Abstract
A 4.5-T cryogenic permanent wavelength shifter based on a flat Halbach array with 5-mm gap at 77 K has been designed. A wavelength shifter is a kind of high-field insertion device. It is composed of one high-field central pole and two relatively weak-field compensating poles arranged at up-stream and down-stream, respectively. The compensating poles are used to eliminate the deflection angle of an electron beam. A wavelength shifter can extend the critical photon energy to higher region in a synchrotron light source. In this work, the wavelength shifter consists of a PrFeB permanent magnet and vanadium permendur pole of which saturation field is about 2.2 T. The PrFeB has extraordinary performance at 77 K. Its residual flux density and intrinsic coercive field intensity are 1.67 T and 6200 kA / m, respectively. It can enhance the central field and avoid the permanent magnet blocks to become irreversibly demagnetized. The overall dimensions of the magnet array are 205 mm in height, 240 mm in length and 120 mm wide. Compared with a superconducting magnet, the cryogenic permanent wavelength shifter is more economic. The cryogenic permanent wavelength shifter is simulated by finite element method with simulation software, OPERA. It shows the magnitude of central field can attain 4.54 T, and the uniformity of the central field is less than 0.1 % in 40 mm with shimming on the central pole. The transverse angle deflection and the transverse tune shift of the electron beam with shimming on both compensating poles have been optimized to be less than 20 μrad and, respectively. An intrinsic reverse field existing in the magnetic circuit will probably partially demagnetize the PrFeB-blocks during their installation at near 295 K. The large reverse field can be facilitated by chamfering the iron poles near the strongly demagnetized regions. In addition, the temperature effect is also be considered. When the overall cryogenic permanent wavelength shifter at 77 K and 85 K, respectively, the integral field rate with respect to temperature is - 10 G．cm．K-1. And temperature gradient on transverse direction will cause a quadrupole field so that the tune shift will increase. This 4.5-T cryogenic wavelength shifter can shift the critical photon energy to 27 keV from 7.1 keV in Taiwan photon source. It will provide high flux density hard X-rays for experiments.