Abstract
Short period, high field undulators are used to produce hard x-rays on synchrotron radiation based storage ring facilities of intermediate energy and enable short wavelength free electron laser. Cryogenic permanent magnet undulators take benefit from improved magnetic properties of ${\mathrm{RE}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ (Rare Earth based magnets) at low temperatures for achieving short period, high magnetic field and high coercivity. Using ${\mathrm{Pr}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ instead of ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$, which is generally employed for undulators, avoids the limitation caused by the spin reorientation transition phenomenon, and simplifies the cooling system by allowing the working temperature of the undulator to be directly at the liquid nitrogen one (77 K). We describe here the development of a full scale (2 m), 18 mm period ${\mathrm{Pr}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ cryogenic permanent magnet undulator (U18). The design, construction and optimization, as well as magnetic measurements and shimming at low temperature are presented. The commissioning and operation of the undulator with the electron beam and spectrum measurement using the Nanoscopmium beamline at SOLEIL are also reported.
Highlights
Accelerator based x-ray sources produce nowadays very intense radiation in a broad spectral range [1,2]
Fourth generation light source generally rely on the free electron laser (FEL) process using relativistic electrons propagating in a periodic magnetic field as a gain medium
Permanent magnets and different parts of the undulator inside the vacuum chamber are equipped with 55 temperature sensors, which are fixed on the girders, LN2 tubes, modules holders, and directly on magnets, in order to measure the temperature during the cooling down and storage ring operation
Summary
Accelerator based x-ray sources produce nowadays very intense radiation in a broad spectral range [1,2]. In order to shift further the emitted radiation toward higher energies; i.e., to the hard x-ray region, the peak magnetic field of the in-vacuum undulators can be increased when operating at cryogenic temperature. The first full scale cryogenic undulator had been developed at European Synchrotron Radiation Facility (ESRF) [29] with a period length of 18 mm using a relatively low remanence Nd2Fe14B magnet grade (Br 1⁄4 1.16 T) cooled down to around 150 K, reaching a gap of 6 mm. Based on the ESRF development, Danfysik build for Diamond [31] a 17.7 mm period cryogenic undulator using high remanence Nd2Fe14B magnets cooled down to 150 K and a magnetic gap of 5 mm. We show some results of photon beam alignment using the Nanoscopium long section beam line, or precise adjustment of the undulator taper with the photon beam itself
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