Numerical Analysis of Coupled Thermal-Electromagnetic Problems in Superconducting Cables

Superconducting materials, being characterized by a negligible electrical resistance under peculiar working conditions, provide extraordinary electromagnetic performances. The research field on electromagnets has taken a lot of advantages from this technology, since the huge electrical current densities that these materials sustain enable to produce very strong magnetic fields, up to more than 10 T, with negligible losses compared to the normal-conducting coils. The development of superconductors technology during the last years has enabled projects that only some decades ago were considered not feasible, both technically and economically. Among them, the most notable are fusion reactors like ITER, presently under construction in Cadarache (France), and particle accelerators for high energy physics such as the Large Hadron Collider (LHC) operating at CERN in Geneva (Switzerland). The present work regards the THELMA code, a coupled thermal-electromagnetic numerical model for the description of superconducting cables and magnets. This software was initially intended for the simulation of the electromagnetic behavior in the so-called Cable-In-Conduit-Conductors (CICC), largely used in fusion machines like ITER. During the PhD activity, a brand-new thermal model has been developed and added to the pre-existing code to describe problems in which the system thermal evolution cannot be assessed a priori. Moreover, the code has been extended to deal also with the Rutherford cables, a type of superconducting cable widely used in accelerator magnets like those of LHC. Finally, the code has been applied to several case studies, both in the field of accelerator and fusion magnets. This thesis is structured in the following way. The first two chapters are a general introduction to superconductivity: the first is a presentation of this phenomenon and its applications, intended for readers that are not familiar with this technology, whereas the second is a more detailed description of the superconducting wires and cables studied during this PhD activity. In the second part of the thesis, the THELMA numerical code is widely described. In chapter 3, the geometrical, electromagnetic and thermal models are presented, with a particular focus on the brand-new parts developed during this PhD activity, such as the Rutherford cable geometrical model, the thermal model and the coupling among electromagnetic and thermal routines. The THELMA model for electrical and thermal contact resistances is instead widely explained in chapter 4, together with the numerical analysis of several experimental measurements on both Rutherford and CICC cables. The third part of the work is instead focused on some examples of the application of the THELMA coupled code, performed during the PhD activity. In chapter 5 the analysis of the voltage-temperature characteristic on a CICC sample is presented, as a validation and an example of the code capability of reproducing non-trivial experimental findings. In chapter 6, the problem of the longitudinal propagation of a thermal-electromagnetic instability (quench) in impregnated Rutherford coils is analyzed with experimental, analytical and numerical tools. In chapter 7, the predictive analyses in terms of current distribution and losses in the CICC magnet NAFASSY are reported. Further details regarding useful material properties and some analytical and numerical models can be found in the appendices.I materiali superconduttori, essendo caratterizzati in particolari condizioni da una resistenza elettrica trascurabile, offrono straordinarie prestazioni elettromagnetiche. La ricerca sugli elettromagneti ha ottenuto notevoli vantaggi da questa tecnologia, in quanto le enormi densit\ue0 di corrente elettrica che questi materiali sopportano possono essere usate per generare campi magnetici estremamente intensi, anche maggiori di 10 T, con delle perdite trascurabili in confronto agli avvolgimenti normoconduttivi. Lo sviluppo della tecnologia dei superconduttori avvenuto negli ultimi anni ha permesso progetti che solo pochi decenni fa erano considerati irrealizzabili, sia dal punto di vista tecnico che economico. Tra questi, i pi\uf9 importanti sono senz\u2019altro i reattori per fusione nucleare come ITER, attualmnente in costruzione a Cadarache (Francia), e acceleratori di particelle per la fisica delle alte energie come il Large Hadron Collider (LHC) del CERN a Ginevra (Svizzera). In questa tesi viene presentato il codice THELMA, un modello numerico per la descrizione accoppiata del comportamento termo-elettromagnetico di cavi e magneti superconduttori. Questo codice era stato inizialmente creato per la simulazione del comportamento elettromagnetico dei cosiddetti Cable-In-Conduit-Conductors (CICC), ampiamente usati in macchine per la fusione nucleare come ITER. Durante l\u2019attivit\ue0 di dottorato, \ue8 stato implementato un nuovo modello termico in aggiunta al codice preestitente, in grado di descrivere problemi nei quali l\u2019evoluzione termica del sistema non pu\uf2 essere prevista a priori. Inoltre, il codice \ue8 stato esteso per descrivere i cavi di tipo Rutherford, usati comunemente nei magneti per acceleratori di particelle come quelli di LHC. Infine, il codice \ue8 stato applicato per l\u2019analisi di diversi casi di studio, sia nell\u2019ambito dei magneti per acceleratori di particelle che per fusione nucleare. La tesi \ue8 strutturata nella seguente maniera. I primi due capitoli sono un\u2019ampia introduzione alla superconduttivit\ue0: il primo \ue8 una presentazione generale di questo fenomeno e sulle sue applicazioni, pensata per chi non dovesse avere familiarit\ue0 con questa tecnologia, mentre il secondo contiene una descrizione pi\uf9 dettagliata dei fili e cavi superconduttori presi in considerazione durante questo dottorato di ricerca. Una descrizione dettagliata del codice numerico THELMA \ue8 invece riportata nella seconda parte della tesi. Nel capitolo 3 vengono presentati i modelli geometrici, elettromagnetici e termici, con particolare dettaglio relativamente alle parti sviluppate durante l\u2019attivit\ue0 di dottorato, quali il modello geometrico del cavo Rutherford, il modello termico e l\u2019accoppiamento tra il modello termico e quello elettromagnetico. Il modello di THELMA per le resistenze di contatto elettriche e termiche \ue8 invece descritto nel capitolo 4, insieme all\u2019analisi numerica di alcuni misure sperimentali sia su cavi Rutherford che CICC. La terza parte della tesi \ue8 invece focalizzata su alcuni esempi di applicazione del codice accoppiato THELMA, svolti durante l\u2019attivit\ue0 di dottorato. Nel capitolo 5 viene analizzata la caratteristica tensione-temperatura di un campione di cavo CICC, quale esempio di validazione sperimentale nella quale il codice \ue8 in grado di riprodurre fenomeni di difficile comprensione. Il capitolo 6 presenta il problema della propagazione longitudinale di un\u2019instabilit\ue0 termo-elettromagnetica in avvolgimenti impregnati di cavi Rutherford, analizzato con strumenti sperimentali, analitici e numerici. Nel capitolo 7 sono invece descritte le analisi predittive in termini di perdite e distribuzione di corrente riguardo il magnete CICC NAFASSY. Ulteriori dettagli riguardanti le propriet\ue0 dei materiali e alcuni modelli analitici e numerici sono infine riportati nelle appendici

