The standard fabrication method for superconducting cavities is to press high RRR niobium sheets to form half cells, which are then joined by EBW (electron beam welding) to form cavities. If the anisotropy of the niobium sheet is too large, gaps will form when the half-cells are joined, so a sheet with low anisotropy is required. To reduce the anisotropy of the sheet, it is essential to apply cross-rolling during fabrication. In this experiment, three types of sheets were produced with different reduction rates during TSCR (Two Sep Cross Rolling). Then, the average anisotropy coefficient r ̅ and planar anisotropy Δr, the evaluation criteria of anisotropy, were compared to find a relationship between anisotropy and cross rolling condition. As a result, it was found that the Δr value was the smallest and the in-plane anisotropy was the smallest when the reduction ratio before and after cross rolling was the same. In addition, half cells of superconducting cavities were press formed using three types of niobium sheets, and the roundness of the equatorial part was measured. There was no difference among the three types.

Tokyo Denkai has been producing niobium for superconducting cavities since 1985. We have also produced niobium for L-band cavities since the beginning of their development, and have a large number of production records. In particular, more than 20,000 pieces have been delivered to TESLA based on the XFEL-007 specifications for the European XFEL, LCLS-II, LCLS-II HE, and SHINE projects. In this report, we present a statistical evaluation of measured data on the actual mechanical properties of niobium sheets in a mass production of niobium sheets based on nearly identical specifications. Specifically, histograms of hardness, RRR, and tensile testing (rolling and transverse direction) of niobium sheets were drawn to evaluate the data variability. The data for all items were normally distributed, indicating that quality was controlled. In addition, the relationship between rolling direction and all tensile test items (yield stress, maximum stress, and elongation) were examined. Positive correlations were observed for yield stress and maximum stress. I report on the quality data and statistical results of the same product over a period of more than 10 years.

Large grain niobium has a higher production yield than fine grain niobium, the price can be lowered. On the other hand, it has a significant reduction in mechanical strength. In this study, tensile tests were conducted on different RRR (RRR185 and RRR495) of large grain sliced discs and two tensile speeds (5 mm/min, 2 mm/min) with more than 50 samples under each condition. In general quality control, three times the mean plus or minus the standard deviation is set as the control value. Using this method, the mean minus 3 standard deviations was reported as the minimum strength of large grain niobium. Since the mechanical strength of large grain niobium is highly dependent on crystal orientation, single crystal round bar tensile test samples were prepared and each crystal orientation was measured before tensile testing. We were able to show that there is a strong correlation between the crystal orientation and the yield stress of niobium single crystals. The purpose of this study is to present the minimum strength of large grain niobium and to provide information to cavity researchers.