Category: PhD Thesis

Author: Cristian Pira; Type of thesis: PhD Thesis
Abstract: A key challenge for the next accelerators is the cost reduction. Bulk niobium cavities performances are closer to their theoretical limits and an alternative technology is mandatory. Niobium thin film copper cavities are the most explored solution, but the Q-slope characteristic of these resonators limits the applications where high accelerating fields are requested. In this work an original approach is adopted in order to enhance the performances of sputtered cavities, exploring the possibility to sputter 70 micron thick films to straighten up the Q-slope in Niobium sputtered copper cavities. The engineering of Nb thick films deposition on 6 GHz copper cavities, necessary to minimize the stress on niobium thick films and avoid film peeling from the substrate, is reported. Thick films show RRR values over 60. Different strategies have been used for stress reduction: deposition at high temperature of 550 °C, deposition at the zero stress pressure point (5∙10-2 mbar), and the development of a multilayer deposition procedure. Rf tests at 4,2 K and 1,8 K on 6 GHz resonant cavities are the tool used to evaluate the influence of the critical deposition parameters on the cavity performances. Two different venting procedures have been tested: a standard one, and a high temperature one, in which pure nitrogen is inserted in the vacuum chamber immediately after the end of the sputtering process. Thick films open the possibility to post treatments on sputtered cavities: the effect of buffer chemical polishing and heat treatments at different temperature have been investigated. The Rf tests on 6 GHz cavities show the fundamental role played by the surface preparation and the possibility to enhance the maximum accelerating field minimizing the film stress. Above all, for the first time, the possibility to obtain flat curve of Q-factor versus accelerating field in sputtered cavities with thick films it is demonstrated.

