Material Investigation For High Gradient RF Superconducting Cavities And Power Couplers

TESLA international collaboration R&D program has been dramatically successful at pushing the gradient of bulk niobium superconducting cavities from the non competitive level around 10MV/m to the level required for linear collider application. The best 9-cell cavity AC71 has reached the highest accelerating gradient over 40MV/m. To the progress of the cavity performance, many technical aspects have made contributions, among which strict material and welding quality control plays a very important role. This thesis is devoted to the investigations of the high purity niobium for high gradient superconducting cavity and copper plated steel for the power coupler. For material investigation, a series of test systems were developed, including a computer controlled cryogenic thermal conductivity measurement apparatus based on a transportable storage dewar, two kinds of RRR measurement methods for samples and an eddy current RRR measurement system for cavity in-situ material property control, eddy current scanning system and SQUID scanning system, tensile test device and gases analysis instruments. Based on the above-mentioned test systems, basic properties of high purity niobium such as thermal conductivity, RRR, microstructures, mechanical properties, foreign inclusions and heat treatments influence on the properties of copper plated steel were investigated. Eddy current RRR measurement system made it available to record the RRR data for over 100 TTF cavities. The investigation shows that for BCP polished cavities the relationship E_acc ∝RRR^1/2 was statistically observed, while for EP polished cavities E_acc seems to be independent on RRR when RRR>200. It suggests that E-polishing probably produces defect-free surface. High RRR and E-polishing has played the main role on pushing the limit of RF superconductivity. The quench driven by local defects, impurity contents and geometrical consideration can be greatly improved or avoided by post purification or e-polishing. The unique DESY electron beam welding facility with the chamber of ultra-high vacuum ( 10^-6 mbar -10^-8mbar), made it available to investigate the behaviour of high purity niobium electron beam welded at ultra high vacuum. The RRR degradation and gases concentration at heat affected area were observed, even at the pressure of 10^-8mbar. At welding seam, RRR improvement at the pressure < 5×10^-6 mbar was seen, but the vacuum higher than 10^-7mbar makes no more contribution. The heat affected area near the overlapped place shows more dangerous due to the high gas concentration. These risks may finally be avoided by the fabrication technology of seamless cavity. Some contamination cases of high purity niobium such as machining, CBP polishing, BCP and EP and homogeneity of high purity niobium were also studied.