Development of soft X-ray flat field laminar-type holographic gratings

Koike, Masato; Sano, Kazuo*; Sasai, Hiroyuki*

The flat field spectrographs are coming into the limelight once again taking the advantages of the rapid progress of the solid state imaging devices, and the needs of the ultra-high speed simultaneous measurements reaching to femto- or atto- seconds. A typical soft X-ray spectrograph employs a mechanically ruled varied-line-spacing spherical grating. We have also developed a holographic grating for this spectrograph,and spectrographs of this type are used in a variety of research areas. However it utilizes a pseudo-flat image plane near the point of inflection of the focal curve called lemniscates. Therefore the obtainable resolving power is limited and it is difficult to reach to several thousands. Also it is still a challenging subject to extend to the multi-keV region due to the degradation of the diffraction efficiency. We describe the recent progress of the soft X-ray flat field holographic grating spectrographs, a design study of a high resolution spectrograph using a varied-line-spacing plane grating, and the possible options to extend them to the multi-keV region.