Studies and analyses for the design of an electrostatic accelerator for the ITER Neutral Beam Injector

Neutral beam injection is considered one of the most effective methods for plasma heating and current drive in fusion experiments around the world. The concept is straightforward: neutral atoms can penetrate through the confining magnetic field and, via collisions with plasma particles can heat the plasma and transfer momentum to sustain the plasma current. ITER, the first fusion experimental reactor under construction in Cadarache, will be equipped with two Neutral Beam Injectors, each of them capable to inject into the plasma up to 16.5 MW, by accelerating negative hydrogen or deuterium ions up to energy of 1MeV. The thesis deals with the design the electrostatic accelerator of the ITER Neutral Beam Injector (NBI). In particular, the thesis focuses on charged particle dynamics for multiple beams modelling, on the issues related to 1 MV dc insulation in vacuum, on the structural behaviour of brittle insulators. Finally, an active steering concept is introduced and its performance and technological issues are discussed. An overview on the physics principles behind the operation of the existing ion sources is given in chapter one, as well as a discussion on the need of negative ions to produce a high energy and high power neutral beam. The electrostatic accelerator, which generates the high energy ion beam, is the second subsystem inside the NBI: chapter two discusses the main criteria adopted to design it and presents how different issues are reciprocally integrated its design. Chapter three reports on the structural verification that has been carried out on the large ceramic insulator rings; in particular the R&D activities related to the fabrication of the large alumina insulator rings used for the bushing of the NBI will be presented. A statistical approach has been applied to evaluate the failure probability of the rings during the brazing procedure. Chapter four explains results obtained on negative ion beam simulation. A Monte Carlo Code has been applied to benchmark the experimental results obtained by the Japanese facility; moreover, a new code to evaluate beam divergence is presented. Finally, Chapter five presents a conceptual design of an Electron Dump & Steering System (EDSS). This subsystem is aimed to filter the electrons that are accelerated together with the negative ions by the electrostatic accelerator; moreover it should deflect the ions in order to deposit the NB power in different plasma regions. Two possible solutions have been analysed; for one of them, a deterministic approach to evaluate the heat flux deposited on the surfaces has been developed and implemented in an ANSYS routine. The EDSS study can be considered as the last step of a virtual travel inside the NBI electrostatic accelerator where the conversion of electrical power into kinetic power takes place.Il riscaldamento mediante fasci di neutri è considerato uno dei più efficaci metodi di riscaldamento e sostentamento della corrente nei plasmi fusionistici di molti esperimenti mondiali. Il concetto è semplice: gli atomi neutri possono penetrare il campo magnetico, mediante collisioni con le particelle cariche, è possibile riscaldare il plasma e sostenerne la corrente. ITER, il primo reattore sperimentale in fase di costruzione a Cadarache, sarà equipaggiato con due iniettori di neutri; ciascuno di essi sarà in grado di trasferire all’interno del plasma di ITER fino a 16.5 MW di potenza mediante l’accelerazione di ioni negativi di idrogeno o deuterio fino ad energie di 1MeV. Questa Tesi tratta il progetto di dispositivi inerenti la tenuta delle alte tensioni in vuoto per l’acceleratore elettrostatico dell’iniettore di neutri per ITER. In particolare, la Tesi si focalizza sulle problematiche legate alla modellistica dei fasci di particelle carichi, sui problemi legati all’isolamento delle alte tensioni in vuoto e sulle analisi strutturali relative ai materiali fragili per gli isolatori ceramici. Infine, un sistema attivo per la deflessione delle particelle cariche verrà presentato e discusso assieme agli aspetti tecnologici che lo riguardano. Una panoramica dei principi fisici che governano le sorgenti a ioni negativi è fornita nel Capitolo 1 evidenziando la necessità di produrre un fascio di neutri ad alta energia e potenza. L’acceleratore elettrostatico, disposto a valle della sorgente, incrementa l’energia degli ioni negativi fino ad 1MV: il Capitolo 2 discute i principali criteri progettuali sottolineando l’interconnessione tra problemi fisici ed ingegneristici. Il Capitolo 3 riporta le verifiche strutturali svolte sugli anelli isolatori di grandi dimensioni in materiale ceramico; in particolare verranno presentate le attività di ricerca e sviluppo riguardanti la fabbricazione degli isolatori di allumina per il passante multistadio dell’iniettore di neutri. Inoltre si è sviluppata una metodologia per calcolare la probabilità di rottura durante il processo di fabbricazione di tali isolatori. Il Capitolo 4 spiega i risultati riguardanti le simulazioni del fascio di ioni negativi, un codice Monte Carlo è stato applicato per confrontare i risultati sperimentali, ottenuti da un acceleratore giapponese, con quanto stimato dai modelli numerici; inoltre sarà presentato nello stesso capitolo un nuovo codice in grado di calcolare la divergenza del fascio di ioni negativi considerando il problema della carica spaziale. Infine il Capitolo 5 presenta uno studio concettuale di un dispositivo che è in grado di filtrare gli elettroni dal fascio di ioni negativi e contemporaneamente curvare leggermente la traiettoria di quest’ultimi (EDSS). Due possibili alternative si sono studiate; per una di esse si è sviluppata, ed applicata, una routine ANSYS per il calcolo del flusso di potenza depositato sulle superfici materiali del EDSS mediante un approccio deterministico. Lo studio del EDSS può essere considerato l’ultimo passo di un viaggio virtuale all’interno dell’acceleratore elettrostatico dell’iniettore di neutri in cui l’energia elettrica viene convertita in energia cinetica di un flusso di particelle cariche

