SPHERICAL-GRATING MONOCHROMATOR SYSTEM WITH CIRCULAR-POLARIZATION CAPABILITY FOR THE U5U UNDULATOR AT BROOKHAVEN

We plan to upgrade the U5U undulator beamline at Brookhaven with a 4-g rating 3.5 m SGM (spherical-grating-monochromator) system offering the following enhancements in performance: (1) Improved imaging especiall y at the sample; (2) Small ''expendable'' gratings; and (3) Circular p olarizer of the 4-reflection type. Ray tracing calculations are offere d with emphasis on spot-size quality at the sample, and on the relativ e merits of scanning the exit- or entrance-slit to maintain focus. The excursion of the entrance slit coincides with the length of the undul ator image. Calculations of the figure of merit TP2 (T = transmission, P = polarization) are presented for a 4-reflection circular polarizer using the candidate mirror materials Al, Ag, Au and Rh. Ag offers the widest range of coverage, homegaBAR = 10-150 eV, with reasonable thro ughput, TP2 = 4-18%.