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Free-electron lasing with compact beam-driven plasma wakefield accelerator


The possibility to accelerate electron beams to ultra-relativistic velocities over short distances by using plasma-based technology holds the potential for a revolution in the field of particle accelerators1,2,3,4. The compact nature of plasma-based accelerators would allow the realization of table-top machines capable of driving a free-electron laser (FEL)5, a formidable tool to investigate matter at the sub-atomic level by generating coherent light pulses with sub-ångström wavelengths and sub-femtosecond durations6,7. So far, however, the high-energy electron beams required to operate FELs had to be obtained through the use of conventional large-size radio-frequency (RF) accelerators, bound to a sizeable footprint as a result of their limited accelerating fields. Here we report the experimental evidence of FEL lasing by a compact (3-cm) particle-beam-driven plasma accelerator. The accelerated beams are completely characterized in the six-dimensional phase space and have high quality, comparable with state-of-the-art accelerators8. This allowed the observation of narrow-band amplified radiation in the infrared range with typical exponential growth of its intensity over six consecutive undulators. This proof-of-principle experiment represents a fundamental milestone in the use of plasma-based accelerators, contributing to the development of next-generation compact facilities for user-oriented applications9.

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Fig. 1: Experimental setup.
Fig. 2: Witness acceleration in plasma.
Fig. 3: Exponential growth of the amplified light.
Fig. 4: Spectral analysis of the amplified light.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.


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This work has been partially supported by the European Commission in the Seventh Framework Programme, grant agreement 312453-EuCARD-2, the European Union Horizon 2020 research and innovation programme, grant agreement no. 653782 (EuPRAXIA), the INFN with the GRANT73/PLADIP grant, SL_COMB2FEL and PLASMAR collaboration with the ENEA FSN-FUSPHY Division. The work of A.Z. was partially supported by the ISF foundation. We thank D. Pellegrini for the development of the high-voltage discharge pulser and F. Anelli, M. Del Franco and A. Liedl for the technical support. We also thank all the machine operators involved in the experimental run.

Author information

Authors and Affiliations



M.F., E.C., A.C., A.P. and R.P. planned the experiment. A.P. and R.P. managed the experiment, with inputs from all the co-authors. A.B. provided the plasma characterization. A.P. and A.S. managed the FEL beamline. G.C. and M.G. managed the FEL diagnostics. A.C. and V.S. managed the beam diagnostics. F.V. managed the photocathode laser system. R.P. carried out the data analysis. A.D.D. provided numerical simulations for the beam–plasma interaction. F.N., M.O. and V.P. provided numerical simulations for the FEL. R.P. and L.G. wrote the manuscript. All authors were involved in the experiment, extensively discussed the results and reviewed the manuscript.

Corresponding author

Correspondence to R. Pompili.

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The authors declare no competing interests.

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Nature thanks James Rosenzeig and the other, anonymous, reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Evolution of the Twiss parameters.

Twiss βx,y (top) and αx,y (bottom) functions downstream of the plasma stage, passing through the second PMQ triplet and the FEL beamline as computed by the dedicated algorithm. The position of the transport quadrupoles and undulators is reported with the red and blue rectangles, respectively.

Extended Data Fig. 2 Energies of the detected FEL radiation.

The total signals, collected downstream of the last undulator and coming from both driver and witness bunches, are reported on the top. The witness signals with subtracted background coming from the driver are reported on the bottom. The intensity fluctuations of the detected radiation are compared with the theoretical Γ function.

Extended Data Fig. 3 Power distribution.

Simulated output power distribution (P) versus s extracted after the sixth undulator. Statistical median in violet, first and third quartiles are reported in blue and red, respectively.

Extended Data Fig. 4 Energy scaling.

Top, analytic approximation of the plasma wakefield reported in Fig. 2d. The red (yellow) line shows the field computed without (with) the witness beam loading. Bottom, scaling of the energy chirp αi needed to minimize the witness energy spread as a function of Lc. The calculation is performed for several witness charges. The red asterisk refers to the configuration used in the current experiment. The x-axis also reports the resulting final energy.

Extended Data Fig. 5 Energy spread evolution.

a, Evolution of the energy spread as a function of the plasma acceleration length Lc for several witness charges. The solid (dashed) lines are computed assuming an initial energy chirp αi ≈ 90 GeV m−1 (αi = 0). b, Energy spread and emittance evolution evaluated for the witness beam parameters used in the experiment. The asterisks refer to the experimentally measured values.

Extended Data Fig. 6 Emittance evolution.

Normalized emittance as a function of the plasma acceleration length Lc with a transversely matched (a, σr = σeq) and unmatched (b, σr = 14 μm σeq) witness beam for several charges. The solid (dashed) lines are computed assuming an initial energy chirp αi ≈ 90 GeV m−1 (αi = 0).

Extended Data Fig. 7 Plasma-accelerated beams at EuPRAXIA.

Longitudinal phase space of both driver and witness. The dashed line shows the beam energy at the plasma entrance.

Extended Data Fig. 8 FEL lasing with the EuPRAXIA witness.

The plot reports the radiation growth along the undulator coordinate z. The inset shows the resulting radiation spectrum.

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Pompili, R., Alesini, D., Anania, M.P. et al. Free-electron lasing with compact beam-driven plasma wakefield accelerator. Nature 605, 659–662 (2022).

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