Complete compensation of criss-cross deflection in a negative ion accelerator by magnetic technique

During 2016, a joint experimental campaign was carried out by QST and Consorzio RFX on the Negative Ion Test Stand (NITS) at the QST Naka Fusion Institute, Japan, with the purpose of validating some design solutions adopted in MITICA, which is the full-scale prototype of the ITER NBI, presently under construction at Consorzio RFX, Padova, Italy. The main purpose of the campaign was to test a novel technique, for suppressing the beamlet criss-cross magnetic deflection. This new technique, involving a set of permanent magnets embedded in the Extraction Grid, named Asymmetric Deflection Compensation Magnets (ADCM), is potentially more performing and robust than the traditional electrostatic compensation methods. The results of this first campaign confirmed the effectiveness of the new magnetic configuration in reducing the criss-cross magnetic deflection. Nonetheless, contrary to expectations, a complete deflection correction was not achieved. By analyzing in detail the results, we found indications that a physical process, taking place just upstream of the plasma grid, was giving an important contribution to the final deflection of the negative ion beam. This process appears to be related to the drift of negative ions inside the plasma source, in the presence of a magnetic field transverse to the extraction direction, and results in a non-uniform ion current density extracted at the meniscus. Therefore, the numerical models adopted in the design were improved by including this previously disregarded effect, so as to obtain a much better matching with the experimental results. Based on the results of the first campaign, new permanent magnets were designed and installed on the Extraction Grid of NITS. A second QST-Consorzio RFX joint experimental campaign was then carried out in 2017, demonstrating the complete correction of the criss-cross deflection and confirming the validity of the novel magnetic configuration and of the hypothesis behind the new models. This contribution presents the results of the second joint experimental campaign on NITS along with the overall data analysis of both campaigns, and the description of the improved models. A general picture is given of the relation among magnetic field, beam energy, meniscus non-uniformity and beamlet deflection, constituting a useful database for the design of future machines

Start of SPIDER operation towards ITER neutral beams

Heating Neutral Beam (HNB) Injectors will constitute the main plasma heating and current drive tool both in ITER and JT60-SA, which are the next major experimental steps for demonstrating nuclear fusion as viable energy source. In ITER, in order to achieve the required thermonuclear fusion power gain Q=10 for short pulse operation and Q=5 for long pulse operation (up to 3600s), two HNB injectors will be needed [1], each delivering a total power of about 16.5 MW into the magnetically-confined plasma, by means of neutral hydrogen or deuterium particles having a specific energy of about 1 MeV. Since only negatively charged particles can be efficiently neutralized at such energy, the ITER HNB injectors [2] will be based on negative ions, generated by caesium-catalysed surface conversion of atoms in a radio-frequency driven plasma source. A negative deuterium ion current of more than 40 A will be extracted, accelerated and focused in a multi-aperture, multi-stage electrostatic accelerator, having 1280 apertures (~ 14 mm diam.) and 5 acceleration stages (~200 kV each) [3]. After passing through a narrow gas-cell neutralizer, the residual ions will be deflected and discarded, whereas the neutralized particles will continue their trajectory through a duct into the tokamak vessels to deliver the required heating power to the ITER plasma for a pulse duration of about 3600 s. Although the operating principles and the implementation of the most critical parts of the injector have been tested in different experiments, the ITER NBI requirements have never been simultaneously attained. In order to reduce the risks and to optimize the design and operating procedures of the HNB for ITER, a dedicated Neutral Beam Test Facility (NBTF) [4] has been promoted by the ITER Organization with the contribution of the European Union\u2019s Joint Undertaking for ITER and of the Italian Government, with the participation of the Japanese and Indian Domestic Agencies (JADA and INDA) and of several European laboratories, such as IPP-Garching, KIT-Karlsruhe, CCFE-Culham, CEA-Cadarache. The NBTF, nicknamed PRIMA, has been set up at Consorzio RFX in Padova, Italy [5]. The planned experiments will verify continuous HNB operation for one hour, under stringent requirements for beam divergence (< 7 mrad) and aiming (within 2 mrad). To study and optimise HNB performances, the NBTF includes two experiments: MITICA, full-scale NBI prototype with 1 MeV particle energy and SPIDER, with 100 keV particle energy and 40 A current, aiming at testing and optimizing the full-scale ion source. SPIDER will focus on source uniformity, negative ion current density and beam optics. In June 2018 the experimental operation of SPIDER has started

Effets synergiques de métaux lourds sur des cultures primaires d'hépatocytes de rat

