The use of particle accelerators as photon sources has enabled advances in science and technology1. Currently the workhorses of such sources are storage-ring-based synchrotron radiation facilities2,3,4 and linear-accelerator-based free-electron lasers5,6,7,8,9,10,11,12,13,14. Synchrotron radiation facilities deliver photons with high repetition rates but relatively low power, owing to their temporally incoherent nature. Free-electron lasers produce radiation with high peak brightness, but their repetition rate is limited by the driving sources. The steady-state microbunching15,16,17,18,19,20,21,22 (SSMB) mechanism has been proposed to generate high-repetition, high-power radiation at wavelengths ranging from the terahertz scale to the extreme ultraviolet. This is accomplished by using microbunching-enabled multiparticle coherent enhancement of the radiation in an electron storage ring on a steady-state turn-by-turn basis. A crucial step in unveiling the potential of SSMB as a future photon source is the demonstration of its mechanism in a real machine. Here we report an experimental demonstration of the SSMB mechanism. We show that electron bunches stored in a quasi-isochronous ring can yield sub-micrometre microbunching and coherent radiation, one complete revolution after energy modulation induced by a 1,064-nanometre-wavelength laser. Our results verify that the optical phases of electrons can be correlated turn by turn at a precision of sub-laser wavelengths. On the basis of this phase correlation, we expect that SSMB will be realized by applying a phase-locked laser that interacts with the electrons turn by turn. This demonstration represents a milestone towards the implementation of an SSMB-based high-repetition, high-power photon source.
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The raw data from this experiment are available at https://doi.org/10.5061/dryad.r7sqv9s9f.
The computer codes used for the data analysis are available at https://doi.org/10.5061/dryad.r7sqv9s9f.
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This work is partially supported by the Tsinghua University Initiative Scientific Research Program number 20191081195, China. We appreciate the continuous support of A. Jankowiak (HZB) and M. Richter (PTB), which made the experiment possible.
The authors declare no competing interests.
Peer review information Nature thanks Jie Gao and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.
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Extended data figures and tables
The magnet lattice and the key are shown at the top. The curves are the model horizontal (red) and vertical (blue) β functions and the horizontal dispersion Dx (green). Operating parameters of the ring: beam energy, E0 = 250 MeV; relative energy spread, σδ = 1.8 × 10−4 (model); horizontal emittance, ϵx = 31 nm (model); horizontal betatron tune, νx = 3.18 (model and measured); vertical betatron tune, νy = 2.23 (model and measured); horizontal chromaticity, ξx = −0.5 (measured).
a, Temporal profiles of two example consecutive laser shots (red and blue) and the averaged waveform of 200 consecutive laser shots (black). b, Statistical distribution of the laser power at t = 0 ns in a for 10,000 consecutive laser shots, where the red curve is a gamma distribution fit. Laser: compact Nd:YAG Q-switched laser (Beamtech Optronics Dawa-200). Detector: ultrafast photodetectors (Alphas UPS-40-UVIR-D; rise time < 40 ps). Measurement system: digital oscilloscope (Teledyne LeCroy WM825Zi-B; bandwidth 25 GHz; sample rate 80 billion samples per second).
Blue dots are the measurement results with the systematic offset subtracted and the red curve is a fit by the sum of two exponential functions, Q(t) = Q1exp(−t/τ1) + Q2exp(−t/τ2), performed at different time intervals, with the fit results connected by a smoothed line.
Extended Data Fig. 4 Linear dependence of the broadband incoherent undulator radiation on the bunch charge.
a, Results corresponding to individual laser shots; the shading (light red) represents 3σ of the detection noise. b, The result after 200-consecutive-laser-shot averaging. The blue dots are the experimental data of a bunch not modulated by the laser and the red curves are linear fits.
Extended Data Fig. 5 Quadratic dependence of the narrowband coherent undulator radiation generated from microbunching on the bunch charge.
a, Results corresponding to individual laser shots; the shading (light red and grey) represents 3σ of the detection noise. b, The result after 200-consecutive-laser-shot averaging; the plot is the same as Fig. 3 and is presented again here for comparison with a and with the incoherent signal in Extended Data Fig. 4. The blue dots represent the experimental data and the red curves are quadratic fits.
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Deng, X., Chao, A., Feikes, J. et al. Experimental demonstration of the mechanism of steady-state microbunching. Nature 590, 576–579 (2021). https://doi.org/10.1038/s41586-021-03203-0
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