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A compact and cost-effective hard X-ray free-electron laser driven by a high-brightness and low-energy electron beam

November 20, 2020

The authors present the first lasing results of SwissFEL, a hard X-ray free-electron laser (FEL) that recently came into operation at the Paul Scherrer Institute in Switzerland. SwissFEL is a very stable, compact and cost-effective X-ray FEL facility driven by a low-energy and ultra-low-emittance electron beam travelling through short-period undulators. It delivers stable hard X-ray FEL radiation at 1-Å wavelength with pulse energies of more than 500 μJ, pulse durations of ~30 fs (root mean square) and spectral bandwidth below the per-mil level. Using special configurations, we have produced pulses shorter than 1 fs and, in a different set-up, broadband radiation with an unprecedented bandwidth of ~2%. The extremely small emittance demonstrated at SwissFEL paves the way for even more compact and affordable hard X-ray FELs, potentially boosting the number of facilities worldwide and thereby expanding the population of the scientific community that has access to X-ray FEL radiation.

Fig. 1: Schematic of SwissFEL. The schematic is not to scale. BC, bunch compressor; TDS, transverse-deflecting structure.

In principle, the electron beam generated at SwissFEL, with its unprecedented low emittance substantially below the original design goals, could drive an ångström-wavelength FEL at even lower beam energies of ~4.5 GeV (assuming feasible undulators with periods shorter than 10 mm). Alternatively, staying at the present electron beam energy, the wavelength range of SwissFEL could be extended towards shorter wavelengths. These results pave the way to yet more compact and affordable FELs, such as the upcoming CompactLight project (, which may boost the number of such facilities, thereby expanding the accessibility of the user community to hard X-ray FEL radiation. Accordingly, further growth in X-ray FEL science as well as potential industrial or medical applications may be envisioned.

In the near future. we plan to study new approaches to produce short and two-colour pulses with enhanced tunability with respect to the methods described in the section of the paper. To this effect, we will explore the generation of short pulses by inducing a transverse chirp to the electron beam. This method will give us better control of the pulse duration, which can be changed simply by tuning the amplitude of the beam tilt. Moreover, we will produce two-colour FEL pulses with two undulator sections tuned to different radiation wavelengths and a chicane between them. This will allow for larger tunability in both time and wavelength separation between the two pulses.

At present, the second accelerator beamline of SwissFEL, dedicated to soft X-ray radiation in the wavelength range between 0.65 and 5.0 nm, is under installation and commissioning. Thanks to the two-bunch operation and a fast bunch separation system, the soft X-ray beamline will be served up to the full SwissFEL repetition rate without disturbing the hard X-ray branch. This beamline will consist of Apple-X undulators capable of providing full polarization control and transverse gradient control. This flexible undulator configuration and the installation of inter-undulator chicanes will allow for many unique operational modes, giving control over FEL properties such as peak power, pulse duration and longitudinal coherence. We intend to offer soft X-ray FEL radiation to scientific users from 2021, with two facility-supported experimental stations: one dedicated to atomic, molecular and optical physics and a second dedicated to condensed matter physics and materials investigation.

See: News & Views, Nature Photonics,
Reference: Prat, E., Abela, R., Aiba, M., Alarcon, A., Alex, J., Arbelo, Y., Arrell, C., Arsov, V., Bacellar, C., Beard, C., Beaud, P., Bettoni, S., Biffiger, R., Bopp, M., Braun, H.H., Calvi, M., Cassar, A., Celcer, T., Chergui, M., Chevtsov, P., Cirelli, C., Citterio, A., Craievich, P., Divall, M.C., Dax, A., Dehler, M., Deng, Y.P., Dietrich, A., Dijkstal, P., Dinapoli, R., Dordevic, S., Ebner, S., Engeler, D., Erny, C., Esposito, V., Ferrari, E., Flechsig, U., Follath, R., Frei, F., Ganter, R., Garvey, T., Geng, Z.Q., Gobbo, A., Gough, C., Hauff, A., Hauri, C.P., Hiller, N., Hunziker, S., Huppert, M., Ingold, G., Ischebeck, R., Janousch, M., Johnson, P.J.M., Johnson, S.L., Juranic, P., Jurcevic, M., Kaiser, M., Kalt, R., Keil, B., Kiselev, D., Kittel, C., Knopp, G., Koprek, W., Laznovsky, M., Lemke, H.T., Sancho, D.L., Lohl, F., Malyzhenkov, A., Mancini, G.F., Mankowsky, R., Marcellini, F., Marinkovic, G., Martiel, I., Marki, F., Milne, C.J., Mozzanica, A., Nass, K., Orlandi, G.L., Loch, C.O., Paraliev, M., Patterson, B., Patthey, L., Pedrini, B., Pedrozzi, M., Pradervand, C., Radi, P., Raguin, J.Y., Redford, S., Rehanek, J., Reiche, S., Rivkin, L., Romann, A., Sala, L., Sander, M., Schietinger, T., Schilcher, T., Schlott, V., Schmidt, T., Seidel, M., Stadler, M., Stingelin, L., Svetina, C., Treyer, D.M., Trisorio, A., Vicario, C., Voulot, D., Wrulich, A., Zerdane, S., and Zimoch, E. (2020). A compact and cost-effective hard X-ray free-electron laser driven by a high-brightness and low-energy electron beam. Nature Photon 14, 748-+. (10.1038/s41566-020-00712-8)

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