Considerations for a soft x-ray spectromicroscopy beamline

The X-1A soft x-ray undulator^1,2 at the NSLS is the source for our experimental programs in spectromicroscopy³. We require both spatial and temporal coherence. Due to the relatively large horizontal divergence of the electron beam in the low β straight section of the x-ray storage ring, it has been possible to split the beam using a scraping mirror into two branches: X-1A used by our program and X-1B used for high resolution spectroscopy4. We are now rebuilding the X-1A beamline to provide improved resolving power and essentially linear trade-off between photon rate at the zone plate and resolving power for the soft x-ray spectromicroscopy experiments. This new beamline will exploit both additional floorspace due to the NSLS building expansion and increases in the brightness of the x-ray ring. Our beam will be further split into two separate beamlines, both of which will use toroidal mirrors to focus the source on the monochromator entrance slits horizontally and to focus on the monochromator exit slits vertically. This separation comes at no loss of coherent flux and permits low thermal loading on the optics, since we need little more than the coherent fraction of the beam at the Fresnel zone plate for microfocusing. Because of the small angular acceptance for spatially coherent illumination of the zone plates and the use of an approximately satisfied Rowland condition, our monochromators have sufficient resolving power with fixed exit arms. Experiments can then be placed near the exit slits, with spatial coherence established by the exit slit size. Resolving power will be controlled by adjusting the entrance slit alone with no change of spatial coherence. The zone plates will be overfilled to be less sensitive to beam vibration and drift.