Defect detection inside superconducting 1.3 GHz cavities by means of x-ray fluorescence spectroscopy

X-ray fluorescence probe for detection of foreign material inclusions on the inner surface of superconducting cavities has been developed and tested. The setup detects trace element content such as a few micrograms of impurities responsible for thermal breakdown phenomena limiting the cavity performance. The setup has been customized for the geometry of 1.3 GHz TESLA-type niobium cavities and focuses on the surface of equator area at around 103 mm from the centre axis of the cavities with around 20 mm detection spot. More precise localization of inclusions can be reconstructed by means of angular or lateral displacement of the cavity. Preliminary tests confirmed a very low detection limit for elements laying in the high efficiency spectrum zone (from 5 to 10 keV), and a high angular resolution allowing an accurate localization of defects within the equator surface

Development of an X-Ray Fluorescence Probe for Inner Cavity Inspection

The ability to detect performance-limiting defects onthe inner surface of superconducting radiofrequency(SRF) niobium cavities, which lead to low quality factorQ0 factor, thermal breakdowns (especially at the equatorwelding seams and the surrounding area), and X-rayradiation (mainly due to sharp geometric defects on theirises) provides a tool of quality control (QC) and failurereason clarification. Detection of failures and defects,especially in early production steps, would significantlyreduce repetition of quite expensive cryogenic RF testsand retreatments of the cavities. Inspection of the innercavity surface by an optical system is an inexpensive anduseful means for surface control and identification ofdangerous or suspicious features [1, 2]. It does notprovide, however, information about material content inthe defect region, which is required for sorting out thecavities with foreign inclusions and for the localisation ofa contamination source in the production cycle.Preliminary diagnostic is usually performed during theQC of niobium sheets resorting to several non-destructivetechniques, e.g. eddy current scanning [3].X-ray fluorescence (XRF) analysis is widely used forelemental and chemical analyses, particularly in theinvestigation of metals. This technique, already employedduring the QC of niobium sheets [4], appears to beentitled for development of a diagnostic tool for thedetection of trace element inclusions on the cavitysurface. Preliminary feasibility tests [5] performed with alow-performance XRF setup, demonstrated that lowamounts (some g) of different metals could be easilydetected when embedded in the niobium matrix. Theseencouraging results have been the first step towards thedevelopment of an XRF tool for the QC of the innersurface of 1.3 GHz SRF cavities. The complicated shape of the cavities and hidden inner surface require, however,development of a special device

An x-ray fluorescence probe for defect detection in superconducting 1.3 GHz cavities

The aim of this project is to develop a system for defect detection by means of X-ray fluorescence (XRF) analysis. XRF is a high sensitivity spectroscopy technique allowing the detection of trace element content, such as the few microgram impurities, responsible for low cavity performances if embedded in the equatorial region during cavity manufacturing. The proposed setup is customized on 1.3 GHz TESLA-type niobium cavities: both the detector and the X-ray excitation source are miniaturized so to allow the probe to enter within the 70 mm iris diameter and aside of the HOM couplers. The detection-excitation geometry is focused on cavity cell equator surface located at about 103 mm from the cavity axis, with an intrinsic spot-size of about 10 mm. The measuring head will be settled on a high angular resolution optical inspection system at DESY, exploiting the experience of OBACHT. Defect position is obtained by means of angular inner surface scanning. Quantitative determination of defect content can be carried out by means of the so called fundamental parameters technique with a Niobium standard calibration

An x-ray fluorescence probe for defect detection in superconducting 1.3 GHz cavities

The aim of this project is to develop a system for defect detection by means of X-ray fluorescence (XRF) analysis. XRF is a high sensitivity spectroscopy technique allowing the detection of trace element content, such as the few microgram impurities, responsible for low cavity performances if embedded in the equatorial region during cavity manufacturing. The proposed setup is customized on 1.3 GHz TESLA-type niobium cavities: both the detector and the X-ray excitation source are miniaturized so to allow the probe to enter within the 70 mm iris diameter and aside of the HOM couplers. The detection-excitation geometry is focused on cavity cell equator surface located at about 103 mm from the cavity axis, with an intrinsic spot-size of about 10 mm. The measuring head will be settled on a high angular resolution optical inspection system at DESY, exploiting the experience of OBACHT. Defect position is obtained by means of angular inner surface scanning. Quantitative determination of defect content can be carried out by means of the so called fundamental parameters technique with a Niobium standard calibration

Production of Superconducting 1.3-GHz Cavities for the European X-Ray Free Electron Laser

The production of over 800 1.3-GHz superconducting (SC) cavities for the European X-ray Free Electron Laser (EXFEL), the largest in the history of cavity fabrication, has now been successfully completed. In the past, manufacturing of SC resonators was only partly industrialized; the main challenge for the EXFEL production was transferring the high-performance surface treatment to industry. The production was shared by the two companies RI Research Instruments GmbH (RI) and Ettore Zanon S.p.A. (EZ) on the principle of “build to print”. DESY provided the high-purity niobium and NbTi for the resonators. Conformity with the European Pressure Equipment Directive (PED) was developed together with the contracted notified body TUEV NORD. New or upgraded infrastructure has been established at both companies. Series production and delivery of fully-equipped cavities ready for cold rf testing was started in December 2012, and finished in December 2015. More than half the cavities delivered to DESY as specified (referred to “as received”) fulfilled the EXFEL specification. Further improvement of low-performing cavities was achieved by supplementary surface treatment at DESY or at the companies. The final achieved average gradient exceeded the EXFEL specification by approximately 25%. In the following paper, experience with the 1.3-GHz cavity production for EXFEL is reported and the main lessons learned are discussed