Abstract
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 $41\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. Most cavities satisfy the XFEL specification. Few cavities with low performance ($15--17\text{ }\text{ }\mathrm{MV}/\mathrm{m}$) 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 $\ensuremath{\mu}\mathrm{m}$ to hundreds of $\ensuremath{\mu}\mathrm{m}$ 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.
Highlights
As described in [1], approximately 50 prototype cavities have been mechanically manufactured at the industry by two vendors, treated and rf-tested at DESY
Optical inspection done on the cavity by the high resolution camera in use at DESY discovered a topographical defect of 1 mm size on the interface between the welding seam and heat-affected zone [Fig. 8(b)]
More details have been observed on the samples, separated from this area: several holes positioned mainly along a grain boundary were clearly visible in the light microscope image [Fig. 11(a)]
Summary
As described in [1], approximately 50 prototype cavities have been mechanically manufactured at the industry by two vendors, treated (partially at the factory and partially at DESY) and rf-tested at DESY. One of the cavities (Z110) was subjected to the attempt to improve performance by post-purification with titanium by annealing at 1400C for 4 hours and subsequent BCP of approximately 100 m. This did not lead to an improvement of the accelerating gradient. Cavity destruction for investigation is a rare action One of those cavities intended for FLASH (D6) with affected performance [see Fig. 1(b)] was destroyed in the past [2]. The samples were taken from cavity quench areas or areas with increased heating (hot spots) that were localized by T-mapping during vertical rf test. 3D microscopy, SEM including electron backscattered diffraction (EBSD), energy dispersive x-ray analysis (EDX), and Auger spectroscopy were used
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