Author: Hanna Skliarova; Type of thesis: PhD Thesis
Abstract: Il 99mTc è un radionuclide estremamente importante, utilizzato in più dell’80% degli esami diagnostici. Generalmente, il 99mTc viene estratto da generatori portatili contenenti il 99Mo (quest’ultimo proveniente dai reattori a fissione che utilizzano 235U altamente arricchito). Il 95% di tutta la produzione mondiale del radionuclide 99Mo è attualmente supportato da cinque impianti nucleari, alcuni dei quali hanno raggiunto più di 50 anni di attività. E sono pertanto esposti a frequenti e improvvise interruzioni che causano periodici cali nella produzione del 99Mo. Presso i Laboratori Nazionali di Legnaro dell’Istituto Nazionale di Fisica Nucleare (LNL-INFN) si è scelta la strategia basata su acceleratore, come alternativa alla via standard per avere una soluzione di riserva in caso di una nuova crisi del 99mTc. Tra le varie possibilità studiate, la migliore, e più promettente, è la produzione diretta del 99mTc attraverso l’irraggiamento, con fascio di protoni, di target di 100Mo arricchito. Un’ulteriore alternativa è la produzione nel nuclide precursore 99Mo nella stessa cornice di irraggiamento. Riguardo la produzione indiretta del 99Mo, il principale svantaggio è l’attività specifica piuttosto bassa se paragonata alla fissione nucleare nei reattori. Pertanto, la produzione del 99mTc a partire dalla reazione 100Mo(p,2n)99mTc, è stata sviluppata e valutata presso LNL-INFN; tuttavia alcune questioni devono ancora essere risolte. Il problema chiave in esame è il design e la costruzione di un target idoneo per la produzione del radionuclide e il recupero del costoso isotopo 100Mo dopo la produzione del radiofarmaco a base di 99mTc. Infatti, solo una tecnologia consolidata e a circuito chiuso può rendere competitiva la via basata sull’acceleratore con la produzione standard basata sul reattore nucleare. Per massimizzare la resa della reazione nucleare, la produzione dovrebbe essere effettuata alle massime correnti protoniche. Questo significa che il target dovrebbe fornire un’elevata efficienza di dissipazione del calore. Un sistema di target solido standard si suppone essere costituito dal materiale target depositato su un backing plate, raffreddato con liquidi. Per massimizzare la dissipazione del calore, il target dovrebbe essere costruito con un materiale avente la massima conducibilità termica, e dovrebbe essere usato un metodo che fornisca un buon contatto termo-meccanico tra il materiale target in sé e il backing plate. Al fine di consentire la massima velocità di produzione di 99mTc attraverso il ciclotrone, la tecnica di deposizione metallica deve garantire: • buona conducibilità termica del Mo; • spessore uniforme e controllato dello strato di Mo; • elevata densità del Mo (bulk-like); • basso livello di ossidazione del Mo; • buon contatto termo-meccanico con il backing plate. Le più comuni tecniche di deposizione per produrre il target di Mo, che includono la sinterizzazione, l’elettrodeposizione, laser melting, laminazione, ecc., non sono in grado di soddisfare i suddetti requisiti. Il gruppo dell’LNL-INFN ha proposto di usare il magnetron sputtering come tecnica base per depositare il 100Mo direttamente su un backing plate. Una delle principali sfide tecnologiche di questa tesi è stata quella di sviluppare un metodo per depositare film ultra spessi (centinaia di micrometri) di metalli refrattari (Nb e Mo) con la tecnica magnetron sputtering. I film depositati con il metodo proposto sono risultati densi (>98% della densità bulk) e aderenti al backing plate ad alta conducibilità termica. Il comportamento termico del sistema (100 µm di spessore di Mo depositato su un backing di rame) è stato testato sotto fascio a 15.6 MeV e 60 µA. Il film di Mo spesso 100 µm depositato con la tecnica magnetron sputtering sul backing di rame ha mostrato un’eccellente stabilità termomeccanica (nessuna delaminazione o rottura) sotto un fascio di protoni di circa 1 kW/cm2 di densità di potenza. Il rame è lontano dall’essere un materiale ideale per il backing plate, poiché si dissolve nella soluzione reattiva di H2O2 nella quale il materiale target è normalmente disciolto dopo l’irraggiamento e prima della separazione chimica, utilizzando un modulo 100Mo/99mTc. Questi svantaggi costringono ad usare moduli di separazione aggiuntivi per ottenere il prodotto finale puro, ma questi sono incompatibili con la necessità di abbreviare e automatizzare le procedure. Il concetto proposto in questa tesi include un sottile strato di materiale ceramico chimicamente inerte, per evitare impurità di radioisotopi. A questo scopo, sono stati proposti i materiali ceramici ad alta conducibilità termica, come parte del target plate. Per minimizzare l’energia persa sulla parte ceramica, è stato minimizzato il suo spessore. Inoltre, per mantenere una rigidità meccanica del sistema, e minimizzare i costi del target, i substrati ceramici sono stati brasati in vuoto a un backing plate di rame. Per realizzare questo prototipo, è stata studiata la brasatura in vuoto dei materiali ceramici con i metalli con diversi filler brasanti. Sono stati usati filler a base di rame in forma pastosa “home-made” per la preparazione del prototipo di target. Al fine di migliorare il contatto brasato, la parte ceramica è stata metallizzata con un sottile strato di Ti depositato con la tecnica magnetron sputtering. I prototipi di target sviluppati sono stati preparati depositando, mediante magnetron sputtering, 100µm di Mo su dischi ceramici che sono stati brasati al supporto metallico ad alta conducibilità termica (rame). I prototipi di target prodotti hanno mostrato buona stabilità termomeccanica sotto la massima potenza di fascio di protoni del ciclotrone GE PETtrace (60 µA, 15.6 MeV, densità di potenza di circa 1 kW/cm2). Sono stati realizzati test brevi (1 minuto) e test lunghi, più vicini alle condizioni di irraggiamento per la produzione. Per diminuire i costi per la produzione di 99mTc con il metodo basato su ciclotrone, è stato sviluppato un sistema per recuperare in forma metallica, il costoso 100Mo dagli scarti del modulo di separazione. In questo modo, il metodo proposto permette il recupero del MoO3 con un’efficienza del 94% partendo dallo scarto, arricchito di Mo, del modulo. La seguente riduzione dell’ossido di molibdeno, in un sistema chiuso in sovrappressione di idrogeno (senza flusso di idrogeno), fornisce più del 95% di Mo metallico. Lo stesso metodo di recupero può essere applicato per minimizzare le perdite del costoso 100Mo durante la deposizione via magnetron sputtering.

