Abstract
The inner surface of superconducting cavities plays a crucial role to achieve highest accelerating fields. The industrial fabrication of cavities for the European X-Ray Free Electron Laser (EXFEL) and the International Linear Collider (ILC) HiGrade Research Project allowed for an investigation of this interplay with a large sample on different cavities undergoing a standardized procedure. 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. New variables to describe the cavity surface were obtained. Two approaches using these variables and images to automatically detect defects has been implemented and tested. In addition, a decision-tree based approach of classifying defect free surfaces regarding their accelerating performance was tested and found to be physically valid.
Highlights
First attempts in automated defect recognition in superconducting radiofrequency cavities To cite this article: M
Laser (EXFEL) and the International Linear Collider (ILC) HiGrade Research Project allowed for an investigation of this interplay with a large sample on different cavities undergoing a standardized procedure
The optical inspection of the inner surface of SRF cavities is a well-established tool at many laboratories [7]
Summary
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 ILCHiGrade research program [14,15,16,17,18]. With a designed accelerating field of 23.6 MV/m for the European XFEL and an aimed average accelerating field on the order of 35 MV/m, the resolution of the optical inspection system should be on the level of 10 μm to resolve the inner surface. An algorithm was developed which enables an automated surface characterization This algorithm delivers a set of optical surface properties, which describe the inner cavity surface and allow for a framework for quality assurance of the fabrication procedures. This framework shows promising results for a better understanding of the observed limitations in defect free cavities
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