXI-6683H PTP timing modules will be used to generate triggers and clocks synchronized with a PTP grandmaster clock. The “Lazy Trigger" technique, then, will be adopted to implement event-driven data acquisition without the need of any hardware link in addition to the Ethernet connections used to transfer data and timing synchronization. To evaluate the accuracy over time that can be achieved with different network topologies and configurations, a test system has been set-up consisting of a grand master clock, two PXI-6683H devices and two PTP aware network switches. In addition, a detailed simulation of the network and the involved devices has been performed using the OMNET++ discrete event simulator. The simulation parameters include not only the network and switches configuration, but also the PID parameters used in the clock servo controllers. A comparison between simulated and measured statistics is reported, together with a discussion on the possible optimal configuration strategies.

The SPIDER Pulse Plant Configuration Environment

At the ITER Neutral Beam Test Facility, the Source for the Production of Ions of Deuterium Extracted from Radio frequency plasma (SPIDER) has been in operation since 2018 aiming at prototyping the heating and diagnostic neutral beam appliances in view of the ITER demanding requirements for plasma burning conditions and instabilities control. For the sake of safety, machine protection and efficiency it is necessary to follow an accurate planning strategy and approval action of the experiment parameter settings. Although the initial tools available for the SPIDER integrated commissioning and early campaigns offered the basic functionality to perform the necessary tasks, there were a set of relevant issues that were identified as needing improvements for an efficient and safe configuration environment. Namely, the fact that there were no tools indicating the parameters change since the previous pulse, or the lack of a comparison tool between a new set of parameters and a previous setup, demanded a tedious and error prone verification of all parameters in the sequence. Moreover, several verifications should be automated according to the machine safety limits, increasing the safety check by human and automated machine (algorithmic) validation. This contribution depicts (i) the approval sequence designed for SPIDER safe operation; (ii) the configuration environment requirements according to the SPIDER pulse preparation procedure; (iii) the decision of the development tools used for implementation and design; (iv) the implementation details and preliminary tests of the global environment.

Functional safety assessment process for MITICA safety system in the ITER neutral beam test facility

The Safety System of the ITER Neutral Beam Test Facility (NBTF) aims to deal with all risks for people and environmental safety related to SPIDER and MITICA operation. A coordinating system, called Central Safety System (CSS), was designed to supervise safety-relevant systems, implement safety actions on the different plant units to guarantee the NBTF personnel safety and provide an effective human interface of the NBTF safety. The SPIDER CSS, the part of CSS dealing with SPIDER and NBTF shared items, is in operation, while the MITICA CSS is in the detailed design phase. The MITICA CSS will have the same architecture and principles as SPIDER CSS. A safety risk analysis was carried out based on a Hazard Identification (HAZID) methodology: MITICA plant was partitioned into 46 nodes, for each node possible hazards were identified based on a list drawn up from EN ISO 12100 standard and, after that, the risk assessment process was performed. As a result, the Safety Instrumented Functions (SIF) to be implemented in the MITICA CSS were defined. Through a Layer Of Protection Analysis (LOPA) approach, in accordance with the IEC 61511 standard, the Safety Integrity Level (SIL) was allocated to each SIF. This paper will describe the MITICA risk analysis process and present the relevant functional safety results for MITICA CSS implementation.

CODAS for long lasting experiments. The SPIDER experience

The SPIDER experiment is the first of two experiments being held at the ITER Neutral Beam Test Facility in Padova (Italy). SPIDER has been operating since 2018, initially with pulse duration of a few seconds and currently with pulses lasting up to 3000 s. The paper reports the CODAS experience gained after three years of operation. In particular, Data Storage and Data Access adopted strategies will be discussed, that proved to be of high impact in overall system performance and maintainability.Regarding Data Storage strategy, a tradeoff must be defined between the continuous and event driven data acquisition. Continuous data acquisition, i.e. sampling data at a constant frequency, represents the normal operation in short experiments, but can easily lead to an unmanageable amount of data for long lasting experiments. On the other side, Data acquisition at a varying rate, that is increased upon the occurrence of given events leading to an improved signal dynamics, is required for a subset of signals that describe physical phenomena with fast dynamics. Several strategies have been adopted in SPIDER to handle varying rate data acquisition and are discussed here.Considering data access strategy, an important Use Case, especially when the pulse duration is long, is the concurrent access to the pulse file for online analysis and visualization. Concurrent data read and write is supported by MDSplus, the adopted data system, but performance can be affected by the required locks in file access. For this reason it is important to limit as far as possible useless data access. This has been achieved in different ways, such as setting a Region of Interest (ROI) in data access and by the extended usage of on-the-fly resampling in conjunction with the availability in the pulse file of different versions of the same data item, acquired at different sampling speeds.