Author: Giorgio Keppel; Type of thesis: PhD Thesis
Abstract: Superconducting radiofrequency resonators have become standard components inside particle accelerators. This paper proposes the research and development of copper cavities internally coated with a niobium thin film, as an alternative to the niobium bulk cavities. This R&D work is part of the ISIDE experiment, of the National Institute of Nuclear Physics, and has been carried out at the Legnaro laboratories. Specifically, at the Material Science and Technologies for Nuclear Physics Service of INFN, it has been built a new system for coating, via magnetron sputtering, a thin film of superconducting niobium inside the TESLA-type 9-cell copper resonators. The work was divided into two main areas. A first phase consisted on the design of a sputtering configuration for coating superconductive cavities based on the construction of a vacuum system using a 3D CAD software and FEM simulations in order to verify the final structure of the vacuum system. Parallel to this a phase for the construction of the induction heating apparatus was followed. Afterward, the assembly of the vacuum system was performed, with connected vacuum tests, and followed by the commissioning of the induction and pumping systems. A second phase, however, focused on the study of coating configurations, particularly on the benefits and/or problems that each configuration can lead. This study ended with the design of four different coating configurations; which will be short-tested in order to check the most suitable for coating the 9-cell copper cavity with good uniformity and excellent superconducting properties. The most promising configuration is definitely the last taken into account, which involves the use of the inductor to heat the cavity during the process and the use of an innovative cylindrical magnetron with a rotating magnet pack inside that confine the plasma on the target surface. The target consists on a Niobium tube, and the study of the magnetic configuration and at the same time rotation of magnet pack permit to have a constant deposition rate along the whole cavity (high cathode erosion rate in the cavity cell regions and low cathode erosion rate along cavity cut-off). Even if the work is still evolving and the deposition tests are just starting, what as been performed is definitely innovative and original; up to now, in literature, there are no reports of coating performed by coupling the induction heating with the magnetron sputtering. The development of a plant that allows coupling these two technologies is the main aim of this work.

Author: Daniel Adrien Franco Lespinasse; Type of thesis: PhD Thesis
Abstract: In the framework of Eucard project, it has been carried out, in collaboration with CERN, the R&D on magnetron sputtering deposition on the HIE-ISOLDE cavity geometry, as an alternative method to deposit niobium thin films. In this research a new magnetron configuration source was tested at the National Institute of Nuclear Physics (INFN-LNL), in order to deposit a uniform niobium thin film onto copper superconducting Quarter Wave Resonator cavities. The methodology was divided in three. A first part, in which a test dummy cavity and a test cathode were used in order to deposit stainless steel onto copper quartz. The purpose of the use of steel has been finding the right parameters of sputtering and also to analyze the uniformity of the film. In this first phase it have been tested several magnetic confinements, which allowed the optimization of the deposition. In parallel it was performed the deposition of stainless steel onto copper strips, to realize the stripping test as a method to analyze the uniformity of the film. The second part was focused on the deposition of niobium thin film onto quartz samples placed along the resonator to improve the superconducting properties, specifically Residual Resistivity Ratio (RRR) and Critical Temperature (Tc); nevertheless other magnetic confinements were tested to maintain the uniformity of the coating. It was studied the influence on superconducting properties of two principal parameters of the sputtering process: the power and the substrate temperature. After setting the deposition parameters, a definitive magnetron confinement was used to deposit the real copper QWR. The RF performance was also measured after the design, construction and installation of a test cryostat. Finally, it was found the magnetic source to deposit a niobium thin film uniformly over QWR cavities. Increasing the substrate temperature and the sputtering power, the transition temperature of the niobium thin film was around 9,3K and it was obtained a maximum RRR of 61. Only 30 min were necessary to deposit the film with a uniformity of 2±1 μm along the cavity. SEM results allowed to analyze the microstructure of the niobium film. Bigger grains were founds on the inner conductor closer to the magnetron source. In addition a test cryostat was successfully built in order to measure the RF performance; the system can be useful to perform measurements at 4.2 and 1.8 K. Respect to the RF performance the first Nb/Cu cavity is under the specifications of CERN with a maximum Q value of 2e8 and an accelerating field of 2MV/m; however this first result is extremely important to start with the optimization phase. Some parameters will be changed in order to improve the performance and push the SRF community to use the magnetron sputtering technique as an economical method to deposit superconducting cavities in short times.

