An Innovative Cylindrical magnetron sputtering source for the deposition of HIE-ISOLDE superconducting Nb/Cu QWRs – Daniel Adrien Franco Lespinasse PhD Thesis

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.

Industrial R&D on Innovative Surface Treatments for an Ecological Descaling of “Acciaierie Valbruna” Stainless Steel Wire Rods in Replacement of the Traditional Acid Etching – Vlada Pastushenko PhD Thesis

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.

Protocol for the preparation of alumina/metal brazed joints-Master Thesis MANUEL RAMONES

Author: MANUEL RAMONES ; Type of thesis: Master thesis
Abstract: Brazing is the joining of materials through the use of heat and a filler metal with a melting temperature above 450°C but below the melting point of the metals being joined. This technique joins parts by creating a metallurgical bond between the filler metal and the surfaces of the two materials joined. There are several factors which have direct influence on the final quality of a brazed piece:
the filler metal selection, the heating technique, the atmosphere in which the process is performed, the cleaning procedure, the assembly of pieces, etc. In metal-ceramic junctions, ceramic surfaces, especially oxides, are very stable chemically, consequently, molten metals do not wet them well. There are several techniques that allow to overcome the chemical challenge. In order to develop a procedure that ensures the conditions to achieve a successful brazing process, several techniques for metal-ceramic brazing in a controlled atmosphere (vacuum) were tested. Brazing using active filler metals and direct brazing using ceramic metallization by sputtering were studied in this work. Alumina (Al2O3) was chosen as the ceramic part, whereas stainless steel 304 and Inconel were chosen as metallic components, cylindrical samples were assembled in lap configuration and were heated by IR lamps. The pieces were cut and the cross-section was studied by Scanning Electron Microscope SEM and energy-dispersive X-ray spectroscopy (EDS, EDX) searching for evidence of filler metal uniform spread and diffusion. The samples brazed using Pallabraze 850 disc of 0.01 mm of thickness for braze Inconel and Metallized alumina with 1m of Titanium and using Argon atmosphere showed the more uniform distribution and diffusion.

Study of the Electropolishing of 6GHz Niobium cavity in ionic liquids-Master Thesis OLGA MALKOVA

Author: OLGA MALKOVA ; Type of thesis: Master Thesis
Abstract: INFN – LNL Laboratories have been already working for several years on the problem of removing fluoride ions from a solution for polishing of niobium. Ionic liquids are one of the possible solutions for this problem. The first results of niobium polishing have already been obtained in ionic liquid – Choline Chloride with Urea. During polishing in the ionic liquid was obtained smooth shiny surface with a roughness comparable as obtained after polishing in classic electrolyte. Various additives such as sulfamic acid, ammonium sulfamate, can improve the condition of the surface.
Our task is to apply this knowledge to polish RF cavities. In our work we try to create new ionic liquid and improve one that was previously obtained. In our laboratory we built new system for electropolishing 6 GHz cavities considering the experience of previous years.

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Development of a new mechanical surface treatment for the internal finishing of 6 GHz superconducting cavities-Master Thesis GUOLONG YU

Author: GUOLONG YU ; Type of thesis: Master Thesis
Abstract: 6 GHz spun seamless Superconducting Radio Frequency (SRF) cavities are a very useful tool for testing alternative surface treatments in the fabrication of TESLA cavity. However, the spinning technique has also some drawbacks like contamination, surface damage in internal part due to the collapsible mandrel line. The first important step of the surface treatments is the mechanical polishing. For this purpose, a new, cheap, easy and highly efficient tumbling approach based on vibration was developed.
Before this approach was conceived, a few other methods, such as Turbula,
Centrifugal Barrel Polishing (CBP), custom Zigzag tumbler and “flower brush” have been studied and tested. But the result was not so satisfactory neither for the low erosion rate nor for the unstableness of the system nor for the complicated polishing process. At last, a vibration system with a simple structure, working stably was created after two experiments.
Another important task of the thesis is to update the optical inspection system for 6 GHz cavities. 3 stepper motors motor was added to move and rotate the cavity and realized auto focus of the miniature camera. A software was developed to achieve a full cavity photographed by one key operation using LabVIEW. A high-efficiency mechanical polishing system is generally judged by two aspects: one is whether the surface property satisfies the demand after polishing; the other is whether the erosion rate can reach and be stabilized at a high value which is comparable or greater than the existing products. The Radio Frequency (RF) test result indicates that the vibration system is feasible. The latest erosion rate 1 gram/hour i.e. removing 13 microns depth of inner surface materials per hour exceeds the performance of CBP, which is widely used in other laboratories in the world. The mechanical polishing process is elaborated and cavities that have been polished are listed. Several influencing factors on the erosion rate, such as tumbling time, media, signal and multi-cavities and plate direction are discussed at the end. A preliminary design of 1.3 GHz vibration system as the future development is provided for the next plan. Continue reading