A Quasi-Periodic Elliptically Polarized Undulator at the National Synchrotron Light Source II

A 2.8 m long quasi-periodic APPLE II type undulator has been commissioned at the National Synchrotron Light Source II (NSLS-II) for the Electron Spectro-Microscopy (ESM) beamline in the framework of the NEXT (NSLS-II Experimental Tools) project [1]. It provides high brilliance photon beams in circularly and linearly polarized radiation from VUV to soft X-Rays. The mechanical structure implemented to achieve the quasi-periodicity in the magnetic field profile is described together with the optimization techniques utilized to correct the undesirable phase-dependent errors. The final magnetic results are presented as well as the spectral performance of the device. Although this EPU (Elliptically Polarizing Undulator) was procured as a turn-key device, the vendor was only responsible for the mechanical frame and the control system. Sorting and assembly of the magnet modules and the magnetic field tuning Virtual Shimming and Magic Finger were performed at the NSLS-II Magnetic Measurement Lab. MAGNETIC AND MECHANICAL STRUCTURE The magnetic structure of EPU105 is configured as four Halbach arrays in four adjacent quadrants. The support structure is equipped with four translation units for the polarization control (linear, circular, and elliptical). The period length (λ) is 105 mm and the minimum magnetic gap of the device is 16 mm. The main magnet dimensions are 34 mm (H) x 34 mm (V) x 26.25 mm (L). The remanence of the NdFeB magnets is 1.25 T. The longitudinal air-gap between the magnets is 50 μm and the gap between the magnet arrays is 1 mm. The full length of the magnetic core is 2654.25 mm, excluding girder movement and the trim magnet holders. The device has two different types of terminations, both composed of one vertically magnetized full block, two vertically magnetized half blocks and four horizontally magnetized half blocks. The end section configuration block width and the space between blocks is optimized to minimize trajectory steering and field integrals. A NEG-coated vacuum chamber with current strip corrections used for dynamic field integral correction is employed [2]. ESM EPU105 is a quasi-period undulator (QPU). The Quasi-Periodicity (Q-P) is obtained by modulating the magnetic field amplitude along the length of the device. This is achieved by vertical displacement of Bmagnets (blocks magnetized longitudinally) at six specific locations. In order to reduce the magnetic field strength at those locations the standard magnet holders were replaced with special holders that displace the magnets vertically by 13 mm with respect to the mid-plane, as shown in Fig. 1. Figure 1: Magnet holders for the Q-P configuration. During the assembly process only those magnets with longitudinal magnetization are shifted in order to minimize the deleterious effects of the Q-P adjustment on the first and second magnetic field integrals. Figure 2 shows the measured magnetic field on-axis and the 3 GeV electron trajectory in the horizontal plane at the minimum gap and phase 0. Figure 2: Measured vertical magnetic field on axis (left) and horizontal trajectory (right) in the Q-P configuration. The quasi-periodicity in the magnetic field profile was introduced in order to modify the properties of the emitted photon beam a reduction in intensity and shift in energy of the higher harmonics compared to the fundamental. Because of the shift in energy the higher harmonics are no longer proportional to an integer multiple of the fundamental energy, which drastically reduces the amount of unwanted higher harmonic radiation transmitted through the monochromator in the ESM beamline. Quasi-periodic undulators were originally proposed as a method to reduce contamination from high order spectral harmonics where optical filtering is not possible or convenient [3]. -1.0 -0.5 0.0 0.5 1.0 Ve rti ca l M ag ne tic F ie ld [T ] -2000 -1000