Draining of Primary Cooling Circuits in Actively Cooled Reactor Components: Modeling the Electrostatic Residual Ion Dump of the ITER Neutral Beam Test Facility

Draining is an important process for in-vessel actively cooled reactor components exposed to the effects of nuclear reactions. Activated corrosion products formed by the water interaction with metals in cooling circuits shall be controlled through coolant evacuation and component confinement in order to avoid spreading of contamination. The process shall be applied to the primary cooling circuits and to the ITER in-vessel actively cooled components, including the components of the neutral beam injectors (NBIs), whose prototype MITICA <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> , <xref ref-type="bibr" rid="ref2" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[2]</xref> is under construction at the ITER Neutral Beam Test Facility in Padua. This work is focused on the electrostatic residual ion dump (ERID) of MITICA, for which the injection of pressurized nitrogen as inert gas is foreseen for blowing out of water from cooling circuits. The process is simulated by means of transient two-phase analyses. Parametric simulations as function of the inlet pressure are carried out to characterize the blowing-out process in terms of evacuation time and draining efficiency.

RF Stray Currents in SPIDER Power Circuits: Model Assessment and Experimental Results

The ITER Neutral Beam Test Facility (NBTF), Padua, Italy, hosts two different experiments: Source for Production of Ion of Deuterium Extracted from Rf plasma (SPIDER), the prototype of the ion source (IS) of ITER neutral beam injector (NBI), and Megavolt ITER Injector and Concept Advancement (MITICA), the prototype of the ITER NBI. The ISs of SPIDER and MITICA are driven by radio frequency (RF) power, for a total of 800 kW at 1 MHz. The RF power is delivered to the inductively coupled plasma drivers of the IS by four tetrode oscillators. Operation of SPIDER at high power pointed out the presence of RF stray currents circulating in the electric system. These currents hinder the correct operation of the system, causing damage to its components. To improve the comprehension of the issue, after an overall circuital investigation and the identification of a possible reclosing path for the RF stray currents, a simplified model of SPIDER electric system was developed, initially focusing on a single RF circuit. The aim of the work presented in this article is to extend the model to four RF circuits, to study the impact of their mutual coupling and of the common potential references on RF stray currents magnitude, with a view to improve the comprehension of the issue and the effect of the provisions to mitigate it. The results obtained with this model are compared to SPIDER experimental measurements for validation.

Techniques to widen the operational space of SPIDER Radio Frequency driven plasma source

SPIDER is the full-size prototype of the ion source of the ITER Neutral Beam Injectors, and is in operation since June 2018 as part of the ITER Neutral Beam Test Facility. The ion source is composed of 8 Radio Frequency drivers supplied by 4 RF oscillators, rated for operation at 1 MHz, for a total power of 800kW. This article presents a strategy for the selection of source biasing and of operating frequencies of the different RF generators to facilitate plasma initiation and to improve plasma stability. This operational strategy improved the plant reliability at high RF power in a broader range of operational conditions in SPIDER.

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.

SPIDER Cs Ovens functional tests

The SPIDER experiment is operating at the ITER Neutral Beam Test Facility PRIMA since June 2018 to study the operation of a full ITER-size negative ion source. In order to increase the negative ion extracted current and reduce the co-extracted electron current, caesium is evaporated inside the source. In fact, the adsorption of Cs onto the plasma grid, i.e. the first grid of the extraction system, lowers the surface work function, enhancing the probability of surface conversion of impinging hydrogen particles into negative ions. At the SPIDER test facility, Cs will be evaporated by heating a liquid Cs reservoir in three Cs ovens located at the back-plate of the ion source. The ovens must assure a controllable and reproducible evaporation. A SPIDER Cs oven prototype was procured and tested in the CAesium Test Stand (CATS) at PRIMA site to validate the design. Following the experimental tests, few improvements were introduced in the final design of the three Cs ovens for the SPIDER source. Then, the ovens were procured in 2019 and commissioned before their installation in SPIDER in 2020. This paper reports the functional tests of the SPIDER Cs ovens and of their embedded diagnostics. These tests were carried out at CATS, with the aim of characterizing the three ovens, in order to validate them for their use in SPIDER in the experimental campaign with caesium evaporation planned for the end of 2020. The ovens were successfully tested and characterized in terms of evaporation rate for different oven temperatures. Moreover, the three ovens show a good reproducibility and reliability.

