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.