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 (http://www.compactlight.eu), 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.