Web-based streamed waveform display using MDSplus events and Node.js

Abstract Streamed data visualization is a requirement for long lasting discharges, such as in ITER and W7X, and more in general for long lasting related experiments, such as the ITER neutral beam test facility. A prerequisite for live visualization is the ability of the underlying data system to support streaming in data acquisition. On-line visualization is routinely performed during long lasting experiments, but implementing streamed data visualization, such as strip charts, would overload the data system, especially if a large number of charts are being displayed. A different approach for streamed data visualization is proposed here, using MDSplus events, rather than directly accessing stored data. Events are implemented as UDP multicast packets and can bring data. A data acquisition program can therefore, in addition to using MDSplus for streaming data segments, generate MDSplus events bringing the most recent chunk of samples. Data carried by events are made available to Web applications by means of a Node.js server, listening for the UDP packets and updating the connected Web clients using HTML5 Server-Sent Events.

Progress in the ITER neutral beam test facility

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

The timing system of the ITER full size neutral beam injector prototype

The ITER Neutral Beam Test Facility (NBTF) is currently under construction at Consorzio RFX, Padova, Italy. The NBTF includes two experimental devices: SPIDER for the study of the ion source and MITICA for the development of the full-size HNB prototype. Even if the two experiments share many architectural aspects in their Control and Data Acquisition System (CODAS), there are nevertheless some significant differences because MITICA must adhere to ITER’s directives for plant control design. This paper describes the architecture of the MITICA timing system that represents one of the first examples of integration of IEEE1588 timing functions in fusion research medium-scale experiments.

Safety systems in the ITER neutral beam test facility

SPIDER, the experimental device of the ITER Neutral Beam Test Facility (NBTF) devoted to the study of the ion source, has recently started operation. The construction of MITICA, the full-size prototype of the ITER heating neutral beam injector, is in progress. SPIDER and MITICA operation presents many possible hazards including radiation, electrical, explosive gas, laser and fire risks. An integrated approach, referred to as the NBTF Safety System, was chosen to deal globally with all risks for people and environment safety. As many different systems collaborate to guarantee the NBTF safety, a coordinating system, called Central Safety System (CSS), was implemented to supervise safety-relevant systems and provide an effective human interface of the NBTF safety. The CSS is also required to implement directly safety instrumented functions. The requirements of the safety systems derived from the safety risk analysis. The CSS uses the Siemens SIMATIC S7 400FH PLC along with safe input/output remote nodes connected through double ring-topology, redundant Profibus network. Complying with the IEC 61508/61511 standards, the system architecture uses the PROFISafe profile and the human machine interface is based on WinCC Open Architecture. Special provisions are used for communication of safety-related information with the PLC.

Simulation of the beamline thermal measurements to derive particle beam parameters in the ITER neutral beam test facility.

Injection of high energy neutral beam particles will be used in the ITER experiment for plasma heating and current drive. In a ITER heating beam injector, a 40 MW electrostatically accelerated negative beam will be neutralised and filtered along the beamline, obtaining a nominal 16.5 MW neutral beam power to be injected in the tokamak plasma or intercepted during conditioning and commissioning. The beam will heat the actively cooled panels of the beamline components with up to 13 MW/m2 surface power density and 18 MW power. These extreme conditions require testing in a ITER full scale neutral beam test facility under construction in Padova where the temperature of the beamline components will be monitored by 610 embedded thermocouples for protection against critical conditions, for recognising beam conditioning, and for deriving beam parameters. Power density maps of the expected beam-component interactions are applied on a parametric non-linear finite element model to simulate fields of expected temperatures. Such thermal maps are analyzed to derive the beam parameters during operation: divergence of 3-7 mrad and misalignment of 0-3 mrad. The sensibility of the temperature measurements is discussed considering a minimum 10% fraction of the nominal beam power.