Coupled thermal and electromagnetic analysis of the NAFASSY magnet

The paper presents an analysis of the current distribution and electromagnetic losses in the NAFASSY magnet carried out with the THELMA code, thanks to a brand-new thermal module coupled with the pre-existing electromagnetic module. The non-linear thermal and electrical properties of both superconducting and copper strands, depending on the local temperature, current density and magnetic field, are taken into account. The model analyses a single turn of the magnet, located in the highest field zone, focusing on the current distribution in the cable, the coupling AC and DC losses during ramped waveforms. The results are then extrapolated to estimate the behaviour of the overall magnet. A description of the models is given, together with a parametric analysis of different boundary conditions and cable discretizations. The analysis shows that, in nominal working conditions, no thermal instability should take place. However, local current redistribution among the strands may occur, mainly driven by the interstrand contact pattern, the local magnetic field and the strand current density

Quench Protection Study of the <formula formulatype="inline"><tex Notation="TeX">Nb3Sn\hbox{Nb}_{3}\hbox{Sn} </tex></formula> Low-<formula formulatype="inline"><tex Notation="TeX">β\beta</tex></formula> Quadrupole for the LHC Luminosity Upgrade

In the framework of the HiLumi program, the development of high field (conductor peak field 12 T) and large aperture (150 mm in diameter) superconducting quadrupoles is under way. These quadrupoles will provide the final focusing of the beam in the interaction region of the Large Hadron Collider (LHC), in the program of the luminosity upgrade. The quench protection of these magnets is a challenging aspect, mainly for the magnet dimension (8 m long), for the large value of the stored magnetic energy (12 MJ) and for the use of Nb3Sn as conductor. In this paper, the quench protection study is reported, comparing results obtained with different codes for quench analysis. The parametric analysis of the transition under different conditions for the protection scheme is also presented

Prevalence of Helicobacter pullorum in Conventional, Organic, and Free-Range Broilers and Typing of Isolates▿ †

Helicobacter pullorum represents a potential food-borne pathogen, and avian species appear to be a relevant reservoir of this organism. In this study, the prevalence of H. pullorum was investigated at 30 conventional farms where 169 ceca from 34 flocks were tested, at eight organic farms where 39 ceca from eight flocks were tested, and at seven free-range farms where 40 ceca from eight flocks were tested. All of the ceca were obtained from healthy broiler chickens. Moreover, amplified fragment length polymorphism, pulsed-field gel electrophoresis, and automated ribotyping were employed to estimate the levels of genetic variability of H. pullorum broiler isolates within and between flocks. Overall, Gram-negative, slender, curved rods, identified as H. pullorum by PCR, were isolated at 93.3% of the farms tested. The percentage of positive free-range farms (54.2%) was significantly lower than that of conventional (100%) or organic (100%) farms (P < 0.001). The level of within-flock genetic variability, calculated as the number of flocks colonized by isolates genetically different by all of the typing methods, was 34.9%. Isolates showing identical profiles by each typing method were observed in 11.6% of the flocks, but they were never detected between flocks. However, groups of isolates clustered together with an overall similarity level of ≥85%. Our results suggest that even though a high level of genetic variability is attributable to H. pullorum broiler isolates, their hierarchical genotyping produces data useful for epidemiological investigations

AC Losses Measurement of the DISCORAP Model Dipole Magnet for the SIS 300 Synchrotron at FAIR

Considerable attention has been paid in the last years to the development of fast-cycled superconducting magnets for future accelerators, leading to the design and construction by INFN and GSI of a 3.8-m-long prototype of a 4.5 T, 1 T/s, dipole magnet, for the SIS300 synchrotron of the FAIR facility (Darmstadt, Germany). This ramp-rate is 20-100 times higher than the one used in other superconducting synchrotrons like RHIC or LHC. Being operated at rather large dI/dt, these magnets are subjected to a wide spectrum of ac dissipation, taking place in the superconductor as well as in the metallic components of the mechanical structure, requiring the development of specialized superconducting cables and a careful consideration of the other aspects of the structural design. Between July and September 2012, the dipole magnet prototype has been subjected to a test at LASA laboratory (INFN Milan, Italy), during which it was successfully operated at current ramp rates as high as 0.7 T/s (the power supply limit). In this paper, we describe the V-I apparatus used to assess the dissipations within the magnet during the ac regime, the measurement results, and their comparison with the values expected from the design and on the basis of superconducting cable qualification results