Quantitative investigation of linear arbitrary polarization in an APPLE-II undulator.

Matthew Hand,Marco Apollonio,Kawal Sawhney,Jingtao Zhu,Sarnjeet S Dhesi,Hongchang Wang,Francesco Maccherozzi

doi:10.1107/s1600577518001960

Matthew Hand, Marco Apollonio + Show 5 more

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https://doi.org/10.1107/s1600577518001960

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Abstract

Insertion devices are utilized at synchrotron radiation facilities around the world for their capability to provide a high-brilliance X-ray beam. APPLE-II type undulators are especially important for their capacity to switch between a variety of photon beam polarization states. A high-precision soft X-ray polarimeter has been used to investigate the polarization calibration of an APPLE-II undulator (period length λu = 64 mm) installed on beamline I06 at Diamond Light Source. Systematic measurement of the beam polarization state at a range of linear arbitrary angles has been compared with the expected result for a given set of undulator gap and row phase parameters calculated from theory. Determination of the corresponding Stokes-Poincaré parameters from the measured data reveals a discrepancy between the two. The limited number of energy/polarization combinations included in the undulator calibration tables necessitates the use of interpolated values for the missing points which is expected to contribute to the discrepancy. However, by modifying the orbit of the electron beam through the undulator by at least 160 µm it has been found that for certain linear polarizations the discrepancies can be corrected. Overall, it is suggested that complete correction of the Stokes-Poincaré parameters for all linear angles would require alteration of both these aspects.

Highlights

Undulator sources are commonly used on beamlines at modern synchrotron radiation facilities owing to their capacity to deliver brilliant soft X-ray beams with variable polarization (Sasaki, 1994; Hwang & Yeh, 1999; Weiss et al, 2001)
It is unlikely that the state of the photon beam generated by a given set of undulator parameters will correspond precisely with that predicted by the theoretical calculation
Many factors related to the undulator can alter the polarization state which is observed at the endstation: inhomogeneity of the magnetic fields, small offsets in the magnet arrays, offsets in the electron beam trajectory, and off-axis alignment of the beamline acceptance

Summary

Introduction

Undulator sources are commonly used on beamlines at modern synchrotron radiation facilities owing to their capacity to deliver brilliant soft X-ray beams with variable polarization (Sasaki, 1994; Hwang & Yeh, 1999; Weiss et al, 2001). Photon energy is controlled over the range 70– 2100 eV by altering the gap between the magnet arrays, and altering the relative phase of the two magnet rows changes the polarization of the beam.

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