Thermal analysis and high heat flux testing of unidirectional carbon–carbon composite for infrared imaging diagnostic

Unidirectional carbon fiber–carbon matrix (CFC) composite tiles, which are proposed to be employed in thermal imaging diagnostic of powerful particle beams, have been designed, manufactured, and tested. The particle beam is intercepted by the tiles, which erodes the carbon surface producing debris, while a plasma forms in front of the tiles due to beam–gas interaction. Carbon fibers are aligned along the tile thickness to transfer the heat flux from the front to the rear surface with limited thermal pattern broadening thanks to the ten times higher thermal conductivity along the fibers. This feature of the tiles allows to detect thermal radiation at the tile rear surface by thermographic cameras, producing curves to be correlated to energy, intensity profile, and duration of the particle beam. Thermal patterns with spatial resolution of a few mm, time resolution up to 40 ms, and maximum temperature of 1300 °C have been measured on CFC tiles exposed to accelerated hydrogen beam pulses with power densities up to 13 MW m−2 in thehigh heat flux test facility GLADIS. Multiphysics transient non-linear parametric finite-element models have been developed to simulate thermal transfer inside tiles, thermal patterns at surfaces, and thermal deformations of tiles by varying spatial distribution and peak value of the power density. Temperature gradients, hoop deformations around the heated region, location of failure region, and failure time have been analysed and discussed to recognise the tile behaviour during the high heat flux tests. Simulation models have been validated by comparing outputs to experimental measurements. Finally, model geometry and parameters have been changed to simulate the behaviour of the complete diagnostic to be used in the Source for the Production of Ions of Deuterium Extracted from a Radio-frequency plasma (SPIDER) test bed of the ITER Neutral Beam Test Facility with expected power density up to 20 MW m−2.

Studies on the requirements and design of the High Voltage Radio Frequency Test Facility

The two projects of the ITER Neutral Beam Test Facility in Padova are MITICA, the full scale prototype of the heating Neutral Beam Injector (NBI) and SPIDER, the full-size negative ion source of the NBI. Both include a Radio Frequency (RF) Ion Source where plasma is produced by the inductive coupling with coils wound around vacuum chambers called driver cases. Each coil is fed at 1 MHz up to a power of 100 kW, which corresponds to a voltage of about 12 kV rms, with nominal plasma parameters.According to the ITER requirements, the ion source is in vacuum to avoid radiation induced conductivity; concern on the voltage hold off of the source components exists, in particular related to the contact zone between the coil and the driver case characterized by intensification of the E-field.The development of a test bed called High Voltage Radio Frequency Test Facility (HVRFTF) has been launched to study the issues related to the voltage hold off in this operational environment and to verify the electrical design of the drivers adopted in the ion sources of SPIDER and MITICA.The paper presents the studies addressed to identify the HVRFTF requirements, to conceive and design suitable driver mock-up to reproduce operative conditions relevant to the RF drivers of SPIDER and MITICA for the characterization of their dielectric strength, to work out a suitable RF circuit for the HVRFTF to generate the required voltage and to design a flexible test arrangement allowing the variation of the quantities which influence the voltage hold off, such as the pressure, geometry and materials of the devices under test, in order to perform parametric analyses.

A portable control and data acquisition solution using EPICS, MARTe and MDSplus

A modern Control and Data Acquisition System (CODAS) must provide an integrated management of supervision, data acquisition and real-time control and a data centric view of the system combining raw, elaborated and configuration data. These requirements have been addressed by the CODAS system of the SPIDER experiment in the ITER Neutral Beam Test Facility. The solution adopted in SPIDER CODAS integrates, by means of few interface components, three frameworks used in nuclear fusion and accelerator communities. A further refinement has recently started, aiming at providing a solution that is complete and generic, and, as such, easily portable to new applications, ranging from small systems to future fusion experiments. The refined system described in the paper will be deployed in the ECT-FALCON Test Facility, used to test the ITER EC launcher components and the European Gyrotron prototype. Its implementation in other test facilities is foreseen, in order to provide a fast track to the development of complete and integrated control and data acquisition.