Investigation of the parameters of a particle beam by numerical models and diagnostic calorimetry

Neutral particle beams are used as auxiliary heating and current drive systemfor magnetically confined thermonuclear plasmas. The design of ITER in particular, the international experiment on nuclear fusion under realisation in Cadarache (France), features two neutral beam injectors (NBI) delivering 16.5 MW each. In order to deposit the beams efficiently at the core part of the ITER plasma, the beam energy has to be in the order of 1 MeV and therefore the option of accelerating positive ions has been discarded since their neutralisation efficiency falls abruptly beyond 100 keV. The generation and acceleration up to 1 MeV of a 40 A deuterium negative ion current still presents many open issues both from the physics and engineering point of view. To work on these issues a dedicated test facility, named PRIMA, is under construction at Consorzio RFX, in Padova (Italy). PRIMA is constituted by two sub-facilities: MITICA, the full injector prototype, and SPIDER, the prototype source dedicated to the optimization of the source performance. The latter, which is going to start operation in 2018, is equipped with many diagnostics to improve the understanding of the negative ion source and beam extraction physics. In such framework, this thesis focuses on the characterisation of a negative ion beam by means of one of such diagnostics, the instrumented calorimeter STRIKE, and by the application of different simulation codes to the interpretation of the experimental conditions. The contents of the thesis chapters are shortly listed below. A brief introduction on nuclear fusion is given in Chapter 1, discussing the use of magnetically confined plasmas, the performance to be achieved in order to have a favourable efficiency in power generation, the main features of ITER and the necessity of additional heating and current drive systems such as radio-frequency waves and neutral beam injectors. In addition, a conceptual design of a NBI in terms of the requirements on its main components is given. Chapter 2 focuses in more detail on the negative ion source and the electrostatic accelerator, introducing the prototype RF source and the main properties of beam optics. A brief insight in the beam modeling activity is also given by presenting the finite element solver and particle-tracing code OPERA and the Monte Carlo particle-tracing code EAMCC3D. Furthermore a brief description is given about the two facilities of particular interest for this thesis: the ITER prototype source SPIDER and the NIO1 (Negative Ion Optimization 1) experiment, currently operated at Consorzio RFX, in which a small-scale version of STRIKE is also installed. The activity regarding the diagnostic calorimeter STRIKE is described in Chapter 3, where the features of the SPIDER beam heat load, the suite of signals collected by STRIKE, the testing and modeling activity on the STRIKE prototype sensors, the tools available for analysing STRIKE data and their application to the data collected with prototype versions of STRIKE in other test facilities are discussed. Chapter 4 describes the application of the codes OPERA and EAMCC3D to simulate the beam features of NIO1. In Chapter 5 a comparison between modeling and data from the NIO1 diagnostic calorimeter is presented for some of the NIO1 experimental campaigns, showing the improvement in the beam physics understanding that a combined approach may guarantee. Finally, the main results and the possible future developments in the synergy between diagnostic calorimetry and beam modeling activity in view of SPIDER are discussed in Chapter 6

Overview on electrical issues faced during the SPIDER experimental campaigns

SPIDER is the full-scale prototype of the ion source of the ITER Heating Neutral Beam Injector, where negative ions of Hydrogen or Deuterium are produced by a RF generated plasma and accelerated with a set of grids up to ~100 keV. The Power Supply System is composed of high voltage dc power supplies capable of handling frequent grid breakdowns, high current dc generators for the magnetic filter field and RF generators for the plasma generation. During the first 3 years of SPIDER operation different electrical issues were discovered, understood and addressed thanks to deep analyses of the experimental results supported by modelling activities. The paper gives an overview on the observed phenomena and relevant analyses to understand them, on the effectiveness of the short-term modifications provided to SPIDER to face the encountered issues and on the design principle of long-term solutions to be introduced during the currently ongoing long shutdown.Comment: 8 pages, 12 figures. Presented at SOFT 202

