TESLA Report 2003-07 RRR Degradation and Gas Absorption in the Electron Beam Welding Area of High Purity

Abstract. Experiences concerning RRR degradation and gas absorption of high purity niobium welded at different electron beam (EB) facilities are summarized. The oxygen and nitrogen content is increased as a rule at the welding seam. The absorption of hydrogen takes place not directly at the welding seam but in the heat-affected area. The RRR degradation is moderate, if the total pressure is better than 5×10-5 mbar and the preparation is done under clean condition. A series of welding was done in the new DESY EB facility, which was specifically designed to meet the requirements for the welding of niobium cavities for TESLA. High vacuum (between 2×10-8 and 2×10-6 mbar) allowed improving the niobium purity at the welding seam, in which the RRR were increased from 350 to ca. 370−380. The distribution of RRR, gases, grain size and thermal conductivity in the welding area are analyzed. The repair of a burned hole in the cavity is also discussed. I

Surface investigation on prototype cavities for the European X-ray Free Electron Laser

The accelerating gradient Eacc of X-ray Free Electron Laser (XFEL) prototype cavities manufactured at the industry and treated at DESY demonstrates wide-range scattering from 15 to 41  MV/m. Most cavities satisfy the XFEL specification. Few cavities with low performance (15–17  MV/m) are limited by quench without field emission. The T-map analysis detected quench areas mainly close to the equator. Optical control by a high resolution camera has been applied and allowed to monitor the defects in some cases with good correlation to T-map observation. In order to understand the cause of reduced performance and get more detailed information of the origin of defects, some samples have been extracted from two cavities and investigated by light microscope, digital light microscope with 3D profile measurement, scanning electron microscope SEM, energy dispersive x-ray analysis, and Auger spectroscopy. The electron backscattered diffraction method in a SEM is used to make localized measurements of the lattice curvature. Several surface flaws with sizes from a few μm to hundreds of μm detected by microscopy. The defects can be separated into two categories. The first category of defects consists of foreign elements (often an increased content of carbon). Inclusions with increased content of carbon adhere on the surface and presumably have a hydrocarbon nature. Deviation from a smooth surface profile characterizes the second type of defects (holes, bumps, and pits). Some holes and bumps were found directly in the welding seam. The hot spots in the heat-affected zone (HAZ) of the equator welds have been partially associated with pits too. The study correlates the location of pits with the presence of plastic strain found to remain after welding. Pits near the HAZ were found either coincident with or near areas of high strain. Pits away from the weld were often found at grain boundary triple junctions