Source For Production Of Ions Research Articles

SPIDER on-line data visualization tool

SPIDER (Source for Production of Ion of Deuterium Extracted from Radio Frequency plasma), has been operating in Padova since May 2018, with the aim to test and develop the ITER-scale radio-frequency negative ion source, to study the beam characteristics and to verify the source operation. The source produces more than 500 gigabyte per operational day. Extracting as much information as possible from a set of selected signals (plasma light, voltages and currents of power supplies, cameras, tomography, beam emission spectroscopy, caesium oven parameters and others) during an experimental day allows a more efficient use of the largest neutral beam operating source. The simultaneous visualization of signals, related to pre-defined times of interest arrays, monitors the state of the beam and source features from quite different points of view. However, the time evolution of signals of interest often does not represent the expected format for data analysis. The experimental pulse can be divided in phases called ‘blips’, which correspond to a given experimental configuration typically lasting 40 s. Blips are separated by time periods, required for changing the experiment settings to different configurations. Some punctual information is required for visualization, while derived information is then stored in a relational database for subsequent analysis.The derivation of punctual values was performed offline, with the disadvantage of not being able to derive results of interest during the experiment itself. The new tool derives blip-related information during the pulse, as soon as required data have been stored in the pulse file. The availability of such tool represents a big improvement in the exploitation of the scientific results of SPIDER that can be used in this way during the experimental session itself. Furthermore new features are being developed regarding the addition of other signals including secondary ones and the access to the database from multiple platforms.

Study of Negative Ion Beamlets Produced in SPIDER by Beam Emission Spectroscopy

Among the beam diagnostics routinely employed in source for production of ion of deuterium (SPIDER), beam emission spectroscopy (BES) provides valuable information for beam characterization, thus driving source optimization methods. In particular, this technique is used to measure beam divergence and uniformity, which are crucial parameters to be optimized for application in International Experimental Thermonuclear Reactor (ITER). In this contribution, we focus on the characterization of H−/D− beamlets extracted during the first SPIDER Cs campaigns. In particular, we show parametric scans and preliminary broad component estimates on single beamlets.

Corrosion and Metal Release of Copper and Stainless Steel Exposed to Ultrapure Water

Source for Production of Ion of Deuterium Extracted from Rf plasma (SPIDER) and Megavolt ITer Injector and Concept Advance (MITICA) experiments are actively cooled by ultrapure water (UPW) to electrically insulate in-vessel components that are polarized to high voltage levels. Water circulating in cooling pipes reacts with wetted surfaces made of copper and stainless steel, causing water resistivity degradation. First investigations on the causes that might determine water degradation highlighted the necessity to measure release rates of the main materials (copper and stainless steel) exposed to UPW as a baseline study to better understand the release rates measured during experimental campaign in the real cooling circuits in SPIDER and MITICA. This article presents first the experimental results of stainless steel and copper samples exposed to stagnant UPW at ambient temperature and high oxygen environment to measure the ion release behavior. Trace metal analysis using inductively coupled plasma mass spectrometry (ICP-MS) was used to quantify metals released in solution, and scanning electron microscope (SEM) was used to characterize the oxide formed on the sample surface after the exposure.

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.

Radio Frequency Generators Based on Solid State Amplifiers for the NBTF and ITER Projects

The article will present and discuss the issues found in Source for Production of Ion of Deuterium Extracted from RF plasma (SPIDER) experiment with the operation of the radio frequency (RF) generators based on self-excited tetrode oscillators and the advantages expected from their replacement with RF solid-state amplifiers. The assessment of the solution based on new solid-state amplifiers for both SPIDER and Megavolt ITER Injector and Concept Advancement (MITICA) experiments operating in Padua, Italy, in the Neutral Beam Test Facility, will be described, outlining the main design challenges and open points.