Investigation of ELISE beam properties by means of the diagnostic calorimeter

Negli ultimi anni molti studi sono stati dedicati allo sviluppo di iniettori di particelle neutre (NBI) come sistema per riscaldare e indurre una corrente in plasmi termonucleari, avendo come riferimento l'esperimento internazionale sulla fusione ITER. Questa tesi si concentra sulla caratterizzazione del fascio di neutri della test facility ELISE di IPP, che rappresenta un passo importante nello sviluppo di NBI per ITER. Lo studio, svolto mediante il calorimetro diagnostico, è rivolto particolarmente alla determinazione di potenza, divergenza e omogeneità del fascio

Investigation of the parameters of a particle beam by numerical models and diagnostic calorimetry

Neutral particle beams are used as auxiliary heating and current drive systemfor magnetically confined thermonuclear plasmas. The design of ITER in particular, the international experiment on nuclear fusion under realisation in Cadarache (France), features two neutral beam injectors (NBI) delivering 16.5 MW each. In order to deposit the beams efficiently at the core part of the ITER plasma, the beam energy has to be in the order of 1 MeV and therefore the option of accelerating positive ions has been discarded since their neutralisation efficiency falls abruptly beyond 100 keV. The generation and acceleration up to 1 MeV of a 40 A deuterium negative ion current still presents many open issues both from the physics and engineering point of view. To work on these issues a dedicated test facility, named PRIMA, is under construction at Consorzio RFX, in Padova (Italy). PRIMA is constituted by two sub-facilities: MITICA, the full injector prototype, and SPIDER, the prototype source dedicated to the optimization of the source performance. The latter, which is going to start operation in 2018, is equipped with many diagnostics to improve the understanding of the negative ion source and beam extraction physics. In such framework, this thesis focuses on the characterisation of a negative ion beam by means of one of such diagnostics, the instrumented calorimeter STRIKE, and by the application of different simulation codes to the interpretation of the experimental conditions. The contents of the thesis chapters are shortly listed below. A brief introduction on nuclear fusion is given in Chapter 1, discussing the use of magnetically confined plasmas, the performance to be achieved in order to have a favourable efficiency in power generation, the main features of ITER and the necessity of additional heating and current drive systems such as radio-frequency waves and neutral beam injectors. In addition, a conceptual design of a NBI in terms of the requirements on its main components is given. Chapter 2 focuses in more detail on the negative ion source and the electrostatic accelerator, introducing the prototype RF source and the main properties of beam optics. A brief insight in the beam modeling activity is also given by presenting the finite element solver and particle-tracing code OPERA and the Monte Carlo particle-tracing code EAMCC3D. Furthermore a brief description is given about the two facilities of particular interest for this thesis: the ITER prototype source SPIDER and the NIO1 (Negative Ion Optimization 1) experiment, currently operated at Consorzio RFX, in which a small-scale version of STRIKE is also installed. The activity regarding the diagnostic calorimeter STRIKE is described in Chapter 3, where the features of the SPIDER beam heat load, the suite of signals collected by STRIKE, the testing and modeling activity on the STRIKE prototype sensors, the tools available for analysing STRIKE data and their application to the data collected with prototype versions of STRIKE in other test facilities are discussed. Chapter 4 describes the application of the codes OPERA and EAMCC3D to simulate the beam features of NIO1. In Chapter 5 a comparison between modeling and data from the NIO1 diagnostic calorimeter is presented for some of the NIO1 experimental campaigns, showing the improvement in the beam physics understanding that a combined approach may guarantee. Finally, the main results and the possible future