Characterization of cesium and H-/D- density in the negative ion source SPIDER

The Heating Neutral Beam Injectors (HNBs) for ITER will have to deliver 16.7 MW beams of H/D particles at 1 MeV energy. The beams will be produced from H-/D- ions, generated by a radiofrequency plasma source coupled to an ion acceleration system. A prototype of the ITER HNB ion source is being tested in the SPIDER experiment, part of the ITER Neutral Beam Test Facility at Consorzio RFX. Reaching the design targets for beam current density and fraction of coextracted electrons is only possible by evaporating cesium in the source, in particular on the plasma facing grid (PG) of the acceleration system. In this way the work function of the surfaces decreases, significantly increasing the amount of surface reactions that convert neutrals and positive ions into H-/D-. It is then of paramount importance to monitor the density of negative ions and the density of Cs in the proximity of the PG. Monitoring the Cs spatial distribution along the PG is also essential to guarantee the uniformity of the beam current. In SPIDER, this is possible thanks to the Cavity Ringdown Spectroscopy (CRDS) and the Laser absorption Spectroscopy diagnostics (LAS), which provide line-integrated measurements of negative ion density and neutral, ground state Cs density, respectively. The paper discusses the CRDS and LAS measurements as a function of input power and of the magnetic and electric field used to reduce the coextraction of electrons. Negative ion density data are in qualitative agreement with the results in Cs-free conditions. In agreement with simulations, Cs density is peaked in the center of the source; a top/bottom non uniformity is however present. Several effects of plasma on Cs deposition are presented.Comment: 17 pages, 9 figures. Paper (Preprint) following the poster contribution at the SOFT 2022 conference. The destination journal is Fusion Engineering and Desig

Preparation of the measurement system for the spatial characterization of SPIDER negative-ion plasma source

Neutral beam injectors are fundamental auxiliary heating systems for nuclear fusion machines. The formation of the negative-ion beam, precursor of the neutral beam, occurs by extracting the ions from a plasma through a multi-aperture, multi-electrode electrostatic accelerator. In the case of the ITER neutral beam injector, to obtain sufficient current, the extraction takes place through 1280 openings distributed over a very large area, which must be illuminated by a sufficiently uniform plasma. The plasma, formed in 8 radio-frequency drivers, passes through an expansion chamber before reaching the extractor. The experimental study of the plasma parameters at the location of the ion extraction, and of its expansion from the driver region, is possible using Langmuir probes on mobile supports. In this dissertation, a preparatory study is conducted for characterizing negative ion plasma in SPIDER negative-ion plasma source. A bibliographic research is performed in order to understand how similar measures were taken in the past in similar devices, at the end of which three different kind of probes are designed and tested: a Mach probe, a double Langmuir probe and a planar Langmuir probe. The Mach probe is composed of 4 cylindrical pins and is designed in order to measure plasma flux and velocity drift. It was tested by assembling it on a rotating system inside an Argon plasma, in order to verify its symmetry and if it is suitable for an RF source like SPIDER. Next, the double probe (named ADEL, i.e. A Double Electrode Langmuir probe) consists of two parallel cylindrical electrodes, and it should give a completely floating measures of the plasma parameters. It was tested inside the CRISPy experiment (Compact RF Ion Source Prototype for emittance scanner testing) in order to give an estimate of electron temperature and plasma density. Regarding the planar Langmuir probe, it is composed of two electrodes: a main electrode and a compensation one. A model in LTSpice is used to simulate its behaviour in an RF plasma, so as to find the optimum capacity to be put in parallel between the two electrodes. Finally, a plan for probes installation and measurements in SPIDER is established in light of the results obtained

Modelling of plasma expansion and interpretation of measured profiles in a negative ion source.

