# 1998 International Union of Crystallography Journal of Synchrotron Radiation Printed in Great Britain ± all rights reserved ISSN 0909-0495 # 1998 780 J. Synchrotron Rad. (1998). 5, 780±782 A plane-grating monochromator beamline for the PTB undulators at BESSY II F. Senf, a * U. Flechsig, a F. Eggenstein, a W. Gudat, a R. Klein, b H. Rabus b and G. Ulm b a BESSY GmbH, Lentzeallee 100, 14195 Berlin, Germany, and b Physikalisch±Technische Bundesanstalt, Abbestrasse 2-12, 10587 Berlin, Germany. E-mail: senf@exp.bessy.de (Received 4 August 1997; accepted 24 December 1997 ) At the BESSY II storage ring, the Physikalisch±Technische Bundesanstalt (PTB) will operate insertion devices dedicated to radiometric use. One branch of the appendant beamline system will be equipped with a grazing-incidence monochromator, described here. The monochromator concept is based on a plane grating operated in parallel light; therefore exact focusing is maintained for all photon energies irrespective of the angular setting at the grating. The monochromator has been optimized for small higher-order transmittance and high power throughput, as required by radiometric applications in the wide photon energy range from 20 eV to 1900 eV. Keywords: radiometry; spectral purity; monochromators. 1. Introduction The Physikalisch±Technische Bundesanstalt (PTB) will operate its own insertion devices in a straight section of the 1.7 GeV electron storage ring BESSY II. In cooperation between PTB and BESSY, three beamline branches have been designed for the radiometric use of undulator radiation: one for experiments using direct undulator radiation, another equipped with a normal-incidence monochromator and a third with a grazing-incidence mono- chromator. For radiometric applications, such as detector calibration with a cryogenic radiometer (Rabus et al. , 1997) as primary detector standard, high spectral purity (less than 0.5% of the radiant power due to stray light from all beamline elements and due to higher orders) and high radiant power (P > 10 mW at 100 mA stored electron current at modest resolution) are required (Ulm & Wende, 1995). Hence, the design targets are slightly different from those for other undulator beamlines at BESSY II, where highest energy resolution and ¯ux density are the centres of interest. In the following, the details and the performance of the grazing- incidence beamline are discussed. 2. Description of the beamline 2.1. Source The beamline will be alternately operated with a U180 (180 mm period) (Klein et al., 1998) and a U49 (49 mm period) (Bahrdt et al., 1996) undulator to cover a wide photon energy range from 20 eV to 1900 eV with maximum performance, i.e. high photon ¯ux and small transmittance of higher diffraction orders. An undulator spectrum consists of discrete harmonics of a fundamental energy with different angular distributions. The radiation in the central cone of the ®rst, third and ®fth harmonics is used as input for the monochromator. The corresponding radiant power output at harmonics with double and triple photon energy must also be considered, since they produce unwanted light in the second and third diffraction orders at the grating, e.g. the sixth and ninth harmonics of the undulator if the third harmonic is used. In most cases, it is suf®cient to consider higher- order radiation up to the third diffraction order. A special case is the third harmonic of the undulator. In the lower energy range, it produces higher-order light in the third diffraction order, whereas Figure 1 Optical layout of the grazing-incidence beamline.