developments in the synergy between diagnostic calorimetry and beam modeling activity in view of SPIDER are discussed in Chapter 6.Al fine di fornire riscaldamento addizionale e indurre una corrente in plasmi termonucleari confinati magneticamente è possibile utilizzare fasci di particelle neutre. In particolare il design di ITER, l’esperimento internazionale sulla fusione nucleare attualmente in costruzione a Cadarache (Francia), prevede due iniettori di particelle neutre (NBI) in grado di fornire 16.5 MW ciascuno. Affinché il fascio possa penetrare efficacemente nel plasma di ITER, la sua energia deve essere nell’ordine di 1 MeV e questo esclude l’opzione di accelerare ioni positivi, la cui efficienza di neutralizzazione si riduce drasticamente per energie superiori a 100 keV. La generazione e accelerazione di 40 A di ioni negativi di deuterio fino a 1 MeV d’altronde, presenta ancora molti punti aperti sia dal punto di vista della fisica che da quello ingegneristico per affrontare i quali la test facility PRIMA è attualmente in fase di realizzazione a Padova presso il Consorzio RFX. PRIMA è suddivisa in due test facility: MITICA, il prototipo di iniettore per ITER, e SPIDER, il prototipo della sorgente di ioni dedicato all’ottimizzazione delle sue performance. Al fine di meglio comprendere i fenomeni che accadono all’interno della sorgente e la fisica dell’estrazione, SPIDER è dotato di diverse diagnostiche. All’interno di questo contesto, questa tesi è dedicata alla caratterizzazione di un fascio di ioni negativi mediante una di queste diagnostiche, il calorimetro STRIKE, e l’applicazione di diversi codici di simulazione per interpretare le condizioni sperimentali. I contenuti dei diversi capitoli sono elencati brevemente di seguito. Nel Capitolo 1 viene introdotto il confinamento magnetico di plasmi termonucleari e si discutono le prestazioni che devono esser raggiunte da un reattore a fusione per produrre energia con una ragionevole efficienza. In questo contesto viene presentato il progetto ITER e viene giustificata la necessità di sistemi di riscaldamento addizionale come onde a radio frequenza o fasci di particelle neutre. Infine viene presentato un design concettuale di un NBI, definendone i componenti principali. Il Capitolo 2 si concentra con maggiore dettaglio su due di questi componenti: la sorgente di ioni e l’acceleratore elettrostatico. In particolare viene descritto il prototipo di sorgente a radio-frequenza e le principali proprietà dell’ottica del fascio. In seguito viene presentata l’attività di modellistica introducendo il codice agli elementi finiti di particle-tracing OPERA e il codice Monte Carlo di particle-tracing EAMCC3D. Infine vengono descritte le due test facility di particolare interesse per questa tesi: il prototipo SPIDER e l’esperimento NIO1 (Negative Ion Optimization 1), in cui è installata una versione ridotta del calorimetro STRIKE. L’attività riguardante il calorimetro diagnostico STRIKE è descritta nel Capitolo 3, in cui sono descritti sia le caratteristiche del carico termico dovuto al fascio di SPIDER che l’insieme di segnali complementari raccolti dal calorimetro STRIKE. Una lunga sezione è dedicata ai prototipi dei sensori di STRIKE, descrivendo l’attività modellistica, i test effettuati sulla loro capacità diagnostica e sulla loro resistenza a carichi termici paragonabili a quelli di SPIDER. Infine, vengono descritti gli strumenti utilizzati per analizzare i dati raccolti con la versione in scala ridotta di STRIKE, il calorimetro mini STRIKE, in diversi esperimenti. Il Capitolo 4 descrive l’applicazione dei codici OPERA ed EAMCC3D per simulare le caratteristiche del fascio di ioni di NIO1. Nel Capitolo 5 i risultati di questi modelli sono confrontati con i dati sperimentali nel caso del calorimetro diagnostico di NIO1, mostrando come un approccio combinato possa favorire l’interpretazione e la comprensione del comportamento dell’acceleratore. Infine i risultati principali e i possibili sviluppi futuri in merito alla sinergia tra calorimetria e modellistica in vista dell’operazione di SPIDER sono discussi nel Capitolo 6

Influence of positive ions on the beamlet optics for negative-ion neutral beam injectors

