Purification technique of 6 GHz Tesla type suoerconducting Nb cavities: A new approach for improving RF performances-Master Thesis RAM KRISHNA THAKUR

Author: RAM KRISHNA THAKUR ; Type of thesis: Master Thesis
Abstract: The lowest possible surface resistivity and higher accelerating field are the paramount considerations, hence are obligatory for accelerating cavities. Since, superconducting materials are used to make radio-frequency cavities for future accelerators. In the case of rf cavities, superconductors are being used in order to minimize the power dissipated and increase the figures of merit of a radio-frequency cavity, such as the quality factor and accelerating gradient.
Hence, these could be achieved by improving surface treatment to the cavity, and processing techniques must be analyzed in order to optimize these figures of merit.
The research work reported in this dissertation mainly carried out on tesla type seamless 6GHz Nb and Cu cavities. We have developed two innovative techniques: firstly, for mechanical polishing of cavities, and secondly for purification of these cavities at atmospheric pressure under cover of 4Helium gas (for protection) and at ultra-high vacuum (UHV) system. These cavities are
fabricated by spinning technology to create seamless cavities.
The main advantages of 6 GHz bulk-Nb cavities are saving cost, materials and time to collect statistics of surface treatments and RF test in a very short time scale. Cavities are RF tested before and after high temperature treatment under atmospheric pressure (under cover of inert gas atmosphere to protect inner and outer surface of cavity) inside transparent quartz tube, and under
UHV conditions. Induction heating method is used to anneal the cavity at temperatures higher than 2000°C and close to the melting point of Nb for less than a minute while few seconds at maximum temperature. Before RF test and UHV annealing, the surface treatment processes like tumbling, chemical, electro-chemical (such as BCP and EP), ultrasonic cleaning and high pressure rinsing (HPR) have been employed. High temperature treatment for few minutes at
atmospheric pressure allow to reduce hydrogen, oxygen and other elemental impurities, which effects on cavity Q-factor degradation, hence recovers rf performances of these cavities. This research work will address these problems and illustrate the importance of surface treatments. Continue reading

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

Protective coatings against liquid metal embrittlement-Master Thesis OLGA CHERENKOVA

Author: OLGA CHERENKOVA ; Type of thesis: Master Thesis
Abstract: A nuclear reactor is a system that contains and controls sustained nuclear chain reactions. Reactors are used for generating electricity, moving aircraft carriers and submarines, producing medical isotopes for imaging and cancer treatment, and for conducting research. Fuel, made up of heavy atoms that split when they absorb neutrons, is placed into the reactor vessel (basically a large tank) along with a small neutron source. The neutrons start a chain reaction where each atom that splits releases more neutrons that cause other atoms to split. Each time an atom splits, it releases large amounts of energy in the form of heat. The heat is carried out of the reactor by coolant, which is most commonly just plain water. The coolant heats up and goes off to a turbine to spin a generator or drive shaft. So basically, nuclear reactors are exotic heat sources.
Cyclotrons continue to be efficient accelerators for radio-isotope production. In recent years, developments in the accelerator technology have greatly increased the practical beam current in these machines while also improving the overall system reliability. These developments combined with the development of new isotopes for medicine and industry, and a retiring of older machines indicates a strong future for commercial cyclotrons. For both systems, efficient cooling is required. Nowadays liquid metals are in wide use for this purpose. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation applications. Liquid metals have safety advantages because they have high heat transfer characteristics, due to high boiling point, no high vapor pressure, and they allow a much higher power density than traditional coolants. Cooling by liquid mercury, sodium, NaK, lead has been studied and used. But due to numerous imperfections of these coolants (such as toxicity, high vapor pressure, not appropriate melting/boiling points, corrosion etc) it is necessary to develop new cooling systems for research and industrial purposes. Thus after choosing the new coolants (liquid metals with desirable properties) for target cooling we will develop the protection coatings for tubes taking into account factors which influence corrosion such as solution pH, oxidizing agent, temperature, velocity, stresses, impurity content. So we can conclude that development of new protective thin films, coatings, claddings are required and mandatory for protection. For its development we propose to use the magnetron sputtering which is a very perspective method for obtaining of thin films. One feature of magnetron sputtering which explains its wide use for the coatings is the low charged particle fluxes reaching a substrate. By this method we can obtain coatings on conductive materials as wellas on nonconductive, also on materials with low melting points. Sputtered films typically have a better adhesion on the substrate than evaporated films. Thereby the producing of protective coatings from liquid metals coolants embrittlement used for nuclear reactors and target cooling in the radiopharmaceutical sphere has been under development within the bounds of this project. Continue reading