# 1998 International Union of Crystallography Journal of Synchrotron Radiation Printed in Great Britain ± all rights reserved ISSN 0909-0495 # 1998 453 J. Synchrotron Rad. (1998). 5, 453±455 Performance of a helical undulator of the UVSOR Shin-ichi Kimura, a * Masao Kamada, a Hiroyuki Hama, a Kazuhiko Kimura, b Masahito Hosaka, a Jun-ichiro Yamazaki, a X.-M. Mare  chal, c Takashi Tanaka c and Hideo Kitamura c a UVSOR Facility, Institute for Molecular Science, Okazaki 444, Japan, b Department of Structural Molecular Science, The Graduate University for Advanced Studies, Okazaki 444, Japan, and c SPring-8, Kamigori, Ako-guo, Hyogo 678-12, Japan. E-mail: kimura@ims.ac.jp (Received 4 August 1997; accepted 2 October 1997 ) A helical undulator was installed in the 0.75 GeV storage ring of the UVSOR facility of the Institute for Molecular Science. The undulator was designed to produce the fundamental of the circularly polarized undulator radiation in the energy range 2± 43 eV, and the higher harmonics with elliptical polarization in the energy range up to 300eV. Recently, the ®rst spectrum from the undulator was observed. The performance of the undulator and the obtained spectrum are reported. Keywords: helical undulators; UVSOR. 1. Introduction Optical excitations with circularly-polarized light are useful for understanding the magnetic moments of electrons. The difference spectrum between absorption for right and left circularly polar- ized light is called circular dichroism. The circular dichroism method is one of the most powerful tools for the investigation of electronic structures, not only of magnetic materials but also of non-magnetic materials. On the other hand, spin-resolved photoelectron spectroscopy can provide information about momentum and spin of electrons. The combination of circularly polarized light and a spin-resolved photoelectron analyzer allows us to observe the electronic structures of materials with selected magnetic moments. For the investigation of electronic structures of magnetic and non-magnetic materials by spin-resolved photoelectron spectro- scopy, we have constructed a beamline, BL5A, in the 0.75 GeV electron storage ring, UVSOR, of the Institute for Molecular Science. The beamline consists of a helical undulator, a high- resolution monochromator and a spin- and angle-resolved photoelectron analyzer. The monochromator, named SGM- TRAIN (spherical grating monochromator with translational and rotational assembly including a normal incidence mount), is an improved version of a constant-deviation and constant-length monochromator (Kamada et al., 1995, 1998). The spin- and angle- resolved photoelectron analyzer is of the low-energy diffuse scattering type (Takahashi et al., 1996). This apparatus has already been completed and reported elsewhere. Recently, the ®rst spectrum of circularly polarized light in the vacuum±ultra- violet region from the helical undulator was observed. In this paper the performance of the helical undulator and the observed spectrum are reported. The UVSOR helical undulator is a modi®ed version of a helical undulator, or an elliptical wiggler (Mare  chal et al., 1995), and an eight-®gure undulator (Tanaka & Kitamura, 1996), which were designed for the 8 GeV storage ring, SPring-8. Since the energy of UVSOR is about one order less than that of SPring-8, special care was taken to optimize the magnetic ®elds. The peak energy of the fundamental radiation can be swept only by changing the undulator gap while keeping the degree of the circular polarization. The same type of helical undulator as UVSOR was installed in the 0.7 GeV storage ring, HiSOR, of Hiroshima University (Hiraya et al., 1998). 2. Parameters The helical undulator has three lanes in the upper and the lower magnet arrays, shown in Fig. 1. The undulator looks like a planar undulator. However, in the helical con®guration the centre lane produces the vertical magnetic ®eld and the side lanes produce the horizontal ®eld. The vertical and horizontal magnetic ®elds were set to be almost equal to each other in an undulator gap of 30±150 mm. In the range of the undulator gap, the range of the de¯ection parameter, K, is 4.6±0.07. The fundamental emission peak is expected to cover the photon energy range 2±43 eV (Kimura et al., 1996). The parameters of the helical undulator are shown in Table 1. Figure 1 Schematic ®gure of the UVSOR helical undulator in the (a) planar and (b) helical con®gurations. Files: s:\acta-doc/el3099/el3099.3d, s:\acta-doc/el3099/el3099.sgm Paper number: S971300^EL3099 Paper type: SC N