Neutral beam injectors are based on the neutralization of ion beams accelerated at the desired energy. In the case of the ITER heating and diagnostic neutral beams, the target heating power translates into stringent requirements on the acceptable beamlet divergence and aiming to allow the beam to reach the fusion plasma. The beamlets composing the accelerated beam are experimentally found to feature a transverse velocity distribution exhibiting two Gaussian components: the well-focused one is referred to as the core component while the rest of the beam, the halo, describes beam particles with much worse optics. The codes that simulate beam extraction and acceleration usually assume that the negative ions move towards the plasma meniscus with a laminar flow (no transverse velocity) or that the transverse velocity distribution can be modelled as a Maxwellian and that the current density is uniformly illuminating the meniscus; under such approximations, the presence of highly divergent components cannot be explained. In this work, we develop a simple test-particle tracing code with Monte Carlo collisions, named ICARO (for Ions Coming Around), to study the transport of negative ions in the extraction region and derive the spatial and velocity distribution of the negative ions at the meniscus (i.e. the plasma boundary where a beamlet is extracted). In particular, the origin of the beamlet halo and its dependence on the source parameters are discussed, highlighting as a key parameter the energy distribution of positive ions in the source plasma

Serpientes y escaleras. Lecciones de expresión creativa y pautas de narrativa gráfica

El presente artículo relata la historia del juego Serpientes y escaleras, su análisis como narrador de ideas y las posibles implicaciones educativas con respecto a la creación de narrativa gráfica en el ámbito de la expresión creativa en alumnos de Artes Visuales del nivel superior en una universidad mexicana. También se señalan estrategias y elementos para crear motivación en proyectos grupales en el área de Artes Visuales

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

Ion beam transport: modelling and experimental measurements on a large negative ion source in view of the ITER heating neutral beam

Neutral beam injectors are among the most important methods of plasma heating in magnetic con nement fusion devices. The propagation of the negative ions, prior to their conversion into neutrals, is of fundamental importance in determining the properties of the beam, such as its aiming and focusing at long-distances, so as to deposit the beam power in the proper position inside the con ned plasma, as well as to avoid interaction with the material surfaces along the beam path. The nal design of the ITER Heating Neutral Beam prototype has been completed at Consorzio RFX (Padova, Italy), in the framework of a close collaboration with European, Japanese and Indian fusion research institutes. The physical and technical rationales on which the design is based were essentially driven by numerical modelling of the relevant physical processes, and the same models and codes will be useful to design the DEMO neutral beam injector in the near future. This contribution presents a benchmark study of the codes used for this purpose, by comparing their results against the measures performed in an existing large- power device, hosted at the National Institute for Fusion Science, Japan. In particular, the negative ion formation and acceleration are investigated. A satisfactory agreement was found between codes and experiments, leading to an improved understanding of beam transport dynamics. The interpretation of the discrepancies identi ed in previous works, possibly related to the non-uniformity of the extracted negative ion current, is also presented

Design and Development of a Diagnostic System for a Non-Intercepting Direct Measure of the SPIDER Ion Source Beamlet Current

Stable and uniform beams with low divergence are required in particle accelerators; therefore, beyond the accelerated current, measuring the beam current spatial uniformity and stability over time is necessary to assess the beam performance, since these parameters affect the perveance and thus the beam optics. For high-power beams operating with long pulses, it is convenient to directly measure these current parameters with a non-intercepting system due to the heat management requirement. Such a system needs to be capable of operating in a vacuum in the presence of strong electromagnetic fields and overvoltages, due to electrical breakdowns in the accelerator. Finally, the measure of the beam current needs to be efficiently integrated into a pulse file with the other relevant plant parameters to allow the data analyses required for beam optimization. This paper describes the development, design and commissioning of such a non-intercepting system, the so-called beamlet current monitor (BCM), aimed to directly measure the electric current of a particle beam. In particular, the layout of the system was adapted to the SPIDER experiment, the ion source (IS) prototype of the heating neutral beam injectors (HNB) for the ITER fusion reactor. The diagnostic is suitable to provide the electric current of five beamlets from DC up to 10 MHz