When, in a tokamak, external power is supplied above a threshold level, the plasma undergoes an abrupt transition which leads to the creation of an edge transport barrier, thus improving the confinement capabilities of the plasma itself. This regime is called High-confinement Mode, or H-Mode; its features are several: the increasing of the temper- ature, density and pressure profiles, leading to the formation of a region of steep gradients at the plasma edge, called pedestal; the generation of an edge current called bootstrap current; and, most importantly, the improvement of the energy confinement time, which approximately doubles in value with respect to the confinement regime preceding the H-Mode, the so-called L-Mode. For these reasons, the H-Mode confinement regime is one of the primary operational scenarios at ITER, the future international project regarding nuclear fusion feasibility at ignition conditions. However, one of the consequences when operating in H-Mode is the arising of instabilities called Edge Localized Modes (or ELMs), which are ejections of plasma from the region where the plasma itself is confined; ELMs are triggered after the confined plasma reaches an MHD instability limit, and even though they have ameliorating effects on some nega- tive aspects of the H-Mode, such as expelling impurities from the highly confined plasma, the plasma ejections correspond to a loss of confinement and pulsed heat loads against key components like the divertor, thus significantly reducing their operating life. Depending on a wide range of conditions, such as power input, impurity and fuel injec- tions, geometric and magnetic configuration parameters and much more, the ELMs that will generate will be different. As such, one would like to understand which are the most significant conditions that will influence the generation of a type of ELM with respect to another, in order to predict which ELMs will generate inside the considered machine. Since there is an extensive bibliography about this topic, but ELMs study is still an open research field, the theory concerning this phenomenon is majorly empyrical. As such, the need scaling laws and behaviors that repeats themselves in different tokamaks is imper- ative, and this means that gathering data from different experiments and from different machines can be a good starting point to reach that goal. For this reason, EUROFusion is promoting the creation of a database concerning ELMy H-Mode plasmas from tokamaks all over the world, focusing in particular in the region of the pedestal, since there is a correlation between this region and the ELMs generation, with the objective to under- stand and predict the ELMs behavior and appearance in ITER. This thesis work aims to build a first version of what the pedestal database will be for the machine operating at the Swiss Plasma Center, the Tokamak à Configuration Variable, or TCV; in order to do so, data from the Thomson Scattering System have been gath- ered, then getting fitted profiles from them and by calculating the plasma at equilibrium by using the CHEASE iterative code, thus using the extrapolated quantities to build the database itself; finally, some considerations over the quantities of the newly built database, such as common behaviors and basic trends, have been made.
Quando, in un tokamak, viene fornita potenza al di sopra di un livello di soglia, il pla- sma subisce una brusca transizione che porta alla creazione di una barriera di trasporto ai bordi, miglio- rando cos`ı le capacità di confinamento del plasma stesso. Questo regime ` e chiamato High-confinement Mode, o H-Mode: i vantaggi di questo regime sono molteplici rispetto al regime di confinamento che precede l’H-Mode, l’L-Mode, in particolare il miglioramento dell’energy confinement time, che raddop- pia approssimativamente di valore. Il regime di confinamento in modalità H sarà uno dei principali scenari operativi di ITER, il futuro progetto internazionale sulla possibilità e la convenienza di effet- tuare reazioni di fusione nucleare. Tuttavia, un plasma in H-Mode ` e l’insorgere di instabilità MHD che si rilassano mediante processi chiamati Edge Localized Modes (o ELMs), espulsioni di plasma dalla regione in cui il plasma stesso ` e confinato; tali espulsioni di plasma corrispondono ad una perdita di confinamento e carichi termici pulsati contro componenti chiave come il divertore, riducendo cos`ı signi- ficativamente la loro vita operativa. A seconda di un ampio intervallo di parametri operativi, gli ELM che verranno generati saranno diversi. Come tale, vorremmo capire quali sono le condizioni più significative che influenzeranno la generazione di un tipo di ELM rispetto ad un altro, al fine di prevedere quali ELM si genereranno all’interno della macchina considerata. Poich´ e lo studio degli ELM ` e un settore di ricerca aperto, dove non si ha ancora una teoria completa, per derivare risultati abbiamo bisogno di profili e comportamenti che si ripetono anche in tokamak diversi, e questo significa raccogliere dati da esperimenti e macchine diverse. Per que- sto motivo, EUROFusion sta promuovendo la creazione di un database riguardante i plasmi H-Mode con generazione di ELM da parte di tokamak in tutto il mondo, concentrandosi in particolare nella regione del pedestal, in quanto esiste una correlazione tra questa regione e la generazione di ELMs, con l’obiettivo di comprendere e prevedere il comportamento e l’aspetto degli ELMs in ITER. Questo lavoro di tesi mira a costruire una prima versione di quello che sarà il database del pedestal in ELMy H-mode per la macchina operante presso lo Swiss Plasma Center, il Tokamak à Configuration Variable, o TCV; per fare ciò, sono stati raccolti i dati dal Thomson Scattering System in servizio nella macchina, ricavando dei fit dai dati e calcolando il plasma in equilibrio utilizzando il codice iterativo CHEASE, utilizzando cos`ı le quantità estrapolate per costruire il database stesso; infine, sono state fatte alcune considerazioni sulle quantità del nuovo database costruito, come profili che si ripetono in altre macchine.
Development of the EUROfusion pedestal database for ELMy H-Mode for the TCV tokamak
ROVERE, EDOARDO
2018/2019
Abstract
When, in a tokamak, external power is supplied above a threshold level, the plasma undergoes an abrupt transition which leads to the creation of an edge transport barrier, thus improving the confinement capabilities of the plasma itself. This regime is called High-confinement Mode, or H-Mode; its features are several: the increasing of the temper- ature, density and pressure profiles, leading to the formation of a region of steep gradients at the plasma edge, called pedestal; the generation of an edge current called bootstrap current; and, most importantly, the improvement of the energy confinement time, which approximately doubles in value with respect to the confinement regime preceding the H-Mode, the so-called L-Mode. For these reasons, the H-Mode confinement regime is one of the primary operational scenarios at ITER, the future international project regarding nuclear fusion feasibility at ignition conditions. However, one of the consequences when operating in H-Mode is the arising of instabilities called Edge Localized Modes (or ELMs), which are ejections of plasma from the region where the plasma itself is confined; ELMs are triggered after the confined plasma reaches an MHD instability limit, and even though they have ameliorating effects on some nega- tive aspects of the H-Mode, such as expelling impurities from the highly confined plasma, the plasma ejections correspond to a loss of confinement and pulsed heat loads against key components like the divertor, thus significantly reducing their operating life. Depending on a wide range of conditions, such as power input, impurity and fuel injec- tions, geometric and magnetic configuration parameters and much more, the ELMs that will generate will be different. As such, one would like to understand which are the most significant conditions that will influence the generation of a type of ELM with respect to another, in order to predict which ELMs will generate inside the considered machine. Since there is an extensive bibliography about this topic, but ELMs study is still an open research field, the theory concerning this phenomenon is majorly empyrical. As such, the need scaling laws and behaviors that repeats themselves in different tokamaks is imper- ative, and this means that gathering data from different experiments and from different machines can be a good starting point to reach that goal. For this reason, EUROFusion is promoting the creation of a database concerning ELMy H-Mode plasmas from tokamaks all over the world, focusing in particular in the region of the pedestal, since there is a correlation between this region and the ELMs generation, with the objective to under- stand and predict the ELMs behavior and appearance in ITER. This thesis work aims to build a first version of what the pedestal database will be for the machine operating at the Swiss Plasma Center, the Tokamak à Configuration Variable, or TCV; in order to do so, data from the Thomson Scattering System have been gath- ered, then getting fitted profiles from them and by calculating the plasma at equilibrium by using the CHEASE iterative code, thus using the extrapolated quantities to build the database itself; finally, some considerations over the quantities of the newly built database, such as common behaviors and basic trends, have been made.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/145958