Design of a helical undulator for UVSOR

Journal of Electron Spectroscopy and Related Phenomena 80 (1996) 437-440

Design of a helical undulator for UVSOR Shin-ichi Kimura ,, M. Kamada ~, H. Hama ~, X. M. Mar6chal 4, T. Tanaka c and H. Kitamura b UVSOR Facility, Institute for Molecular Science, Okazaki 444, Japan b JAERI-RIKEN SPring-8 Project Team, The Institute of Physical and Chemical Research (RIKEN), Wako 351-01, Japan cFaculty of Engineering, Kyoto University, Kyoto 606-01, Japan

The design concept and the expected performance of a helical undulator which is to be installed in the 0.75 GeV storage ring, UVSOR, of the Institute for Molecular Science are reported. The undulator should produce perfectly circularly polarized light in the energy range of 2 - 45 eV and elliptically and linearly polarized light with energies up to 300 eV.

1. INTRODUCTION Circularly polarized light in the vacuumultraviolet region is a powerful source for the investigation of the electronic structure of materials. The light can be obtained by offaxis synchrotron radiation from bending magnets and also by insertion devices such as a helical undulator and a helical wiggler. Since the light intensity of the latter is some orders greater than that of the former, insertion devices have been installed in storage rings. We plan to install a helical undulator in 0.75 GeV electron storage ring, UVSOR, of the Institute for Molecular Science. The main purpose is to investigate the electronic structure of magnetic and non-magnetic materials by spin resolved photoelectron spectroscopy. A monochromator for the helical undulator, which is named SGM-TRAIN, has already been designed and it is currently under construction [1]. The SGM-TRAIN is an improved version of a constant-deviation and constant-length monochromator proposed by Ishiguro e t al. [2, 3]. The monochromator covers the energy range of 5 - 250 eV using two glancing incidence and one normal incidence mounts. The energy range of the light 0368-2048/96/$15.00 (c5~1996 Elsevier Science B.V, All rights rcscr~ ed PII S0368- 2048 (96) 03010-1

from the undulator is therefore required to be in the range of 5 - 250 eV. Several kinds of helical undulators and wigglers have been installed in other storage rings [4-6]. They have some disadvantages for our purpose. The main reason is the low acceleration energy of UVSOR. Hence we adopt a new type which is planned for the SPring-8 of JAERI-RIKEN at Nishi-Harima [7]. The advantage compared to other helical undulators or other helical wigglers is follows; (1) the undulator is retractable from the storage ring, (2) the peak energy of the helical undulator radiation can be changed with keeping the degree of helical polarization. The undulator for the UVSOR storage ring becomes not only a helical undulator but also a multipole planar wiggler because of the strong remanent field and the small period length of the permanent magnet. A helical optical klystron set up to produce a circularly polarized free electron laser is also possible by rearranging a part of the magnet arrays of the undulator. In the following, the design concept and the expected performance of the undulator are reported.

2. DESIGN CONCEPT OF A HELICAL

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Table 1 Parameters of the helical undulator. Number of periods Period length, ~u Total length Permanent magnet Remanent field, Br Width of magnet pole for vertical field Width of magnet pole for horizontal field Gap between upper and lower magnets Magnetic field of helical mode Magnetic field of planar mode Deflection parameter, K~. y, of helical mode Deflection parameter, K, of planar mode

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Figure 2. The expected brilliance spectra of the helical undulator for the UVSOR storage ring. is obtained below 250 eV, (3) the throughput photon flux after the post-mirror at the photon energy of 100 eV is 1012 photons/sec with the resolving power E/AE ~ 103, and (4) the length of the straight section in which the undulator is inserted is about 2.9 m and the energy of the electron beam is 0.75 GeV. The designed alignment and the dimension of permanent magnets are shown in Fig. 1. Fundamentally, the undulator has planar configuration of magnets with 21 periods. It looks like a planar undulator but each magnet array consists with three lanes. Center lane and side lanes provide vertical and horizontal magnet fields, respectively. The phase between the horizontal and the vertical fields can be changed by shifting the side lanes. Hence the direction and the degree of circular polarization can be changed. The detailed parameters of the magnet array are given in Table 1. Since the form of the undulator is like a planar one, the undulator is retractable from the storage ring. Hence the undulator can be removed when the undulator is not used.

