Within the framework of magnetic nuclear fusion, the study of plasma-wall interaction (PWI) plays a key role in the way towards the successful realization of nuclear fusion reactors. Erosion processes on plasma facing components (PFCs) and the following contamination of the core by the eroded impurities can compromise the performances of the magnetic confinement devices. To study these phenomena, modelling tools, such as the ERO2.0 code, prove to be a precious resource to complement experimental activities. The goal of this thesis is to employ the code to study erosion and transport processes both in a linear device, GyM, and in a tokamak, ASDEX Upgrade (AUG). The thesis focuses on the role of helium as a plasma species, as its presence in future reactors as a D-T reaction product requires the study of its influence on PWI. In GyM, simulations with a global approach were used to assess the influence of an additional PFC, the sample-holder, not included in previous studies, while also giving an estimate of the erosion rates of the lateral wall and the samples, assuming a variety of different wall materials and tuning the ion impact energy by applying a biasing voltage. Simulations were also conducted at a local micro-scale level, observing a tiny volume on the samples surface, allowing to evaluate the surface morphology of real samples employed in experimental campaigns, thus establishing a link between simulations and experiments. In AUG, simulations were mostly aimed at reproducing PWI processes observed during an experimental campaign involving L-mode He plasma discharges. The main goal was to match the measured net eroded thickness on the outer divertor with the one computed by the simulations. Although preliminary results showed a large gap, a parametric analysis of electron and ion temperatures and the oxygen impurities concentration allowed to obtain a better agreement. Finally, the model was improved by considering a Zeff distribution in the plasma, describing the local concentration of the two He charge states. Differences in impurity transport were observed with the new model leading to a lower tungsten impurity content in the volume.
Nel contesto della fusione nucleare magnetica, lo studio dell’interazione plasma-parete (PWI) riveste un ruolo chiave nel percorso verso la realizzazione di successo dei reattori a fusione nucleare. I processi di erosione sui componenti a contatto con il plasma (PFC) e la conseguente contaminazione del nucleo da parte delle impurità erose possono compromettere le prestazioni dei dispositivi di confinamento magnetico. Per studiare questi fenomeni, strumenti di modellazione, come il codice ERO2.0, si dimostrano risorse preziose per integrare le attività sperimentali. L’obiettivo di questa tesi è impiegare il codice per studiare i processi di erosione e trasporto sia in una macchina lineare, GyM, sia in un tokamak, ASDEX Upgrade (AUG). La tesi si concentra sul ruolo dell’elio (He) come specie di plasma, poiché la sua presenza nei futuri reattori come prodotto della reazione D-T richiede lo studio della sua influenza sulla PWI. In GyM, le simulazioni con un approccio globale sono state utilizzate per valutare l’influenza di un ulteriore PFC, il portacampioni, non incluso negli studi precedenti, fornendo anche una stima dei tassi di erosione della parete laterale e dei campioni, assumendo una varietà di materiali per la parete e regolando l’energia di impatto degli ioni mediante l’applicazione di una tensione. Le simulazioni sono state condotte anche localmente a livello microscopico, esaminando un piccolo volume sulla superficie dei campioni, consentendo di valutare la morfologia superficiale dei campioni impiegati realmente nelle campagne sperimentali, stabilendo così un collegamento tra simulazioni ed esperimenti. In AUG, le simulazioni sono state principalmente mirate a riprodurre i processi di PWI osservati durante una campagna sperimentale che coinvolgeva scariche di plasma di He in L-mode. L’obiettivo principale era confrontare lo spessore netto eroso misurato sul divertore esterno con quello calcolato dalle simulazioni. Sebbene i risultati preliminari abbiano mostrato un ampio divario, un’analisi parametrica delle temperature degli elettroni e degli ioni e della concentrazione di impurità di ossigeno ha permesso di ottenere un migliore accordo. Infine, il modello è stato migliorato considerando una distribuzione di Zeff nel plasma, che descrive la concentrazione locale dei due stati di carica dell’elio. Con il nuovo modello, sono state osservate differenze nel trasporto delle impurità, portando a un contenuto inferiore di impurità di tungsteno nel volume.
Erosion and impurity migration modelling of helium plasmas in tokamaks and linear devices
Tuccari, Carlo
2022/2023
Abstract
Within the framework of magnetic nuclear fusion, the study of plasma-wall interaction (PWI) plays a key role in the way towards the successful realization of nuclear fusion reactors. Erosion processes on plasma facing components (PFCs) and the following contamination of the core by the eroded impurities can compromise the performances of the magnetic confinement devices. To study these phenomena, modelling tools, such as the ERO2.0 code, prove to be a precious resource to complement experimental activities. The goal of this thesis is to employ the code to study erosion and transport processes both in a linear device, GyM, and in a tokamak, ASDEX Upgrade (AUG). The thesis focuses on the role of helium as a plasma species, as its presence in future reactors as a D-T reaction product requires the study of its influence on PWI. In GyM, simulations with a global approach were used to assess the influence of an additional PFC, the sample-holder, not included in previous studies, while also giving an estimate of the erosion rates of the lateral wall and the samples, assuming a variety of different wall materials and tuning the ion impact energy by applying a biasing voltage. Simulations were also conducted at a local micro-scale level, observing a tiny volume on the samples surface, allowing to evaluate the surface morphology of real samples employed in experimental campaigns, thus establishing a link between simulations and experiments. In AUG, simulations were mostly aimed at reproducing PWI processes observed during an experimental campaign involving L-mode He plasma discharges. The main goal was to match the measured net eroded thickness on the outer divertor with the one computed by the simulations. Although preliminary results showed a large gap, a parametric analysis of electron and ion temperatures and the oxygen impurities concentration allowed to obtain a better agreement. Finally, the model was improved by considering a Zeff distribution in the plasma, describing the local concentration of the two He charge states. Differences in impurity transport were observed with the new model leading to a lower tungsten impurity content in the volume.File | Dimensione | Formato | |
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2024_04_Tuccari_Tesi_01.pdf
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https://hdl.handle.net/10589/219316