New record photon pulse energies at SwissFEL

The very large number of coherent photons produced by free-electron lasers is one of the key qualities of such facilities, attracting users from numerous research fields including chemistry, biology and materials science. Recently, the two branches of PSI's free-electron laser SwissFEL each have reached new record pulse energies, packing more photons than ever before into ultrashort X-ray pulses delivered at rates of 100 Hz to the users of both beamlines.

At the hard-X-ray beamline, called “Aramis”, it was possible to reach 1 mJ of pulse energy at a photon wavelength of 0.18 nm (corresponding to 7 keV photon energy). The measured pulse energy means that each pulse delivered to the users contains over one billion coherent photons (10¹²)! In a later experiment requiring the shortest wavelength nominally available at SwissFEL, 0.10 nm or 1 Angstrom (12 keV photon energy), the FEL delivered pulse energies of up to 0.78 mJ.

Still higher pulse energies are possible at longer wavelengths. At the “Athos” beamline, designed to deliver soft-X-ray pulses in the wavelength range between 0.65 and 5 nm and still under commissioning, a record pulse energy of 2 mJ was reached at a wavelength of 1.44 nm (860 eV).

These record pulse energies are the result of the continuous efforts of numerous PSI expert groups to push the performance of the technical subsystems of SwissFEL and the continuous progress with the procedures to set-up and tune the SwissFEL accelerator and FELs. In particular the enhanced stability of  laser and radiofrequency formed the basis for further optimizations towards exploiting the facility's full potential. These optimizations profit from the steadily improving experience of the people who set-up and tune the machine, the advances with programs for automated set-up and optimization, specific measures for reducing the electron beam’s energy spread, improved orbit control with beam-based alignments of the undulator lines, and the thorough tuning of undulator configurations and strength profiles.

The achieved lasing levels establish the SwissFEL beamlines well among the most powerful free-electron lasers worldwide. Much effort is now going into ensuring the reproducible delivery of such power levels to users on a regular basis.