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Pilan , Nicola (2009) Studies and analyses for the design of an electrostatic accelerator for the ITER Neutral Beam Injector. [Ph.D. thesis]

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Abstract (english)

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.

Abstract (italian)

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.

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EPrint type:Ph.D. thesis
Tutor:Gnesotto , Francesco
Ph.D. course:Ciclo 21 > Scuole per il 21simo ciclo > INGEGNERIA INDUSTRIALE > INGEGNERIA ELETTROTECNICA
Data di deposito della tesi:31 January 2009
Anno di Pubblicazione:January 2009
Key Words:neutral beam insulation charged brittle ceramic probabilistic adimensional
Settori scientifico-disciplinari MIUR:Area 09 - Ingegneria industriale e dell'informazione > ING-IND/31 Elettrotecnica
Struttura di riferimento:Dipartimenti > pre 2012 - Dipartimento di Ingegneria Elettrica
Codice ID:1814
Depositato il:31 Jan 2009
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