Development of x-ray mirrors for XFEL sub-10 nm focusing system based on Wolter type III geometry

Focusing x-ray free-electron lasers (XFEL) allows us to study nonlinear optics within the xray region. Recently, we challenged the focusing XFELs to below 10 nm. However, the conventional multilayer Kirkpatrick-Baez(KB) mirrors require too strict alignment accuracy of the incident angle. To solve this problem, we propose advanced KB (AKB) mirrors, based on Wolter type III geometry. Because the configuration satisfies the Abbe sine condition, AKB mirrors enables a tolerance of incident angle error 1000 times greater than conventional KB mirrors. The remaining problem is how such mirrors are to be fabricated, because required shape accuracy is below 1 nm and the small radius of curvature on the mirrors makes high accuracy shape measurement difficult. In this work, we performed a mirror fabrication procedure based on a combination of a grating interferometer and a differential deposition. Experiment at BL29XUL of SPring-8 demonstrated AKB mirrors with an accuracy of λ/4 fabricated.