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Physics of negative ion extraction and acceleration in neutral beam injectors
Full text linkMost of the work reported in this PhD thesis was carried out at Consorzio RFX, where the most important neutral beam facility worldwide is under construction. This test bed will host 2 experiments, an ion source and a full injector, representing the state of art technology in the NBI context. In this initial phase of design and optimization the modeling plays a key role in defining the characteristics of the devices and in anticipating their performance. In this perspective the work reported here has the scope to analyze some issues that are critical for the operation of such experiments, with particular attention to the phase of extraction and acceleration of ions from the source, before they are neutralized and injected into the plasma core. Many issues strictly connected to this basic topic will be discussed as well, as the production of secondary particles by interaction of the beam with the gas flowing in the injector. The development, implementation and use of dedicated models are described and their results discussed and, in some cases benchmarked with existing experiments.Gran parte del lavoro descritto in questa tesi è stato svolto presso il Consorzio RFX, dove è in corso di costruzione quello che diverrà il piu` grande laboratorio di prova del mondo dedicato agli iniettori di neutri. Questo impianto ospiterà due esperimenti, una sorgente di ioni ed un acceleratore completo, che rappresentano la tecnologia piu` all’avanguardia nel contesto degli NBIs. Nella fase iniziale di progettazione e ottimizzazione di tali dispositivi la modellizzazione gioca un ruolo chiave, aiutando a definire le caratteristiche dell’iniettore e dando la possibilità di prevedere le sue prestazioni. In questa prospettiva il lavoro riportato in questa tesi ha lo scopo di analizzare alcuni aspetti cruciali per il funzionamento degli iniettori, prestando particolare attenzione alla fase di estrazione ed accelerazione degli ioni dalla sorgente, prima che vengano neutralizzati ed iniettati nel plasma. Allo stesso tempo saranno analizzati diversi aspetti strettamente correlati a questi argomenti di base, come, ad esempio, la produzione di particelle secondarie durante l’interazione tra il fascio ed il gas presente nell’iniettore. Verranno dunque presentati lo sviluppo, l’implementazione e l’uso di specifici modelli, i cui risultati verranno discussi ed, in alcuni casi, confrontati con le evidenze sperimentali di dispositivi esistenti
Multi-beamlet investigation of the deflection compensation methods of SPIDER beamlets
No full textSPIDER (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 tim
The influence of grid positioning on the beam optics in the neutral beam injectors for ITER
No full textDesign of the new extraction grid for the NIO1 negative ion source
Full text linkNIO1 is a compact source of negative ions jointly developed by RFX and INFN, to study the physics of production and acceleration of H- beams. Negative ions, up to 120 mA of current, are extracted from a radiofrequency driven plasma, by means of a gridded electrode (plasma grid, PG) featuring 9 apertures arranged in a 3x3 square lattice. The same aperture pattern is replicated in the following electrodes, allowing ion acceleration up to 60 keV. All electrodes are realized in copper, by electro-deposition technique, leaving empty slots in the metal to place magnets and to flow water for the grid cooling. The first set of electrodes was completed, installed in the source and tested. At the same time, an upgrade of the extraction system was carried out, in order to optimize the beam optics and to explore alternative electrostatic configurations. In particular, the accelerator will be modified by completely replacing the EG grid, exploiting the modularity of NIO1. The new electrode will feature other slots in between apertures, to place additional magnets. This allows testing different magnetic configurations, to optimize electron filtering and residual ion deflection. The present paper describes the theoretical activities driving the design of these new extractors, carried out with most updated numerical codes, and exploiting the synergy with the refined modeling of the 40 A ITER negative ion sources, under development at Consorzio RFX. Beam simulations are performed both with tracing codes (SLACCAD and OPERA) and with particle in cell codes (ACCPIC
Influence of positive ions on the beamlet optics for negative-ion neutral beam injectors
Full text linkNeutral 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
Cancellation of the ion deflection due to electron-suppression magnetic field in a negative-ion accelerator
No full textA new magnetic configuration is proposed for the suppression of co-extracted electrons in a negative- ion accelerator. This configuration is produced by an arrangement of permanent magnets embedded in one accelerator grid and creates an asymmetric local magnetic field on the upstream and downstream sides of this grid. Thanks to the “concentration” of the magnetic field on the upstream side of the grid, the resulting deflection of the ions due to magnetic field can be “intrinsically” cancelled by calibrating the configuration of permanent magnets. At the same time, the suppression of co-extracted electrons can be improved
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