A Fixed Angle Double Mirror Filter for Preparing a Pink Undulator Beam at the Advanced Photon Source

Recent advances in X-ray Photon Correlation Spectroscopy (XPCS) use the full bandwidth of an undulator harmonic in order to maximize the coherent flux for small angle X-ray scattering experiments. X-ray mirrors and filters are typically used to select a given harmonic of the spectrum. At the University of Michigan/Howard University/Lucent Technologies, Bell Labs, Collaborative Access Team (MHATT-CAT) undulator beamline of the Advanced Photon Source, we have designed a fixed-angle Double Mirror Filter which will provide a "pink beam" (i.e., 2-3 % bandwidth) for XPCS experiments. This device uses two small mirrors which vertically reflect a 0.1 mm x 0.1 mm white beam in a symmetric geometry. The doubly reflected beam propagates parallel to the incident white beam, but is offset vertically by 35 mm. Using the standard offset of the APS allows one to stop the white beam with a standard APS beam stop. In this report, we will describe our design considerations for this instrument. We also report the results of preliminary tests of the performance. The mirrors preserve the transverse coherence of the source, and filter the undulator spectrum as expected. X-ray Photon Correlation Spectroscopy (XPCS) can often benefit from using the full bandwidth of an undulator harmonic in order to maximize the coherent flux for small-angle X-ray scattering (SAXS) experiments [1]. In a typical set up, a combination of vertical and/or horizontal grazing incidence mirrors is used to filter out the high energies of the undulator spectrum. This provides a "pink" beam, i.e. one whose energy and bandwidth are essentially those of the undulator fundamental. Optimum operation involves tuning the mirror reflection angle so as to place the mirror cut-off energy just above the fundamental energy This typically places the mirror reflection angle between 5 to 15 mrad. It is also desirable to have a substantial offset between the white beam and the pink beam to facilitate shielding from the white beam high energy radiation. The standard monochromatic beam vertical offset at the Advanced Photon Source (APS) is 35 mm. A combination of two vertical reflections serves to preserve the beam direction, which obviates the need for an articulated beamline. Together, these requirements dictate a large CP521, Synchrotron Radiation Instrumentation: Eleventh US National Conference, edited by P. Pianetta, et al. © 2000 American Institute of Physics l-56396-941-6/00/$17.00