Characterisation of impurity behaviour in RFX-mod and MST reversed-field pinches

The work presented in this thesis is inserted in the field of the research on plasma physics conducted with magnetically confined devices, whose ultimate goal is the demonstration of the feasibility of the energy production through fusion reactions between nuclei of hydrogen isotopes. One of the major issues in the operation of a magnetically confined plasma, both in a present device and in a future reactor, is the presence in the plasma of other elements besides the main gas: the so called impurities. These originate mainly by the interaction of the hot plasma with the plasma facing materials, which can both be eroded and release atmospheric gases previously adsorbed. Once these impurities enters the plasma they get ionised and start emitting radiation through bremsstrahlung and atomic processes (line and recombination radiation). If impurities accumulate in the core plasma, where the temperature is highest, this can lead to a dramatic increase in radiation losses, drive radiative instabilities and dilute the plasma with negative consequences on plasma reactivity. In this regard it is extremely important to keep as low as possible the total impurity content and to control the impurity peaking, i.e. where impurities accumulate. The first point can be addressed by a correct choice of the plasma facing components and by conditioning the first wall. The second point needs a study of the impurity transport in different scenarios, understanding what prevents the accumulation of ions in the core. The radial impurity flux is usually expressed by means of two transport coefficients: the diffusion coefficient and the convective velocity. The determination of these coefficients in different scenarios allows to understand and control the impurity peaking. The Ph.D. activity developed during these three years is focused on this topic: the understanding of the impurity transport behaviour in different scenarios by means of the determination of the transport coefficients. This analysis has been conducted on a particular magnetic configuration: the reversed-field pinch. It has been carried out on the two main devices operating in this configuration: RFX-mod located in Padua (Italy) and MST located in Madison (WI, USA). The analysis method consists in the reproduction by means of a 1D impurity transport code of the plasma signals which are mainly affected by the presence of impurities. In order to discriminate the two coefficients, transients in the plasma signals are needed, which are then reproduced by the code. These transients are usually induced by means of artificially introduced impurities; the code reproduces the variations in the emission patterns or in the impurity density (depending on the available measurements) which are caused by the introduced atoms. By matching the simulation with the measurements an estimation of the transport coefficients is obtained. Once the coefficients have been estimated, they are compared with theoretical models in order to understand the nature of the transport in a particular plasma scenario. For both machines, through the application of this method, the transport of different species of impurities (intrinsic and not) in different magnetic regimes has been studied. In RFX-mod, experiments with injected pellets of carbon and lithium have been carried out and subsequently reproduced with the code, allowing to determine the transport coefficients for the two elements. Moreover, first experiments with introduced tungsten have been conducted; its emission spectrum has been acquired and its penetrating capability investigated. In MST time-dependent impurity density measurements were available for different intrinsic species in two magnetic regimes; their reproduction with the transport code allowed to estimate the transport coefficients. This manuscript is organised in two main parts: first three chapters form the introductory part, chapters four and five form the original work of this thesis. Chapter 1 introduces the role of impurities in fusion plasmas. The production processes are described as well as the impurity effects on the plasma properties. Emitted radiation from impurities is divided into its components (continuum and produced by atomic processes) and briefly characterised. Then the different models used to describe the ion populations are presented: coronal, local thermodynamic and collisional-radiative. Finally the impurity radial transport is depicted in its analytical form, according to the classical and neo-classical formulation. Chapter 2 describes the reversed-field pinch magnetic configuration. Particle transport in this configuration is analytically presented. Chapter 3 gives the details of the simulation tool used in this thesis for reproducing experimental measurements. The code consists of a transport model coupled to a collisional-radiative model for various species. Both models are detailed as well as the inputs and outputs to the code. Chapter 4 reports the analysis performed at RFX-mod on impurity transport. The transient experiments conducted through pellets injection of carbon and lithium are described and the results presented. Transport coefficients are estimated for the different species in different magnetic scenarios. Then experiments with tungsten injection through the laser-blow off technique are presented and the results discussed and compared with reports from other devices. Chapter 5 deals with the analysis realised at MST on impurity transport. Impurity density measurements carried out for different intrinsic species in different plasma regimes are reproduced with the code. The estimated transport coefficients are presented and compared with theoretical models. Chapter 6 concludes the thesis with a discussion of the most important results achieved and an overview of possible future developments