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>- X displacement(mm) The value of ku is 0.11 m and the total length of the undulator including the orbital correction magnet is 2.3512 m. The remanent field of the permanent magnet (Br) is assumed to be 1.3 T. The expected photon energy range of the perfectly circularly polarized undulator radiation is 2 - 45 eV. The higher order radiation in the operation of the elliptic undulator radiation is obtained up to 200 eV. The expected maximum deflection parameter, K, is 8.5 in the planar configuration. The critical photon energy of the planar multipole wiggler operation is about 310 eV. The expected brilliance spectra of some operations compared with the bending radiation are shown in Fig. 2 [8]. In the helical configuration, the K~ and the Ky values can be changed without changing the ratio Kx / Ky. Here K~ and Ky are the parallel and perpendicular components, respectively, of the K of the undulator with respect to the horizontal plane. The magnetic field and the K value as a function of the magnet gap are plotted in Fig. 3. The figure shows that the decreasing rates of K~ and Ky are almost equal to each other. This means that the peak energy of the helical undulator

Figure 4. The calculated magnetic field and the electron beam trajectory in the helical configuration at magnet gap of 30 ram.

radiation can be changed with keeping the degree of helical polarization only by changing the distance between the upper and the lower permanent magnet arrays. The expected trajectory of the electron beam in the helical undulator operation is shown in Fig. 4. The electron beam runs around the z-axis because of the existence of the beam correction magnet. The center of the trajectory of the electron beam does not move even ff the magnet gap is changed. This indicates that the emission point of the undulator light source does not move at each peak energy. The undulator becomes an optical klystron for free electron laser (FEL) with circular polarization. Magnets of three periods at the center part of the undulator need to be rearranged to make a dispersive section for the optical klystron mode. The schematic figure of magnets of the optical klystron is shown in Fig. 5. The helical optical klystron is favor-

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ergy of the helical undulator can be swept by .changing only the magnet gap and (4) a circularly polarized free electron laser is also possible using a helical optical klystron set up by rearranging the magnet array of the undulator. This work is a UVSOR Special Project of the Institute for Molecular Science. One of authors (S. K.) thanks Dr. Cai for his help in writing this paper.

REFERENCES Figure 5 The magnet array of the hehcal optical klystron for circularly polarized free electron laser using the helical undulator.

able not only to obtain higher gain but also to avoid mirror degradation because of more fundamental and less higher harmonic radiation on the axis than a planar optical klystron. The expected photon energy at the magnet gap of 50 mm and an acceleration energy of 600 MeV is about 5.3 eV (~. = 235 nm). The detail is reported elsewhere [9]. It may hopefully bring wide applications for study of molecular chemistry by employing two-color experiments with synchrotron radiation and of spin states of electrons of valence band.

4. SUMMARY The design concept of a helical undulator for UVSOR 0.75 GeV storage ring was reported. The expected spectral features are as follows; (1) the helical undulator light is obtained in the energy range of 2 - 45 eV, (2) the intensity is about 400 times stronger than the bending radiation, (3) the peak en-

1. M. Kamada, K. Sakai, S. Tanaka, S. Ohara, S. Kimura, A. Hiraya, M. Hasumoto, K. Nakagawa, K. Ichikawa, K. Soda, K. Fukui, Y. Fujii and E. Ishignro, Rev. Sci, Instrum. 66 (1995) 1537. 2. E. Ishiguro, M. Suzui, J. Yamazaki, E. Nakamura, K. Sakai, O. Matsudo, N. Mizutani, K. Fukui and M. Watanabe, Rev. Sci. Instrum. 60 (1989) 2105. 3. A. Hiraya, E. Nakamura, M. Hasumoto, T. Kinoshita, K. Sakai, E. Ishiguro and M. Watanabe, Rev. Sci. Instrum. 66 (1995) 2104. 4. S. Yamamoto, H. Kawata, H. Kitamura, M. Ando, N. Saki and N. Shiotani, Phys. Rev. Lett. 62 (1989) 2672. 5. H. Onuki, N. Saito, T. Saito and M. Habu, Rev. Sci. Instrum. 60 (1989) 1838. 6. Y. Miyahara and S. Sasaki, Synchrotron Radiation News 7 (1994) 18. 7. X. M. Mar6chal, T. Tanaka and H. Kitamura, Rev. Sci. Instrum. 66 (1995) 1937. 8. The spectra were calculated using the Synchrotron Radiation Calculation Program, SPECTRA, written by Kitamura and using our original program. 9. H. Hama, Proc. Int. Conf. FEL '95 (Nucl. Instr. and Meth A).