A Novel Tool for Breakdown Probability Predictions on Multi-Electrode Multi-Voltage Systems

An innovative approach for the voltage breakdown prediction in high-voltage systems, insulated by large vacuum gaps, is presented. It is based on the correlation between the clump mechanism and a statistical approach to the breakdown probability. The aim of this paper is twofold. First, the numerical solution of 3-D electrostatic problems by a couple of complementary formulations is presented. Second, an efficient post-processing tool is introduced, based on the analytical solution of the equations of motion in a domain covered by a tetrahedral mesh, to estimate the breakdown probability associated to the electrically charged microparticles leaving one electrode and clashing to the electrode with opposite polarity with sufficient energy to get vaporization. This approach has been benchmarked on a reference configuration (sphere/plane) problem and applied to calculate the particle trajectories in a very complex multi-electrode multi-voltage system

Design of a System for Performing High-Voltage Holding Test Campaigns on a Mockup of MITICA Negative Ion Source

Megavolt ITER Injector and Concept Advance- ment (MITICA) is the full-size prototype of the neutral beam injector (NBI) for the ITER currently under construction at the neutral beam test facility (NBTF) (Consorzio RFX, Padova, Italy). One of the main issues of such a complex machine is to secure a stable voltage holding of the ion source\u2014accelerator complex\u2014which is biased at 121 MV and constitutes a very large (cathodic) area, with respect to the ground (the vessel). Coordinated with the experimental program of the HV test facilities in Japan (QST, Naka) and in Italy (high voltage Padova test facility, Consorzio RFX), a test campaign is foreseen for 2020\u20132021, using the real MITICA vacuum vessel and a mockup of the ion source. This mockup can constitute a simple geometry, where the MITICA ion source is represented by a sphere (cathode), connected to the high-voltage bushing. A planar electrode (anode) will cover the lower part of the beam source vessel (BSV) and will be positioned at an adjustable distance from the sphere, to assure fine control of the electrostatic configuration. The experiments will be devoted to characterize the breakdown voltage as a function of the gap length and vacuum pressure. In this article, the design activity of the electrodes to be used during the campaign is presented

Magnetic Field Effect on Voltage Holding in the MITICA Electrostatic Accelerator

MITICA is the complete full-scale prototype of a 17 MW heating neutral beam injector for ITER. This experi- mental device, presently under construction in Padova, includes a negative ion source (H- or D-), and an electrostatic accelerator (1 MV, 40 A, 3600 s). Voltage holding is recognized to be one of the most critical issues for the 1 MV accelerator operations, not only owing to the complex multistage electrostatic accelerator structure, but also for the presence of magnetic field, which is necessary for deflecting the coextracted and secondary electrons as early as possible, before they are accelerated. The required magnetic field is produced by a combination of several sources, such as permanent magnets and current-carrying conductors. To avoid gas breakdown between electrodes, the design of the accelerator shall guarantee that the electrostatic field configura- tion and the pressure distribution correspond to operating points located far enough on the left side of the well-known Paschen breakdown curve. For this reason, MITICA has been designed so that the (H2 or D2) gas pressure multiplied by the distance between electrode ( p \ub7 d ) shall not exceed 0.1\u20130.3 Pa \ub7 m. However, indications have been found in literature that the presence of magnetic field might shift part of the left branch of the Paschen curve more to the left, thus reducing the above-defined limit and possibly affecting the voltage holding criteria to be used in MITICA design. To support the design of MITICA at low gas pressure and in the presence of magnetic field, an experimental campaign has been carried out at the high-voltage padova test facility; during this campaign, a field distribution similar to that expected in MITICA has been realized. The magnetic field has been produced using permanent magnets located inside the electrodes or outside the vacuum tank. This paper describes the test assembly, procedure adopted, and the experimental findings; the results have been also successfully compared with simulations of the breakdown process. In certain magnetic field configurations, a clear effect has been recognized, indicating a nonnegligible shift to the left of the lower part of the Paschen Curve

A new deflection technique applied to an existing scheme of electrostatic accelerator for high energy neutral beam injection in fusion reactor devices

