Superconducting cavity material for the European XFEL

Abstract

Analysis of the strategy for superconducting cavity material procurement and quality management is done on the basis of the experience with the cavity production for the European X-ray Free Electron Laser (EXFEL) facility. An adjustment of the material specification to EXFEL requirements, procurement of material, quality control (QC), documentation, and shipment to cavity-producers have been worked-and carried out by DESY. A multistep process of qualification of the material suppliers included detailed material testing, single-and nine-cell cavity fabrication and cryogenic radiofrequency tests. Production of about 25,000 semi-finished parts of high purity niobium and niobium-titanium alloy in a period of three years has been divided finally between companies Heraeus, Tokyo Denkai, Ningxia OTIC, and PLANSEE. Large-grain (LG) material as a possible option for the EXFEL has been considered and resulted in the production of one cryogenic module consisting of seven (out of eight) LG cavities. They fulfilled the EXFEL requirements and showed even 25 to 30% higher unloaded quality factor. A possible shortage of the required quantity of LG material on the market leaded, however, to the choice of conventional fine grain material. The eddy-current scanning has been applied as an additional QC tool for the niobium sheets and contributed significantly to the material qualification and sorting. 2% of the sheets have been rejected what potentially could affect up to 1/3 of the cavities. The main imperfections and defects in the rejected sheets have been analyzed. Samples containing foreign material inclusions have been extracted from the sheets and electrochemically polished. Some inclusions remained even after 150 µm surface layer removal. Indications of foreign material inclusions have been found in the industrially fabricated and treated cavities and a deeper analysis of the defects has been performed.

Performance in the vertical test of the 832 nine-cell 1.3 GHz cavities for the European X-ray Free Electron Laser

“…Production included both mechanical fabrication and surface preparation [3,8] together with the required extensive quality assurance, quality management, and associated documentation [9]. Details about the niobium and niobium-titanium material used can be found in [4]. The Nb material for the accelerating cells was delivered in differing amounts by three companies (Tokyo Denkai, 1 Ningxia OTIC, 2 and SE Plansee 3 ).…”

“…Cavity delivery began in December 2012, ramping up to full production rate in October 2013, and then continued until the end of 2015 [3]. The niobium material was procured by DESY and supplied to the vendors [4].…”

Production of superconducting 1.3-GHz cavities for the European X-ray Free Electron Laser

“…Material inspection based on eddy current scanning of the delivered material, and supplemented by nondestructive detailed investigation like x-ray element analysis and digital microscopy, allowed to avoid diminished rf performance of cavities caused by material. The details of QC are described in [14].…”

“…(i) Foreign material inclusions have been suspected on the inner surface of several EXFEL cavities. As mentioned above, all niobium sheets for EXFEL cavities have been eddy-current scanned to exclude the presence of inclusions relevant for EXFEL performance [14]. However, inclusions can also be introduced during one of the cavity production steps, such as deep drawing, clamping for machining or during assembly for EBW.…”

“…Some aspects of the EXFEL cavity production have been presented and discussed at several conferences and workshops and can be found in the respective conference proceedings [4,[13][14][15][16][17][18][19]. The aim of this paper is to summarize all the important production issues and to discuss the adopted strategy for the cavity procurement and quality management.…”