Water Degradation in ITER Neutral Beam Test Facility Cooling Circuits

Source for production of ion of deuterium extracted from rf plasma (SPIDER) is the 100 keV negative ion source prototype of the ITER neutral beam injector. The cooling plant is one of the SPIDER auxiliary systems where ultrapure water (UPW) is used as the cooling media, thanks to its advantageous properties such as good cooling performance and high resistivity. Water resistivity needs to be maintained above certain values during SPIDER operation to enable the electrical insulation of in-vessel components that operate at different voltages. Nonetheless, degradation of water resistivity was observed in some circuits during operation. Water resistivity degradation had a negative impact on the SPIDER experimental campaign since it limits cooling plant availability. To understand the cause for the observed water degradation, sampling points were installed to sample water during operation. Ad hoc experiments have been performed on the worst affected circuit, and water samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Circulation experiments were conducted at constant temperature, pressure, and flow rate to assess water degradation by monitoring water conductivity increase. ICP-MS analyses on water from the worst affected circuit revealed the presence of Cu in relevant quantities and Zn. Corrosion induced by the presence of a non-compatible component with SPIDER requirements was found. The identified component was made of a galvanized steel plate, carbon steel, and brass. This was found responsible for the contamination of the cooling loop and its water properties’ degradation.

Characterization of SPIDER beam optics in volume operations with visible cameras

SPIDER (Source for Production of Ion of Deuterium Extracted from RF plasma) is the full-size prototype of the negative ion source for future ITER neutral beam injectors. With a maximum acceleration voltage of 100 kV, a negative ion beam is extracted and accelerated, and different diagnostics are developed to characterize the performance, shape, and deflection of the beam as well as the width of the beamlets. In particular, the optics of the accelerated beam is studied with a set of 15 cameras installed all around the beam, and they measure the visible emission; the spatial profile of the emission gives an estimate of the beamlets that compose the beam and also information about its optics. In this paper, experiments performed in volume operation (without cesium) are analyzed, studying the role of the extraction and acceleration voltages, polarization of plasma grid and bias plate, and magnetic filter field. It is shown how the width of the beam is not spatially uniform. Two types of non-uniformity are pointed out: a top/bottom asymmetry at large spatial scale and a smaller one at the single driver scale.

A soft X-ray unipolar charger for ultrafine particles

The performance of a unipolar particle charger using soft X-ray as the ion production source was optimized and characterized in this study. The charger design was based on the high efficiency particle charger reported by Chen and Pui (1999), in which the radioactive material (210Po) as the ion source and sheath flow for minimizing the particle loss were used. The use of 210Po source makes it challenging for field investigation due to the regulation of controlled substances. The inclusion of sheath flow in the high efficiency charger inevitably dilutes the output particle concentration, which is an undesired effect for the measurement of atmospheric ultrafine particles. The current soft X-ray charger was designed to ease the above concerns. The penetration efficiency, intrinsic and extrinsic charging efficiency, and extrinsic charge distributions of particles passing through the charger were measured under the optimal voltage operation. It was found that the loss of ultrafine particles in the charger was higher than that in the high efficiency charger (due to the lack of applying sheath flow). At the flow rate of 5.0 lpm, the measured extrinsic charging efficiency was comparable to that of the charger applied in the thermal desorption chemical ionization mass spectrometer (Smith et al., 2004). The measured extrinsic charge distribution of particles further showed that more than 50% of particles in the size range of 20–40 nm and 20–50 nm were singly charged at the flow rate of 1.5 lpm and 5.0 lpm, respectively.

Neural network based prediction of heat flux profiles on STRIKE

The instrumented calorimeter STRIKE (Short-Time Retractable Instrumented Kalorimeter Experiment) has been designed with the main purpose of characterizing the SPIDER (Source for Production of Ion of Deuterium Extracted from Radio Frequency plasma) negative ion beam in terms of beam uniformity and divergence during short pulse operations. STRIKE is made of 16 1D Carbon Fiber Composite (CFC) tiles, intercepting the whole beam and observed on the rear side by infrared (IR) cameras. The front observation presents some drawbacks due to optically emitting layer caused by the excited gas between the beam source and the calorimeter, and the material sublimated from the calorimeter surfaces due to the heating itself. This paper proposes a Neural Network-based approach to solve the inverse non-linear problem of determining the energy flux profile impinging on the calorimeter, considering the 2D temperature pattern measured on the rear side of the tiles. Most of the conventional methods used to evaluate the inverse heat flux are unbearably time consuming; since the objective is having a tool for heat flux evaluation for STRIKE real time operation, the need to have a ready-to-go instrument to understand the beam condition becomes stringent. For this reason, in this paper, a Multi-Layer Perceptron has been used to solve the problem. Once properly trained, the neural networks provide a fast evaluation of the impinging flux. Furthermore, there is no need to optimize any parameter since this operation is already included in the self-adjustment of the network weights during the training. The achieved results show the reliability of the proposed method both with stationary and non-stationary heat fluxes.

