Progress of R&D on SRF cavities at DESY towards ILC performance goal

An extensive R&D programme on cavity investigationsand treatments towards ILC performance goal has beenestablished at DESY. Aims and details of the program aswell as the detailed results of the optical inspections andreplica investigations of the inner cavities surface will bereported

Second sound quench detection of dressed TESLA-shape SRF cavities

A compact detector and numerical algorithm for secondsound measurements has been developed. The detector allowsprecise 3D quench localisation within a single unit andcan be used even for cavities with mounted helium tank. Thecompact device is easily mounted and requires minimumspace. It can be used as a part of the standard cold test ofcavities. The results obtained with the new detector and a3D algorithm have been cross-checked by optical inspectionand resistor-based temperature mapping. The resolution ofthe detector is seen to be limited by the sampling rate andthe lateral extent of the quench induced heated area on theNb superconductor

Camera Control for the OBACHT system

A high resolution camera and sophisticated illumination system has been mountedin a robot that facilitates the acquisition of images of the inner surface of superconductingcavities, which otherwise are hardly accessible. The OBACHT (OpticalBench for Automated Cavity inspection with High resolution images on shortTimescales) system in use at DESY has been optimized for acquisition time andresolution. This note describes the algorithms used to recognize the characteristicwelding seam and adjust the focus

Surface study of centrifugal barrel polished 1.3 GHz Nb cavities

Superconducting radio-frequency (SRF) cavities are the key components of accelerators such as the European X-ray Free Electron Laser and the planned International Linear Collider (ILC). Centrifugal barrel polishing (CBP) is a promising technique both for repairing and improving the performance of such cavities due to ability to remove large defects effectively and obtaining mirror-like surface without chemistry. In spite of numerous attempts, the CBP-treated cavities show yet worse SRF performance than expected. The present study explores the multi-step recipe last developed at FNAL/JLab by using a coupon cavity with removable samples. It allows investigation of the interior surface after each polishing step by microscopic techniques such as laser profilometry, SEM/EDX, AFM etc. and measurement of the roughness and material removal rates at the most relevant areas. The study reveals some polishing media (e.g. Al2O3) to be embedded into the surface which causes new surface scratches in the final polishing step and * being normal conducting * most probably the worsening of the SRF performance. Additionally, a possibly detrimental shearing and deformation of the upper surface layer is observed. An improvement of the recipe is under study

Experience of LCLS-II Cavities Radial Tuning at DESY

Radial tuning (rolling) was applied to three LCLS-II cavities to prevent that their lengths exceed the technical limits. The cavities have a reduced frequency due to additional material removal during cavity treatment well beyond the baseline recipe. The mechanical condition of the cavities was relatively soft because of the thermal history and the niobium manufacture requirement of an optimal flux expulsion. The niobium was highly recrystallized by 3 hours annealing at 900°C and 975°C respectively. Each cavity received an inner surface treatment of 200 µm electro-polishing (EP) and an external 30 µm buffered chemical polishing (BCP) as part of the baseline recipe. Each cavity received an addition ~100 µm of chemical removal along with a second annealing treatment before the radial tuning process. Detailed information about the accuracy and homogeneity of LCLS-II cavities rolling is presented as well as results of field distribution analysis for TM011 zero-mode with a comparison to standard cavities

Inspection and repair techniques for the EXFEL superconducting 1.3 GHz cavities at Ettore Zanon S.p.A.

The quality control of the inner surface ofsuperconducting RF cavities is essential in order to assurehigh accelerating gradient and quality factor.Ettore Zanon S.p.A. (EZ) has implemented in the serialproduction an optical system that use an high-resolutioncamera, in order to detect various types of defects. Thissystem is added to a grinding machine, that wasspecifically designed and built to repair imperfections ofthe cavities inner surface.This inspection and repair system is applied to recoverperformance limited cavities of the 1.3 GHz EuropeanEXFEL project, where surface irregularities are detected,either by the Obacht inspection system at Desy or theoptical system at EZ.The optical system and the grinding procedure arequalified using two series cavities limited in gradient andshowing different types of surface defects. Theperformances of these cavities have been recovered toreach the specifications of the project. Until now, all theseries EXFEL cavities built by EZ, repaired with thistechnique, have shown an accelerating gradient wellabove the EXFEL goal.The paper describes the equipment installed in thegrinding machine and it analyzes the performance of thecavities that have been repaired using this system