ITER is an international project aiming to demonstrate the feasibility of energy production through controlled thermonuclear fusion. In order to trigger the required fusion reactions, an extremely hot plasma has to be confined for a sufficiently long time. One of the primary methods for plasma heating is the Neutral Beam Injection (NBI), which amounts to depositing additional power in the plasma by means of a highly energetic neutral beam, obtained through the neutralization of a precursor ion beam. The ITER NBI will employ radiofrequency (RF) driven ion sources to generate and extract negative ions; this kind of source displays some important advantages, such as low need for maintenance and moderate energy consumption, even though the research on the application of this technology to fusion experiments is less mature than the more consolidated arc-filament discharge. In this framework, the main purpose of this thesis project is the investigation of the most important physical processes underlying the plasma generation and expansion in a RF negative ion source, with a particular focus on plasma uniformity: indeed, the presence of a magnetic filter field inevitably introduces drift motions inside the source, ultimately affecting several properties of the extracted beam such as current intensity, stability and convergence. A pre-existent Particle-In-Cell simulation code was adapted and developed so as to study the plasma generation and expansion mechanisms in the SPIDER source. The numerical results have been compared with recent experimental measurements obtained with movable electrostatic probes, with the intention of both validating the code and providing an interpretation for the experimental trends, clarifying the relations between plasma properties and variations of the background gas pressure, of the magnetic field and of the voltages of the plasma-facing electrodes. SOMMARIO: ITER è un progetto internazionale volto a dimostrare la fattibilità della produzione di energia tramite fusione nucleare termocontrollata. Affinché le reazioni di fusione possano avere luogo, è necessario confinare un plasma ad alta temperatura per un tempo sufficientemente lungo. Gli iniettori di neutri sono uno dei metodi più diffusi per il riscaldamento del plasma: in questo caso si utilizza un fascio di neutri ad alta energia, ottenuto tramite neutralizzazione di un fascio precursore di ioni, per depositare potenza nel plasma stesso. Il progetto di ITER prevede sorgenti a radiofrequenza (RF) per la produzione e l'estrazione di ioni negativi; questa tecnologia porta molti vantaggi, come il ridotto bisogno di manutenzione e un moderato consumo di energia ma, d'altra parte, la ricerca sulla sua applicazione a esperimenti di fusione nucleare è ancora in evoluzione. In questo contesto, l'obiettivo principale di questo lavoro di tesi è lo studio dei processi fisici alla base della generazione ed espansione del plasma in una sorgente di ioni negativi a radiofrequenza, con particolare attenzione all'uniformità di plasma: infatti, la presenza di un filtro magnetico introduce inevitabilmente dei moti di deriva all'interno della sorgente e, di conseguenza, comporta l'alterazione di alcune proprietà del fascio come l'intensità di corrente, la stabilità ed anche la convergenza. Un codice Particle-In-Cell già esistente è stato adattato e sviluppato con l'obiettivo di studiare i meccanismi di generazione ed estrazione di plasma nella sorgente SPIDER. I risultati numerici sono stati confrontati con misure sperimentali ottenute tramite sonde elettrostatiche mobili, con l'obiettivo di validare il codice e di analizzare le correlazioni tra le proprietà del plasma e variazioni della pressione del gas di background, del campo magnetico e del potenziale della griglie esposte al plasma

Characterisation of the BATMAN beam properties by H-Doppler shift spectroscopy and mini-STRIKE calorimeter

Questo lavoro si inserisce nell'ambito della ricerca delle sorgenti di ioni negativi, atti a creare un fascio di particelle negative che serviranno, opportunamente neutralizzati, per il riscaldamento addizionale del plasma nel reattore a fusione ITER. Il lavoro di tesi è stato svolto all'istituto IPP di Garching, presso la test facility BATMAN. La tesi si concentra sullo studio delle proprietà del fascio di ioni, studiato tramite metodi spettroscopi (BES) e calorimetria (mini-STRIKE).ope

Investigation of a Magnetically Enhanced Inductively Coupled Negative Ion Plasma Source

Experiments and numerical models were used to investigate an inductively coupled plasma source (ICPS) operating with a magnetic filter field. The work shows that applying magnetic filters transversely to the plasma offers several new control parameters to help enhance the properties of a plasma source. The application of these new results using magnetic enhancement is discussed with respect to both industrial plasma fabrication processes and neutral beam injection for fusion power. Experimental measurements of the power transfer efficiency of the ICPS were undertaken comparing the effect of the magnetic field for both hydrogen and argon plasmas. The location and strength of the magnetic field was varied while measurements of the plasma resistance and power transfer efficiency were performed. The changes in forward power transfer were correlated with plasma density measurements and a numerical model of the electrical plasma circuit was used to guide the optimal choice for the power system components. The results demonstrate that the magnetic field increases the total efficiency of the plasma source and that the gains are strongly dependant on the choice of location for the magnetic field. Plasma properties were then investigated across the plasma source 1 cm intervals. Experimental measurements comparing the effect of the magnetic filter on the plasma properties include: electron densities using a hairpin probe, electron energy probability functions using a compensated Langmuir probe, negative ion densities by laser photo detachment and rotational gas temperatures by optical emission spectroscopy. These measurements revealed interesting new properties of the plasma when a magnetic filter is applied including: the formation of a high density cold particle trap, changes in particle transport and drift motions, increased gas temperatures, and a peak in negative ion density under the magnetic filter center. Pulsing the plasma can greatly affect the plasma dynamics, leading to electron cooling in the afterglow and increased negative ion production. A combination of a pulsed plasma with a magnetic filter was then investigated. Measurements of the negative ion and electron populations were performed in the plasma afterglow with the magnetic filter applied. The results reveal a complex and dynamic afterglow process including strong spatial dependencies measured for diffusive transport, ambipolar breakdown and ion-ion plasma formation. The applications for this work include offering new avenues for control over processing plasma chemistry as well as initial results toward the future viability of a caesium-free pulsed negative ion neutral beam source