Characterisation of impurity behaviour in RFX-mod and MST reversed-field pinches

The work presented in this thesis is inserted in the field of the research on plasma physics conducted with magnetically confined devices, whose ultimate goal is the demonstration of the feasibility of the energy production through fusion reactions between nuclei of hydrogen isotopes. One of the major issues in the operation of a magnetically confined plasma, both in a present device and in a future reactor, is the presence in the plasma of other elements besides the main gas: the so called impurities. These originate mainly by the interaction of the hot plasma with the plasma facing materials, which can both be eroded and release atmospheric gases previously adsorbed. Once these impurities enters the plasma they get ionised and start emitting radiation through bremsstrahlung and atomic processes (line and recombination radiation). If impurities accumulate in the core plasma, where the temperature is highest, this can lead to a dramatic increase in radiation losses, drive radiative instabilities and dilute the plasma with negative consequences on plasma reactivity. In this regard it is extremely important to keep as low as possible the total impurity content and to control the impurity peaking, i.e. where impurities accumulate. The first point can be addressed by a correct choice of the plasma facing components and by conditioning the first wall. The second point needs a study of the impurity transport in different scenarios, understanding what prevents the accumulation of ions in the core. The radial impurity flux is usually expressed by means of two transport coefficients: the diffusion coefficient and the convective velocity. The determination of these coefficients in different scenarios allows to understand and control the impurity peaking. The Ph.D. activity developed during these three years is focused on this topic: the understanding of the impurity transport behaviour in different scenarios by means of the determination of the transport coefficients. This analysis has been conducted on a particular magnetic configuration: the reversed-field pinch. It has been carried out on the two main devices operating in this configuration: RFX-mod located in Padua (Italy) and MST located in Madison (WI, USA). The analysis method consists in the reproduction by means of a 1D impurity transport code of the plasma signals which are mainly affected by the presence of impurities. In order to discriminate the two coefficients, transients in the plasma signals are needed, which are then reproduced by the code. These transients are usually induced by means of artificially introduced impurities; the code reproduces the variations in the emission patterns or in the impurity density (depending on the available measurements) which are caused by the introduced atoms. By matching the simulation with the measurements an estimation of the transport coefficients is obtained. Once the coefficients have been estimated, they are compared with theoretical models in order to understand the nature of the transport in a particular plasma scenario. For both machines, through the application of this method, the transport of different species of impurities (intrinsic and not) in different magnetic regimes has been studied. In RFX-mod, experiments with injected pellets of carbon and lithium have been carried out and subsequently reproduced with the code, allowing to determine the transport coefficients for the two elements. Moreover, first experiments with introduced tungsten have been conducted; its emission spectrum has been acquired and its penetrating capability investigated. In MST time-dependent impurity density measurements were available for different intrinsic species in two magnetic regimes; their reproduction with the transport code allowed to estimate the transport coefficients. This manuscript is organised in two main parts: first three chapters form the introductory part, chapters four and five form the original work of this thesis. Chapter 1 introduces the role of impurities in fusion plasmas. The production processes are described as well as the impurity effects on the plasma properties. Emitted radiation from impurities is divided into its components (continuum and produced by atomic processes) and briefly characterised. Then the different models used to describe the ion populations are presented: coronal, local thermodynamic and collisional-radiative. Finally the impurity radial transport is depicted in its analytical form, according to the classical and neo-classical formulation. Chapter 2 describes the reversed-field pinch magnetic configuration. Particle transport in this configuration is analytically presented. Chapter 3 gives the details of the simulation tool used in this thesis for reproducing experimental measurements. The code consists of a transport model coupled to a collisional-radiative model for various species. Both models are detailed as well as the inputs and outputs to the code. Chapter 4 reports the analysis performed at RFX-mod on impurity transport. The transient experiments conducted through pellets injection of carbon and lithium are described and the results presented. Transport coefficients are estimated for the different species in different magnetic scenarios. Then experiments with tungsten injection through the laser-blow off technique are presented and the results discussed and compared with reports from other devices. Chapter 5 deals with the analysis realised at MST on impurity transport. Impurity density measurements carried out for different intrinsic species in different plasma regimes are reproduced with the code. The estimated transport coefficients are presented and compared with theoretical models. Chapter 6 concludes the thesis with a discussion of the most important results achieved and an overview of possible future developments.Il lavoro presentato in questa tesi si inserisce nel campo della ricerca sulla fisica del plasma condotta con dispositivi confinati magneticamente, il cui obiettivo finale è la dimostrazione della fattibilità della produzione di energia attraverso