Precision control of gamma-ray polarization using a crossed helical undulator free-electron laser


Polarized gamma-ray beams are a precise and selective probe for studying fundamental questions about nuclear structure and hadron properties. Improvements to this probe require new experimental approaches that can produce high-flux gamma-ray beams with easily switchable pure polarization states. Here, we report an optics-free method to precisely control the polarization of a Compton gamma-ray beam. Using a free-electron laser (FEL) oscillator with two helical undulator magnets of opposite helicities, we have produced a linearly polarized FEL beam with a variable polarization direction and an unprecedented degree of linear polarization, PLin = 0.997. With this FEL as a photon drive, we are able to generate Compton gamma-ray beams having either left/right-circular polarization or rotatable linear polarization. The linearly polarized gamma-ray beam has been characterized and shows PLin = 0.97. This demonstrated polarization control technique is well suited for high-flux gamma-ray production with any level of FEL power.

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Fig. 1: Experimental set-up for the FEL and gamma-beam polarization control.
Fig. 2: Rotation of the FEL beam linear polarization by continuously varying the phase delay ΔϕBun.
Fig. 3: Gamma-beam transverse profile measured using an imaging system during one cycle rotation of the linear polarization.
Fig. 4: Measurement of gamma-ray linear polarization at 6 MeV using a photonuclear-reaction-based gamma-ray polarimeter.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


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This research was carried out at the High Intensity Gamma-ray Source facility at Triangle Universities Nuclear Laboratory (TUNL), a US Department of Energy (DOE) Centre of Excellence. Y.K.W., J.Y., H.H., P.L., S.F.M., V.G.P., M.W.A. and J.M.M. acknowledge DOE support under grant no. DE-FG02-97ER41033. M.W.A., J.M.M. and Y.K.W. acknowledge support from the US Domestic Nuclear Detection Office, Academic Research Initiative grants nos. 2010-DN-077-ARI-02 and 2008-DN-077-ARI1010. M.W.A. also acknowledges DOE support under grant no. DE-SC000536. The authors thank G. Swift for designing and building the mechanical system for FEL polarization diagnostics, P. Morcombe and P. Wallace for proofreading the manuscript, and H. Greenside and D. Straub for helping improve the abstract. The authors are also grateful for the support provided by the engineering and technical staff at Duke Free Electron Laser Laboratory/TUNL.

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V.N.L. and Y.K.W. conceived and proposed several crossed helical undulator schemes. V.N.L., N.A.V., S.F.M., Y.K.W., H.H., V.G.P. and J.L. were responsible for designing, constructing and measuring helical undulators, as well as assessing the impact of electron beam dynamics, and developing magnet power supplies and related accelerator controls for the experiment. Y.K.W., J.Y. and V.G.P. conceived, developed, installed and calibrated the high-precision FEL beam polarimeter. J.M.M., M.W.A. and M.H.S. conceived and developed the gamma-ray beam polarimeter. J.Y., Y.K.W., H.H., S.F.M., J.M.M. and M.H.S. conducted the experiments. V.N.L., N.A.V. and S.H. provided theoretical analysis and projections. J.Y., P.L., J.M.M. and Y.K.W. interpreted data and produced figures. J.Y. and Y.K.W. wrote the paper. All authors discussed the results and suggested improvements.

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Correspondence to Ying K. Wu.

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Yan, J., Mueller, J.M., Ahmed, M.W. et al. Precision control of gamma-ray polarization using a crossed helical undulator free-electron laser. Nat. Photonics 13, 629–635 (2019).

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