Beam physics via tomographic diagnostics

The goal of this thesis work is the study of the beam physics of the negative ion beam for ITER HNB. A new diagnostics is installed on SPIDER, the full-size prototype of ITER negative ion source: the visible tomography. It is composed by a set of visible cameras which measure the light emitted by the beam particles when they interact with the background gas. An algorithm to reconstruct through tomographic inversion the two-dimensional pattern of the beam emission is developed, and the reconstructed profiles are used to study the homogeneity of the beam current, also with the support of a model to directly correlate the beamlet emission with the beamlet current density. Thanks to the masking of most of the apertures composing SPIDER multi-beamlets negative ion beam, the single-beamlet divergence is estimated through the Gaussian fit of the 1D beam profiles. \\ The results obtained by this new technique are used to investigate the beam features as a function of the main source and accelerator parameters, integrating the information provided by all the other diagnostics available. A strong correlation between the beam properties and the plasma features is found, thus a deep investigation of the source plasma is carried out. The beam homogeneity depends on the uniformity of both electrons and negative ions at the extraction region, in order to obtain identical beamlet optics at all the apertures and to avoid localised heating of the extraction grid due to the co-extracted electrons. The estimation of the single-beamlet current density is exploited to better interpret the spectroscopic measurements both close to the grid system (together with the electrostatic probes data), and inside the drivers. This experience is fundamental for the future operation at full performances, when the characterization of the single beamlet will be more challenging. The various operational regimes explored, both with and without caesium evaporation, are investigated to improve the understanding of the physics behind the generation and extraction of a large negative ion beams, when the principal source and accelerator control parameters are varied. This is the first comprehensive analysis of the experimental data measured during the experimental campaign of SPIDER experiment, and the performances achieved in term of beam divergence and homogeneity, as well as beamlet current density and co-extracted electrons currents are presented.The goal of this thesis work is the study of the beam physics of the negative ion beam for ITER HNB. A new diagnostics is installed on SPIDER, the full-size prototype of ITER negative ion source: the visible tomography. It is composed by a set of visible cameras which measure the light emitted by the beam particles when they interact with the background gas. An algorithm to reconstruct through tomographic inversion the two-dimensional pattern of the beam emission is developed, and the reconstructed profiles are used to study the homogeneity of the beam current, also with the support of a model to directly correlate the beamlet emission with the beamlet current density. Thanks to the masking of most of the apertures composing SPIDER multi-beamlets negative ion beam, the single-beamlet divergence is estimated through the Gaussian fit of the 1D beam profiles. \\ The results obtained by this new technique are used to investigate the beam features as a function of the main source and accelerator parameters, integrating the information provided by all the other diagnostics available. A strong correlation between the beam properties and the plasma features is found, thus a deep investigation of the source plasma is carried out. The beam homogeneity depends on the uniformity of both electrons and negative ions at the extraction region, in order to obtain identical beamlet optics at all the apertures and to avoid localised heating of the extraction grid due to the co-extracted electrons. The estimation of the single-beamlet current density is exploited to better interpret the spectroscopic measurements both close to the grid system (together with the electrostatic probes data), and inside the drivers. This experience is fundamental for the future operation at full performances, when the characterization of the single beamlet will be more challenging. The various operational regimes explored, both with and without caesium evaporation, are investigated to improve the understanding of the physics behind the generation and extraction of a large negative ion beams, when the principal source and accelerator control parameters are varied. This is the first comprehensive analysis of the experimental data measured during the experimental campaign of SPIDER experiment, and the performances achieved in term of beam divergence and homogeneity, as well as beamlet current density and co-extracted electrons currents are presented