Author: Vlada Pastushenko; Type of thesis: PhD Thesis
Abstract: This work deals with an industrial research on ecological innovative descaling treatments for stainless steels, in substitution of the acid etching process: from the study and the research on samples, the most efficient techniques and their application to industrial purpose are explained. The research has basically covered the study of two pre-finishing treatments (high pressure water blasting and “dry ice”-blasting) and two etching treatments (electropolishing with ionic liquids and in aqueous media). In the field of stainless steels, the surface oxide of iron Fe (III) is combined with the multiple elements added to alloys in order to increase their characteristics (carbon, chromium, nickel and other). Moreover, the surface oxide is presented as a layer very tenacious and compact. In addition, heat treatment leads to the formation of a layer without chromium more readily attacked by oxygen from the atmosphere. Surface treatments are required to remove the surface oxide and recover the
chromium layer. The chemical pickling, that is one of the most common etching processes, depends on many factors, such as the size of the pieces, the type of plant, the type of alloy et al. In general, the traditional solutions contain from 10% to 20% by weight of nitric acid, and 1% to 5% by weight of hydrofluoric acid. The oxidizing environment is provided by nitric acid, which effectively removes the oxide surface, and is subsequently used without the hydrofluoric acid to restore the passive layer (passivation). From the environmental point of view, however, the use of this reagent is very costly:
• Air pollution: the formation of nitrogen oxides (NOx) during the process causes fumes and vapours. These gases are harmful to health, highly polluting (production of acid rain) and extremely aggressive towards metals.
• Water pollution: the high concentration of nitrates and nitrites is one of the causes of eutrophication. In particular, nitrites may form carcinogenic compounds such as nitrosamines, which can enter the food chain through fish.
• Health and safety: hydrofluoric acid is highly corrosive and a poison. It should be handled with extreme attention, using protective equipment and safety precautions. Once absorbed into the blood through the skin, it reacts with blood calcium and may cause cardiac arrest. In addition, it combines with calcium and magnesium of the bones. Since its action can be delayed for many hours, it can distribute throughout the body, causing the erosion of bones. These features have shown how the study of alternative “green” treatments is crucial.

Author: SILVIA MARIA DEAMBROSIS ; Type of thesis: PhD Thesis
Abstract: For superconducting alloys and compounds, at a given operating temperature, the best rf performances (low surface resistance and high relevant critical fi elds) are obtained for high Tc and low resistivity materials. Among the possible candidates, A15 compounds appear to be the most promising.
We needed a fast, easy and performing way to characterize A15 superconducting materials for their potential application to accelerating resonators. The idea is to build microcavities completely equal in shape to the real scale model. The rf characterization of samples is an useful diagnostic tool to accurately investigate local properties of superconducting materials. However, a common limitation of systems used for this, often consists in the di culty of scaling the measured results to the real resonator.
In this work we will proof that 6 GHz resonators can simply become our cavity shaped samples. Our attention was focused on two materials: V3Si that has a really high RRR value and Nb3Sn that is the only A15 material already used for a resonant accelerating structure.
The process parameters optimization necessary to improve the A15 phase superconducting properties, crystal structure and morphology is going on through the small sample production: this is fundamental but still not enough. Continue reading →

Author: Lanza Giulia ; Type of thesis: PhD Thesis
Abstract: Superconducting radiofrequency resonators for particle acceleration have become a standard component for particle accelerators. This work described an in-depth study and development of an alternative to the more frequently used bulk niobium cavities: niobium thin film coated into a copper cavity. The first niobium-coated copper cavity was produced at CERN in the early eighties. The sputter technology was chosen first in the pure diode configuration and subsequently in the magnetron configuration. The latter was adopted for the successful series production of the LEP and LHC cavities. In this work an intensive R&D effort has been undertaken to study the coating technique, to improve it and understand the correlation between the coating system applied and the film morphology, the superconducting properties and the RF film quality. Four different coating configurations for sputtering niobium films into 1.5 GHz copper cavities has been explored. First, the standard technique applied for several years at CERN to coat the LEP cavities has been reproduced. Then, in order to improve the Nb film quality,the application of three main ideas to the sputtering process was investigated: i) making niobium atoms imping perpendicularly to the substrate surface, ii) promoting the effect of plasma bombardment on the growing film, iii) increasing the sputtering rate. Therefore, three different and new sputtering configurations are described: the effect of Nb atoms arriving perpendicularly to the substrate was explored either by using a cathode that follows the cavity shape (Large Area Cavity Shaped Cathode) or by increasing the plasma confinement efficiency by means of a target parallel to the magnetic field lines (Ringed Shaped
Cathode). The removal of adsorbed impurities from the film surface and the increase of the film density were investigated by using a biased third electrode that promoted the positive ion bombardment of the growing film. A mixed Bias-Magnetron system was built using a positively charged metal grid which surrounded the cathode. Different film characteristics were studied and compared, focusing mainly on superconducting and resistive properties. Also morphological and microstructural properties were analyzed in a very valuable collaboration with “Interdepartmental Laboratory of Electron Microscopy” (LIME), University of Rome “Roma Tre”, at the Mechanical and Industrial Engineering Department. Four RF test on different accelerating cavities are reported and commented. In addition a 3-cell bulk niobium 1.3 GHz cavity were prepared and measured in order to compare bulk and thin film results. Even though the work is still in progress all of the partial results to-date have been viii Abstract analyzed and commented, in order to extrapolate every possible information. The final result is a global overview of the sputtering coating techniques and a of the results obtained using each system. Suggestions for future efforts have been included as part of the conclusions. Continue reading →