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.

Neutron measurements at the ELISE neutral beam test facility and implications for neutron based diagnostics at SPIDER.

Along the route to the development of a neutral beam injector for ITER, the Padua based Source for Production of Ion of Deuterium Extracted from Rf plasma (SPIDER) and megavolt ITER injector and concept advancement facilities will make use of neutron diagnostics to quantify the homogeneity of the neutral beam profile by measuring the map of the neutron emission from the beam dump with the close-contact neutron emission surface mapping (CNESM) system. Neutrons are here produced from beam-target reactions between the deuterium beam and the deuterons previously adsorbed in the calorimeter. In order to aid the interpretation of the diagnostic data, a dedicated experiment on neutron emission from beam-target reactions with beam parameters approaching those expected at SPIDER has been performed at the Extraction from a Large Ion Source Experiment (ELISE) neutral beam test facility. The time trace of neutron emission has been measured using a calibrated liquid scintillator detector at increasing power densities on the target. Compared to calculations based on the local mixing model, a systematic discrepancy was observed exceeding the statistical accuracy of the measurements and increasing as a linear function of the power density. The data are used to derive an empirical temperature dependent correction for applications to neutron measurements at SPIDER.

SPIDER CODAS Evolution Toward the ITER Compliant Neutral Beam Injector CODAS

The ITER neutral beam test facility, currently under the construction at Padua, Italy, consists of two experiments: source for production of ions of deuterium extracted (SPIDER) to test the ITER size ion source and megavolt iter injector and concept advancement (MITICA) to test the full ITER neutral beam injector. While the former represents an intermediate step, also finalized to the ITER Diagnostic Neutral Beam, the latter will represent the final design and implementation of the full prototype of the ITER heating neutral beam (HNB) injectors. Consequently, for the control and data acquisition system (CODAS) of SPIDER, there are no requirements for compliance with ITER, whereas the MITICA HNB Plant System will comply with the ITER directives for plant instrumentation and control because final system will be eventually integrated in ITER. SPIDER CODAS is currently under commissioning and the integration of the different frameworks that operate in an integrated way is proven to be successful. The experience and solutions gained in SPIDER CODAS development will be reused as far as possible in MITICA and to provide at the same time the required ITER plant system compatibility, a set of ITER-like layers (networks) will be defined, using exactly the same protocol defined for ITER interfaces.

Multi-beamlet investigation of the deflection compensation methods of SPIDER beamlets.

SPIDER (Source for Production of Ions of Deuterium Extracted from a Rf plasma) is an ion source test bed designed to extract and accelerate a negative ion current up to 40 A and 100 kV whose first beam is expected by the end of 2016. Two main effects perturb beamlet optics during the acceleration stage: space charge repulsion and the deflection induced by the permanent magnets (called co-extracted electron suppression magnets) embedded in the EG. The purpose of this work is to evaluate and compare benefits, collateral effects, and limitations of electrical and magnetic compensation methods for beamlet deflection. The study of these methods has been carried out by means of numerical modeling tools: multi-beamlet simulations have been performed for the first time.

Castellated tiles as the beam-facing components for the diagnostic calorimeter of the negative ion source SPIDER.

This paper presents the results of numerical simulations and experimental tests carried out to assess the feasibility and suitability of graphite castellated tiles as beam-facing component in the diagnostic calorimeter of the negative ion source SPIDER (Source for Production of Ions of Deuterium Extracted from Radio frequency plasma). The results indicate that this concept could be a reliable, although less performing, alternative for the present design based on carbon fiber composite tiles, as it provides thermal measurements on the required spatial scale.

