Polarization control of a free-electron laser oscillator using helical undulators of opposite helicities

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Polarization control of a free-electron laser oscillator using helical undulators of opposite helicities

Jun Yan, Hao Hao, Senlin Huang, Jingyi Li, Vladimir N. Litvinenko, Peifan Liu, Stepan F. Mikhailov, Victor G. Popov, Gary Swift, Nikolay A. Vinokurov, and Ying K. Wu

Phys. Rev. Accel. Beams 23, 060702 – Published 29 June 2020

Abstract

Polarized photon beams provide a unique experimental tool for the study of various polarization-dependent physical processes. Here, we report the experimental demonstration of full polarization control of an oscillator free-electron laser (FEL) using helical undulators of opposite helicities. Using two helical undulator magnets of opposite helicities and a buncher magnet in between, we have generated a linearly polarized FEL beam with any desirable polarization direction. With the development of a high-precision FEL polarimeter, we are able to optimize the highly polarized FEL beams in visible wavelengths and measure the polarization with high accuracy, demonstrating linear polarization $P_{lin} > 0.99$ on the routine basis and with the maximum polarization reaching $P_{lin} = 0.998$ . In this paper, we describe the FEL configuration, experimental setup, and related beam diagnostics, including the newly developed high-precision FEL polarimeter. We report our experimental approaches to generate, tune up, and characterize the polarization controllable FEL beams and share a new insight into how high-degree polarization is realized based upon our investigation of the temporal structure of the FEL beam. This FEL polarization control technique has been used successfully to generate a polarization controllable Compton $γ$ -ray beam for nuclear physics experiments.

Received 8 April 2020
Accepted 18 June 2020

DOI:https://doi.org/10.1103/PhysRevAccelBeams.23.060702

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Polarization of light

Synchrotron radiation & free-electron lasers

Accelerators & Beams

Authors & Affiliations

Jun Yan ^1,2,*, Hao Hao^1,2, Senlin Huang ³, Jingyi Li⁴, Vladimir N. Litvinenko ⁵, Peifan Liu ^1,2, Stepan F. Mikhailov^1,2, Victor G. Popov ^1,2, Gary Swift^1,2, Nikolay A. Vinokurov ^6,7, and Ying K. Wu ^1,2,†

¹Department of Physics, Duke University, Durham, North Carolina 27708, USA
²Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA
³Institute of Heavy Ion Physics, School of Physics, Peking University, Beijing 100871, China
⁴Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
⁵Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA
⁶Budker Institute of Nuclear Physics SB RAS, Novosibirsk 630090, Russia
⁷Novosibirsk State University, Novosibirsk 630090, Russia

^*junyan@fel.duke.edu
^†wu@fel.duke.edu

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Issue

Vol. 23, Iss. 6 — June 2020

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Images

Figure 1
Undulator configuration for the polarization control of FEL beams. The two helical undulators in the middle of the FEL straight section (OK-5B and OK-5C) are turned on and operated in opposite helicities. Buncher B is used as a phase shifter.
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Figure 2
(a) Layout of the prototype FEL polarimeter with two nonpolarizing beam splitters (BS1 and BS2), a focusing lens (F), a linear polarizer (P), a commercial polarimeter, and a photodiode (PD). (b) Polarization angle measured in arm B as a function of the polarization angle of an incoming linearly polarized beam at 548 nm.
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Figure 3
(a) Layout of the new FEL polarimeter. After the FEL beam passes through a nonpolarizing beam splitter (BS1), it is split into two arms by a second nonpolarizing beam splitter (BS2), each comprised of a focusing lens, a Glan Thompson beam splitter, and two photodiodes. (b) Measured degree of linear polarization $P_{lin}$ in different operational modes when the incoming beam has nearly perfect linear polarization as it is prepared using a high-quality linear polarizer. The inset shows the deviation from a perfect linear polarization, $Δ P_{lin} = 1 - P_{lin}$ .
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Figure 4
Degree of FEL beam linear polarization $P_{lin}$ measured using (a) the prototype and (b) the new FEL polarimeter. The polarization angle is rotated by tuning buncher setting $N_{B}$ . The insets show the measured FEL polarization ellipses. All measurements are indexed by a number from 1 to 17. In (a), the storage ring is operated with a single-bunch electron beam at an energy of 550 MeV and beam current of about 45 mA. The FEL is lasing at about 547.8 nm with an FEL detuning of about $7 \times 10^{- 7}$ . In (b), a 533 MeV single-bunch electron beam is used with a beam current of about 30 mA. The FEL is operated with zero detuning. The lower part of (b) shows the measured spectra of the FEL beam with their maximum intensity scaled to unity. The measured FEL central wavelength is $547.94 \pm 0.093 nm$ , and the measured average rms spectral width is 0.25 nm with an rms variation of 0.066 nm. Note that the dataset shown in (b) is different from the one reported in Ref. [43].
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Figure 5
FEL macropulses of the two orthogonal circular polarization components decomposed from an FEL beam with good average horizontal polarization. (a) FEL intensity measured in the time domain over one second. The inset is an enlarged plot, showing six consecutive FEL macropulses. (b) Relationship between the pulse difference $D_{i}$ and the pulse height for 455 macropulses collected from four one-second measurements (gray dots). The pulse height is normalized in a way to equalize the average intensity of the two macropulse signals. The data sorted into 13 bins using the pulse height are analyzed to show the averaged pulse difference (blue lines) and rms spread (red rectangles). For this measurement, the electron beam energy is 533 MeV and beam current is 33 mA in the single bunch. The FEL is operated at 544 nm with close to zero FEL detuning.
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