Optical surface properties and their RF limitations of European XFEL cavities

Abstract

The inner surface of superconducting cavities plays a crucial role to achieve highest accelerating fields and low losses. The industrial fabrication of cavities for the European X-Ray Free Electron Laser (XFEL) and the International Linear Collider (ILC) HiGrade Research Project allowed for an investigation of this interplay. For the serial inspection of the inner surface, the optical inspection robot OBACHT was constructed and to analyze the large amount of data, represented in the images of the inner surface, an image processing and analysis code was developed and new variables to describe the cavity surface were obtained. This quantitative analysis identified vendor specific surface properties which allow to perform a quality control and assurance during the production. In addition, a strong negative correlation of ρ = −0.93 with a significance of 6 σ of the integrated grain boundary area A versus the maximal achievable accelerating field E acc,max has been found.

First attempts in automated defect recognition in superconducting radio-frequency cavities

“…In the scope of the ILC HiGrade Research Project and the European XFEL cavity fabrication, a new framework has been developed which enables this automated analysis of a large amount of images of the inner surface of cavities [33]. First applications of the newly developed framework were investigation of optical surface properties of the two cavity vendors for the European XFEL and significant differences in the quantitative characterization have been identified and a standard for a cavity surface has been established [18]. The next steps, and first results as shown in this work, are to detect defects on an irregular surface automatically and classify their potential limitations as well as to classify the "goodness" of a defect free cavity surface.…”

“…With the optical inspection system OBACHT, a manual analysis was done during the European XFEL production [39] and a classification of types of defects was done. But even cavities without a localized defect can quench at lower fields and first attempts in correlating global surface properties and not localized defects with performance parameters were done [18]. Here, an automated approach of a defect-free surface classification will be presented and is based on a classification tree [40].…”

“…Two defects squares (two yellow squares on the left) have been identified, five other squares are false positives (shown as the 5 yellow squares on the right side of the welding seam). limiting cells in cavities and are defect free [18]. The results is shown in a confusion table in Table 3.…”

“…A misclassification of (35 ± 7) % is expected. The decision tree was then applied to a another set of 17 cells produced within the European XFEL fabrication and which were found to be the 12/19 limiting cells in cavities and are defect free [18]. The results is shown in a confusion table in Table 3.…”

“…In addition, a correlation of surface structures of defect free cavities and their RF performance was done. During the construction of the European XFEL [13], more than 100 TESLA cavities underwent subsequent surface treatments, acceptance tests at 2 K, and optical inspections within the ILC-HiGrade research program [14][15][16][17][18]. The optical inspection of the inner surface of SRF cavities is a well-established tool at many laboratories [7].…”