A scheme of a neutral beam injector (NBI), based on electrostatic acceleration and magneto-static deflection of negative ions, is proposed and analyzed in terms of feasibility and performance. The scheme is based on the deflection of a high energy (2 MeV) and high current (some tens of amperes) negative ion beam by a large magnetic deflector placed between the Beam Source (BS) and the neutralizer. This scheme has the potential of solving two key issues, which at present limit the applicability of a NBI to a fusion reactor: the maximum achievable acceleration voltage and the direct exposure of the BS to the flux of neutrons and radiation coming from the fusion reactor. In order to solve these two issues, a magnetic deflector is proposed to screen the BS from direct exposure to radiation and neutrons so that the voltage insulation between the electrostatic accelerator and the grounded vessel can be enhanced by using compressed SF6 instead of vacuum so that the negative ions can be accelerated at energies higher than 1 MeV. By solving the beam transport with different magnetic deflector properties, an optimum scheme has been found which is shown to be effective to guarantee both the steering effect and the beam aiming

Prediction of lightning impulse voltage induced breakdown in vacuum interrupters

In this work, we investigate the Lightning Impulse Voltage LIV breakdown probability of different medium-voltage Vacuum Interrupter (VI) tubes manufactured by Siemens; for this investigation, the Voltage Holding Prediction Model (VHPM) has been employed. The study focuses on results of systematic LIV experiments (up to 250 kV) carried out in the Siemens VI factory on different VI configurations and aimed at validating the VHPM under impulsive voltage and in well-controlled conditions. In order to assess the predictive capability of the VHPM, the model has been applied to a tube configuration with unknown voltage breakdown characteristics exploiting Weibull parameters obtained from the analysis of tube configurations with well-known LIV features. Experiment and simulation showed fair agreement concerning the overall VI breakdown probability as a function of the voltage applied to the contact electrodes

Optimization of the Accelerators for the ITER Neutral Beam Injector Project

A joint Japan-EU R&D activity is in progress to optimize the accelerator for the ITER NBI. The accelerator baseline design is based on a five grids system which can be adapted to operate with three grids for initial operations at low voltage (500 kV). Moreover, in order to speed up the test of the NBI system at the Test Facility, a negative ion source with extraction voltage up to 100 kV will be operated in parallel to the full injector. In this contribution the three accelerators mentioned above are presented discussing the procedure to optimize the grid geometry in order to assure optimum optics during operation when the grids undergo deformations and thermal stresses due to the particles that hit their surface

Progress in the MITICA beam source design

In the framework of the development of the ITER neutral beam (NB) system, a test facility is planned to be built in Padova. A full size prototype of the ITER heating NB injector (MITICA) shall be built and tested at full beam power (17 MW) as per ITER requirements. The design of the MITICA beam source has further progressed following updated optimization and overall integration criteria. In the paper, the major design choices and revisions are presented, together with some results of numerical analyses carried out in order to assess the electrostatic and thermo-mechanical behaviour of the source

Physics design of the injector source for ITER neutral beam injector (invited)

Two Neutral Beam Injectors (NBI) are foreseen to provide a substantial fraction of the heating power necessary to ignite thermonuclear fusion reactions in ITER. The development of the NBI system at unprecedented parameters (40 A of negative ion current accelerated up to 1 MV) requires the real- ization of a full scale prototype, to be tested and optimized at the Test Facility under construction in Padova (Italy). The beam source is the key component of the system and the design of the multi-grid accelerator is the goal of a multi-national collaborative effort. In particular, beam steering is a chal- lenging aspect, being a tradeoff between requirements of the optics and real grids with finite thickness and thermo-mechanical constraints due to the cooling needs and the presence of permanent magnets. In the paper, a review of the accelerator physics and an overview of the whole R&D physics program aimed to the development of the injector source are presented

HV Holding in Vacuum, a Key Issue for the ITER Neutral Beam Injector

This overview summarizes the R&D activities aimed to secure for the ITER Neutral Beam Injectors a stable voltage holding, one of the most challenging issue for the NBI systems operation. The huge size of the accelerator and the highest voltage (1 MV) ever used in such systems make prominent some aspects that are, in other application, almost not influent or well under control, like the Area Effect, Total Voltage Effect, the highly complex electrostatic configuration and, last but not least, the cleaning degree of the electrodes surfaces. A clear explanation of the mechanism leading to the voltage breakdown in these conditions is far from being well established. But in the last years a great effort have been deployed by the NBI community to reliably achieve 1 MV voltage holding that is required for the NB injectors for ITER. Theoretical models and new tools have been derived to explain the mechanism of voltage conditioning and for the design of the complex electrodes geometries that supersede the simple electric field optimization. Presently, the milestone of the 1 MV voltage holding has been achieved but only with multi-gap configuration; activities are nevertheless in progress aimed to obtain a reliable single gap 1 MV voltage holding in the accelerator