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 $41\mathrm{MV}/\mathrm{m}$. Most cavities satisfy the XFEL specification. Few cavities with low performance ($15–17\mathrm{MV}/\mathrm{m}$) 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 $\mu \mathrm{m}$ to hundreds of $\mu \mathrm{m}$ 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.

Figure 1

(a) Cavities of XFEL prototype manufactured with Eacc below $20\mathrm{MV}/\mathrm{m}$; (b) cavities S7-S12 and D6 of the 1st generation of FLASH fabrication.

Figure 2

Separation of samples for surface investigation

Figure 3

(a) T-map for $\pi$ mode with hot spots in the equator area of cavity Z130 and away from the equator caused quench at $16\mathrm{MV}/m$; (b), (c) images of defects detected in the hot-spot areas shown in (a).

Figure 4

(a) SEM image of the foreign material inclusion; (b) EDX element analysis of the image of (a).

Figure 5

(a) SEM image of the black spot represented in Fig. 3b; (b) EDX element analysis of the black spot represented by (a).

Figure 6

(a) Area of black spots detected in the cavity Z111; (b) Auger spectrum of the black spots of (a) indicating the presence of carbon.

Figure 7

(a) Area shown in Fig. 6a, but after ion sputtering and removal of the layer of 500 nm; (b) Auger spectrum of the area shown in (a) (conventional spectrum of pure niobium).

Figure 8

(a) T-map for ($3/9$) $\pi$ mode excitation of the cavity Z130; (b) optical inspection image of the hot spot represented in (a).

Figure 9

(a) Light microscope image of the hot spot represented in Fig. 8a; (b) 3D microscope image of the hot spot represented in Fig. 8a.

Figure 10

(a) T-map of cavity Z111; (b) high resolution camera inspection of the hot spot of (a).

Figure 11

(a) Light microscope image of the hot spot represented in Fig. 10a; (b) magnified SEM image of holes represented in (a).

Figure 12

SEM and EBSD results from pit near the HAZ, sample #2. ($200\times 200\mu \mathrm{m}$, $0.5\mu \mathrm{m}$ resolution): (a) inverse pole figure (pit near [111]); (b) GROD map; (c) key for (a); d) key for (b). Maximum lattice rotation rate within pit of $3°/\mu \mathrm{m}$.

Figure 13

SEM and EBSD results from pit far from the HAZ, sample #4. ($200\times 200\mu \mathrm{m}$, $1\mu \mathrm{m}$ resolution): (a) image quality map; (b) inverse pole figure (pit near [001]); (c) GROD map; (d) key for (b); (e) key for (c). Maximum lattice rotation rate within pit of $\sim 0.2°/\mu \mathrm{m}$. Note “beach” markings.

Figure 14

Examples of images and depth profiles of pits. Pits are shallow and flat.

Figure 15

RRR in welding area (electron beam welding at pressure $2.3\times {10}^{-8}\mathrm{mbar}$).