Development of figure correction system for axisymmetric x-ray mirrors

Abstract

Axisymmetric mirrors, such as ellipsoidal mirrors, are used as nanoscale focusing elements for soft X-rays. High figure accuracy is required to prevent distortion of the wavefronts of reflected X-ray beams. Although a mirror fabrication technique based on electroforming has been developed, figure correction of the inner surface of the mirror with most of the conventional machining methods is difficult. In this study, we constructed a processing system specialized for the inner surface of axisymmetric mirrors. This system is based on a fluid jet processing method. A fluid containing abrasives flows out from a very small nozzle placed inside the mirror and impinges against its inner surface. The surface layer of the mirror is locally removed with a spatial resolution of about 2 mm using this system. Abrasives used in our system contain organic resin and silica; therefore, they have a sufficient machining rate and a good dispersibility in water. We also applied X-ray ptychography to measure the 3-dimensional figure error of the mirror surface. We performed numerically controlled processing and measured the surface figure of the mirror with both a contact-type roundness measurement machine and a soft X-ray ptychographic system. The result of X-ray ptychography agreed well with the profile obtained by contact measurement.