Self-induced steady-state magnetic field in the negative ion sources with localized rf power deposition.

The study is in the scope of a recent activity on modeling of SPIDER (Source for Production of Ions of Deuterium Extracted from RF plasma) which is under development regarding the neutral beam injection heating system of ITER. The regime of non-ambipolarity in the source, established before, is completed here by introducing in the model the steady state magnetic field, self-induced in the discharge due to the dc current flowing in it. Strong changes in the discharge structure are reported.

Manufacturing and Testing of Grid Prototypes for the ITER Neutral Beam Injectors

A comprehensive set of research and development activities has been carried out at Consorzio RFX regarding the construction of the grids for the ITER neutral beam injectors, in order to validate the proposed manufacturing methodologies, to further develop the details of the engineering design, and to adapt existing production techniques to the specific case. In the framework of this research and development program, two multichannel prototypes have been designed and manufactured. These prototypes feature all the possible manufacturing issues of the source for production of ion of deuterium extracted from RF plasma (SPIDER) grids. In fact, they reproduce the geometry of the extraction grid (EG) for SPIDER by having the same thickness, same cooling channels, same distributors, same aperture shape, and same slots for the magnets of an EG segment. The differences from an actual EG segment regard only the dimensions, i.e., reduced width (one fourth) and height (one half). The construction and testing of these prototypes are here described in detail.

First negative ion beam measurement by the Short-Time Retractable Instrumented Kalorimeter Experiment (STRIKE)

The Source for Production of Ion of Deuterium Extracted from Rf plasma (SPIDER) test facility is under construction in Padova to optimise the operation of the beam source of ITER neutral beam injectors. The SPIDER beam will be characterised by the instrumented calorimeter STRIKE, whose main components are one-directional carbon-fibre-carbon-composite tiles. A small-scale version of the entire system has been employed in the BAvarian Test MAchine for Negative ions (BATMAN) testbed by arranging two prototype tiles in the vertical direction. The paper presents a description of the mini-STRIKE system and of the data analysis procedures, as well as some results concerning the BATMAN beam under varying operating conditions.

Non-ideal operating conditions of the ion source prototype for the ITER neutral beam injector due to thermal deformation of the support structure

The beam formation and acceleration of the ITER neutral beam injector will be studied in the full-scale ion source, Source for Production of Ions of Deuterium Extracted from a RF plasma (SPIDER). It will be able to sustain 40 A deuterium ion beam during 1-h pulses. The operating conditions of its multi-aperture electrodes will diverge from ideality, as a consequence of inhomogeneous heating and thermally induced deformations in the support structure of the extraction and acceleration grids, which operate at different temperatures. Meeting the requirements on the aperture alignment and distance between the grids with such a large number of apertures (1280) and the huge support structures constitute a challenge. Examination of the structure thermal deformation in transient and steady conditions has been carried out, evaluating their effect on the beam performance: the paper describes the analyses and the solutions proposed to mitigate detrimental effects.

Caesium oven design and R&D for the SPIDER beam source

In the ITER neutral beam injectors caesium will be used in the negative ion source to enhance the ion yield, by lowering the surface work function, and to reduce the co-extracted electrons. Fresh Cs has to be deposited almost continuously inside the plasma source and its injection is obtained by means of Cs ovens.Three independent Cs ovens have been integrated in the SPIDER (Source for Production of Ion of Deuterium Extracted from Rf plasma) beam source (BS). Since the entire BS is in-vacuum, a completely new design of the Cs ovens was required, assuring their vacuum and high temperature compatibility, and remote operation. They feature an ON–OFF valve and two independent heating systems to control the Cs evaporation and to avoid vapour condensing on the injection duct, by controlling the temperature. A Cs level indicator to measure the quantity of liquid caesium inside the reservoir, based on the electric resistance variation of an electrode, is proposed for the first time and a proof of concept is given.The oven design and the R&D activity on the level measurement system are presented.

Effects of Negative Ion Source Characteristics on Beam Optics: The Case of SPIDER

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