Nuclear Instruments and Methods in Physics Research A266 (1988) 425-429 425 North-Holland, Amsterdam PERFORMANCE OF THE SURF-II HIGH-THROUGHPUT TOROIDAL GRATING MONOCHROMATOR Richard L. KURTZ, David L. EDERER, Jochen BARTH * and Roger STOCKBAUER National Bureau of Standards, Gaithersburg, MD 20899, USA The performance of the "high-flux" toroidal grating monochromator (HFTGM) at the NBS SURF-II synchrotron storage ring is assessed. Two gratings are studied: one with a ruled profile and the other having a laminar profile. The laminar profile is shown to reduce substantially the intensity of higher-order diffracted light with only a small decrease in the intensity of the first order light. The dependence of the energy resolution as a function of the area of the grating illuminated is also discussed. I. Introduction Since the introduction of the toroidal grating mono- chromator (TGM) by Madden and Ederer [1] in 1972, the implementation of numerous TGM designs has made it one of the most popular instruments at synchrotron radiation sources. The primary reason for this popular- ity is the simplicity of the optical design: a single active optical element, a single rotation required to set the photon energy, and stationary entrance and exit slits. The net result is that the light exits at a constant angle regardless of the photon energy, and the toroidal grat- ing performs both the diffraction and focussing. The high-flux TGM (HFTGM), so named because it accepts 51 mrad of horizontal orbit from the NBS SURF-II synchrotron light source, also includes movable vertical and horizontal baffles in front of the grating that allow us to select the portion of the grating that is il- luminated. This results in an easy method for trading intensity for higher resolution. In addition, these baffles reduce the intensity of light scattered from the edges of the grating. In this paper, we discuss the performance of the HFTGM installed at the SURF-II light source from two standpoints: comparison of ruled versus laminar gratings and comparison of resolution versus il- luminated area. The design of this monochromator has been described earlier by Stockbauer and Madden [2]. The following section describes the mechanical layout and the two gratings studied, the third section the measurements made and the performance obtained, and the fourth section summarizes the results. * NBS Guest Scientist. Present address: BESSY, Lentzeallee 100, D-1000 Berlin 33, Germany. 2. Description of instrumentation The simplicity of the design of the HFTGM is evidenced by the small number of essential components. No entrance slit is needed since the synchrotron light appears as a line source. The height of the electron beam orbiting in the storage ring substitutes for the slit height. While SURF-II may be run with a beam height of under 100 ~m, in these measurements the beam height was intentionally broadened to increase the beam lifetime. The size used in these measurements was 250 /~m. Between the electron beam and the grating, a pair of vertical and horizontal baffles are used to define the illuminated grating area. The distance from the tangent of the orbit to the grating is 1.59 m; the light disperses vertically, travelling 3.73 m from the grating to the exit slit. A 1 mm fixed slit was used in the measurements described here. In addition, a tungsten grid, located 12 cm in front of the exit slit is used to measure photon flux from the monochromator. This has been calibrated against an NBS photodiode and is used to normalize the data to the instantaneous photon flux. Two gratings with similar figures have been studied. The first grating was ruled in a conventional manner with a groove density of 600 l/mm. This results in a groove profile that is an asymmetric sawtooth. As dis- cussed below, the contribution of higher-order light components can be substantial with this configuration, particularly at the lower photon energies. For experi- ments such as measurements of secondary-electron yield (SEY), constant-final-state spectroscopy (CFS), and photon stimulated desorption (PSD), the corrections to the data required by the presence of these higher orders often can be substantial and produce uncertainties. The effects of the higher order light can be reduced, how- ever, by producing a grating with a laminar (flat top IlI(c). GRATING MONOCHROMATORS