New optical setup for the generation of variable spot size on third generation synchrotron beamlines

In third generation synchrotron radiation beamlines, a focussed X-ray beam is often employed. However, in some cases users need to modify their spot size in order to match the broad range of samples sizes. This is the case for XPEEM microscopy beamline which need a homogeneous beam with a spot size varying from 2 to 50 μm. These specifications requires that the beamline works out of focus, and in this case the spot becomes non homogenous (as already observed experimentally on several synchrotron beamlines). In this paper, we will explain, using a geometrical approach, that this effect on the spot is produced by mirrors slope errors. We propose a new optical solution that overcomes these difficulties. Our optical solution has been validated experimentally on the second branch of the Nanospectroscopy beamline at Elettra, where we have obtained homogenous spot sizes of 10, 20 and 30 μm with the same optical design.