Le but de l'étude était d'examiner la possibilité d'interactions synergiques entre les métaux lourds qui peuvent être retrouvés comme contaminants de l'eau. A cette fin. des cultures primaires d'hépatocytes de rats ont été mises à profit la détection des effets toxiques parce qu'elles représentent un système cible pertinent. Les cellules furent exposées au plomb (Pb), au cuivre (Cu) et au cadmium (Cd) seuls ou en mélanges binaires de Cd-Pb ou Cd-Cu. Les concentrations de métaux sélectionnés se sont situées entre 100 ppb et 5 ppm et le temps d'incubation de 20 h. La cytotoxicité a été suivie par la libération par les cellules de lactique (LDH) dans le milieu extracellulaire ainsi que par l'incorporation d'arginine tritiée([3H] Arg) dans les protéines. Il a été observé, en accord avec des études antérieures, que le Cd était le plus cytotoxique des métaux. En effet, à des concentrations aussi basses que 100 200 ppb, il y a eu un accroissement marqué de l'activité LDH dans le milieu extracellulaire ainsi qu 'une réduction significative de la quantité de tritium protéique. Quand le Pb fut ajouté au Cd, la cytotoxicité mesurée fut comparable à celle observée avec le Cd seul. Au contraire, quand le Cu fut appliqué en mélange avec le Cd, les effets qui ont résulté furent plus importants que ce qui pouvait être anticipé à partir de la sommation des toxicités individuelles. La potentialisation de la toxicité du Cd fut détectable même à des niveaux non cytotoxiques de Cu. Ceci a été confirmé tant avec les données sur la libération de LDH que sur la synthèse protéique. La présente étude montre l'applicabilité du modèle «hépatocyte» pour la détection d'effets synergiques d'agents fréquemment retrouvés comme contaminants toxiques des écosystèmes aquatiques. De plus, les résultats obtenus ouvrent la porte à l'examen des mécanismes d'action en jeu comme étape ultérieure dans la mise à profit de ce modèle cellulaire

Design of electrodes for high voltage tests in MITICA

The NBTF (Neutral Beam Test Facility) in Padova, Italy, is dedicated to testing and optimizing the Neutral Beam Injector for ITER. A full-scale prototype known as MITICA is currently being constructed at NBTF to demonstrate all the design parameters for ITER, including stable beam operation at 1MeV energy. To achieve stable high voltage holding of 1MV, a dedicated campaign is planned for 2023, which will utilize realistic mockups of the MITICA ion source and accelerator. During this campaign, the mockup of the ion source will be biased with an increasing potential up to -1MV, and the discharge towards the grounded electrodes will be observed. This will help identify any potential weak points in the geometry that can be mitigated. This work briefly describes the design process of the mock-up electrodes, including electrostatic and structural analyses. The construction of the electrodes has started in 2022 and the installation is foreseen for 2023

Experimental and numerical investigation on the asymmetry of the current density extracted through a plasma meniscus in negative ion accelerator

In multi-beamlet negative ion accelerators for neutral beam injectors, the transverse magnetic eld necessary for suppressing the co-extracted electrons induces a de ection of the negative ion beamlets that must be corrected. For the design, particle-tracing simulation codes are used to compute ion trajectories and optical properties of the beamlets in the acceleration stage. In these codes, uniform boundary conditions are normally assumed for the ion current density distribution at the surface (called meniscus), where negative ions are extracted from the plasma and form beamlets, which are accelerated across the apertures of the accelerator grids. Recently, experimental campaigns dedicated to the accurate measurement of the beamlet de ection in the acceleration phase revealed higher de ection than foreseen by simulations. In this work, we demonstrate that an agreement with the experimental data can be obtained by incorporating in the numerical simulations a non-radially symmetric distribution of the ion current density extracted across the meniscus surface. In the rst part, the asymmetry of the ion current density at the meniscus is studied in an empirical way, by analysing and tting the experimental results obtained with different operating parameters. In the second part, we show that the ion current asymmetry estimated by this procedure is well consistent with the ow pattern of H 12 ions calculated in the meniscus zone using a detailed particle in cell (PIC) model of the ion source in the presence of transverse magnetic eld

Electrostatic Design of the MITICA Intermediate Electrostatic Shield

Two heating Neutral Beam Injectors (NBIs), required for plasma heating and current drive, are foreseen for ITER operation. Each beam will be generated by a 40A current of Deuterium negative ions, accelerated up to the specific energy of 1MeV and then neutralized, delivering to the plasma a power up to 16.5 MW each. The beam source (BS) will be constituted by an RF-driven negative ion source at -1 MV potential and by an electrostatic accelerator (consisting of 5 stages at intermediate potentials). All components will be installed in a vacuum vessel, together with a high-capacity cryo-pumping system that controls the background gas pressure.In order to validate the ITER NBI design and address all the outstanding issues related to these demanding requirements, a full-scale prototype called MITICA (Megavolt ITER Injector & Concept Advancement) is under construction in Padova at Consorzio RFX. Voltage insulation in vacuum and/or very low-pressure gas on a single gap is indeed one of the expected issues that MITICA will have to deal with. An effective solution for increasing the voltage holding capability of the system consists in the use of an additional intermediate electrostatic shield, biased at an intermediate potential, placed between the ion source and the vacuum vessel. In this paper, the electrostatic design of the shield is presented, by considering the voltage holding capability. A novel 3D version of a numerical tool, called Voltage Holding Prediction Model (VHPM), is applied to the additional intermediate shield design to assess the expected voltage holding capability of the experiment in high vacuum