reazioni di fusione tra nuclei di isotopi di idrogeno. Uno dei principali problemi nell’operare un plasma a confinamento magnetico, sia in un esperimento attuale che in un futuro reattore, è la presenza nel plasma di altri elementi oltre il gas principale: le cosiddette impurezze. Esse si originano principalmente dall’interazione del plasma caldo con i materiali che si affacciano sullo stesso; questi materiali possono essere sia erosi sia rilasciare gas atmosferici precedentemente adsorbiti. Una volta che queste impurezze entrano nel plasma, esse vengono ionizzate ed emettono radiazione attraverso bremsstrahlung e processi atomici (radiazione di linea e di ricombinazione). Se le impurezze si accumulano nel centro del plasma, dove la temperatura è più alta, questo può portare a un drammatico aumento delle perdite tramite radiazione, si possono generare instabilità radiative e il plasma viene diluito con conseguenze negative sulla sua reattività. A questo proposito è estremamente importante mantenere il più basso possibile il contenuto totale di impurezze e controllarne il picco, cioè la posizione in cui si accumulano. Il primo punto può essere affrontato con una corretta scelta dei materiali che si affacciano al plasma e condizionando la prima parete. Il secondo punto necessita di uno studio del trasporto di impurezze in diversi scenari, comprendendo cosa ne impedisce l’accumulo nel centro del plasma. Il flusso radiale di impurezze è generalmente espresso mediante due coefficienti di trasporto: il coefficiente di diffusione e la velocità convettiva. La determinazione di questi coefficienti in diversi scenari permette di capire e controllare il picco delle impurezze. L’attività di dottorato sviluppata durante questi tre anni è incentrata su questo argomento: la comprensione del comportamento del trasporto di impurezze in diversi scenari mediante la determinazione dei coefficienti di trasporto. Questa analisi è stata condotta su una particolare configurazione magnetica: la strizione a campo rovesciato. Lo studio è stato effettuato sui due principali esperimenti operanti in questa configurazione: RFX-mod situato a Padova (Italia) e MST situato a Madison (WI, USA). Il metodo di analisi consiste nella riproduzione per mezzo di un codice unidimensionale di trasporto di impurezze dei segnali di plasma che sono principalmente interessati dalla presenza di impurezze. Per discriminare i due coefficienti sono necessari dei transienti nei segnali del plasma, che vengono poi riprodotti dal codice. Questi transienti sono solitamente indotti mediante l’introduzione artificiale di impurezze nel plasma; il codice riproduce le variazioni nelle emissioni o nella densità delle impurezze (a seconda delle misurazioni disponibili), causate dagli atomi introdotti. Riproducendo le misurazioni con la simulazione, si ottiene una stima dei coefficienti di trasporto. Una volta che i coefficienti sono stati stimati, essi sono confrontati con modelli teorici per comprendere la natura del trasporto in un particolare scenario del plasma. Per entrambe le macchine, attraverso l’applicazione di questo metodo, è stato studiato il trasporto di diverse specie di impurezze (intrinseche e non) in diversi regimi magnetici. In RFX-mod, sono stati eseguiti e successivamente riprodotti con il codice diversi esperimenti con iniezione di pellet di carbonio e litio, consentendo di determinare i coefficienti di trasporto per i due elementi. Inoltre sono stati condotti i primi esperimenti con iniezione di tungsteno; il suo spettro di emissione è stato acquisito e la sua capacità penetrante è stata indagata. In MST erano disponibili delle misurazioni della densità di impurezze per le varie specie intrinseche; queste misurazioni sono dipendenti dal tempo ed eseguite in due diversi regimi magnetici. La loro riproduzione con il codice di trasporto ha permesso di stimare i coefficienti di trasporto. Questo manoscritto è organizzato in due parti principali: i primi tre capitoli costituiscono la parte introduttiva, i capitoli quattro e cinque formano il lavoro originale di questa tesi. Capitolo 1 introduce il ruolo delle impurezze nei plasmi fusionistici. Ne sono descritti i processi di produzione come pure i loro effetti sulle proprietà del plasma. La radiazione emessa dalle impurezze è suddivisa nelle sue componenti (continua e prodotta da processi atomici), le quali sono brevemente caratterizzate. Successivamente vengono presentati i diversi modelli utilizzati per descrivere le popolazioni di ioni: corona, termodinamico locale e collisionale-radiativo. Infine il trasporto radiale di impurezze è raffigurato nella sua forma analitica, secondo la formulazione classica e neo-classica. Capitolo 2 descrive la configurazione magnetica della strizione a campo rovesciato. Il trasporto di particelle in questa configurazione è presentato analiticamente. Capitolo 3 fornisce i dettagli dello strumento di simulazione utilizzato in questa tesi per la riproduzione delle misure sperimentali. Il codice è costituito da un modello di trasporto accoppiato ad un modello collisionale-radiativo per varie specie. Sono descritti entrambi i modelli nonché gli ingressi e le uscite del codice. Capitolo 4 riporta l’analisi effettuata su RFX-mod circa il trasporto di impurezze. Sono descritti gli esperimenti transitori condotti attraverso l’iniezione di pellet di carbonio e litio e ne sono presentati i risultati. I coefficienti di trasporto sono stimati per le diverse specie in diversi scenari magnetici. In seguito sono presentati gli esperimenti effettuati con l’iniezione di tungsteno attraverso la tecnica dello sfogo di laser; i risultati sono discussi e confrontati con i resoconti di altri esperimenti. Capitolo 5 affronta l’analisi realizzata su MST circa il trasporto di impurezze. Le misurazioni della densità di impurezze effettuate per le varie specie intrinseche sono riprodotte con il codice in diversi regimi di plasma. Sono presentati i coefficienti di trasporto stimati e sono confrontati con i modelli teorici. Capitolo 6 conclude la tesi con una discussione dei risultati più importanti conseguiti ed una panoramica dei possibili sviluppi futuri

