OPTICAL LAYOUT AND RESOLUTION OF THE ASTRID PLANE FOLDING MIRROR SPHERICAL GRATING MONOCHROMATOR

We present the optical layout, the theoretically expected and experime ntally obtained resolution of a plane, folding mirror spherical gratin g monochromator at the ASTRID storage ring in Denmark. The monochromat or has a plane mirror in front of the,orating and uses a variable incl uded angle on the grating to obtain a zero defocus term. The advantage s of this design are a fixed exit slit position, which makes it easier to refocus behind the exit slit, and the possibility to optimize the resolution for the energy regions, using different ranges of included angles. The maximum resolving power E/Delta E for 10 mu m slits with f ull vertical acceptance increases from about 5700 for the high-energy grating to about 14000 for the low-energy grating. The resolution has been tested using absorption features of helium, argon and nitrogen. A resolution of 68 meV was obtained at the N-2 K edge around 400 eV. At the He double resonances at 65 eV the 17+ structure was observed. In second order, at the He 7+ double resonance, a resolution of 1.4 meV w as obtained. The resolving power E/Delta E at this energy (32 eV) is a round 23000.