Isolation of different erythromycin-resistance Streptococcus pneumoniae phenotypes in Piemonte Region

The frequency of erythromycin resistance in Streptococcus pneumoniae isolates from human specimens, in different Piemonte Region hospitals between 2005-2007, has been studied. Erythromycin-susceptible isolates were 137/198 and erythromycin-resistant 61/198.Among resistant S. pneumoniae isolates 26/61 belonged to constitutive resistance phenotype, 26/61 to M phenotype and 9/61 to inducible resistance phenotype

Effect of toroidal plasma currents on the Wendelstein 7-X Scrape-Off Layer

The role of toroidal plasma currents for the island divertor scrape-off layer in the stellarator Wendelstein 7-X is investigated using reciprocating electric probes. Experiments show that small amounts (of a few kA) of plasma current are sufficient to significantly affect the scrape-off layer plasma conditions, whereas higher plasma currents above 10kA result in more drastic changes. This behavior is linked to the effect of the plasma current on the rotational transform profile, which can result in significant shifts of the edge magnetic islands. These shifts affect the interaction of the islands with the divertor and can eventually result in a transition from a diverted to a limited plasma configuration. The probe observations are complemented by further edge diagnostics including plasma flow measurements, divertor Langmuir probes, divertor thermography and impurity spectroscopy.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement number 633053

Overview of the RFX-mod fusion science activity

This paper reports the main recent results of the RFX-mod fusion science activity. The RFX-mod device is characterized by a unique exibility in terms of accessible magnetic con gurations. Axisymmetric and helically shaped reversed- eld pinch equilibria have been studied, along with tokamak plasmas in a wide range of q(a) regimes (spanning from 4 down to 1.2 values). The full range of magnetic con gurations in between the two, the so-called ultra-low q ones, has been explored, with the aim of studying speci c physical issues common to all equilibria, such as, for example, the density limit phenomenon. The powerful RFX-mod feedback control system has been exploited for MHD control, which allowed us to extend the range of experimental parameters, as well as to induce speci c magnetic perturbations for the study of 3D effects. In particular, transport, edge and isotope effects in 3D equilibria have been investigated, along with runaway mitigations through induced magnetic perturbations. The rst transitions to an improved con nement scenario in circular and D-shaped tokamak plasmas have been obtained thanks to an active modi cation of the edge electric eld through a polarized electrode. The experiments are supported by intense modeling with 3D MHD, gyrokinetic, guiding center and transport codes. Proposed modi cations to the RFX-mod device, which will enable further contributions to the solution of key issues in the roadmap to ITER and DEMO, are also brie y presented

Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100,000

Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but recent advances in plasma theory and computational power have led to renewed interest in stellarators. The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation, with the aim to show that the earlier weaknesses of this concept have been addressed successfully, and that the intrinsic advantages of the concept persist, also at plasma parameters approaching those of a future fusion power plant. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy

Major results from the first plasma campaign of the Wendelstein 7-X stellarator

After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3 1019 m-3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.Peer reviewe