The elusive 1022 W cm–2 milestone has been reached by focusing hard X-rays down to transverse dimensions of ~7 nm x 7 nm at the SACLA X-ray free-electron laser (XFEL) in Japan, enabling a two-order increase in the intensity of photons. The findings, published in Nature Photonics, demonstrate exquisite control to achieve intense, short-wavelength electromagnetic radiation that can be used to probe atomic, molecular, and optical physics with unprecedented resolution.

Credit: David Pile

Meanwhile, in California, terawatt-scale, sub-femtosecond, soft X-ray pulses have been demonstrated at the Linac Coherent Light Source (LCLS) XFEL. These findings are also published in Nature Photonics, along with another LCLS effort in which researchers demonstrated the generation and control of sub-femtosecond pulse pairs from a two-colour XFEL set-up and conducted pump-probe experiments in core-ionized molecules.

LCLS is seeing a slew of upgrades, as noted by LCLS director Mike Dunne in a Q&A published in Nature Photonics. LCLS-II, which achieved first lasing in late 2023, is ramping up specifications. The upgrade uses a new 700-m superconducting linear accelerator, eventually enabling a ~1 MHz repetition rate, compared to the 120 Hz of LCLS-I. Additional upgrades include LCLS-II HE (High Energy), a superconducting electron gun called LEI (Low Emittance Injector), upgrades to the Matter in Extreme Conditions (MEC) instrument — which is focused on fusion science, plasma science, and high-energy-density science — and the big LCLS upgrade, LCLS-X. LCLS-X will exploit the HE and LEI upgrades, expand from the present two undulators to ten undulators, and feed dozens of instruments at a time rather than just a couple.

Aside from the accomplishments at SACLA in Japan and LCLS in the USA, there is a growing number of XFEL facilities worldwide, each with their own specialities. For example, PAL-XFEL in South Korea provides excellent peak brightness and photon beam stability. Heung-Sik Kang, director of PAL (Pohang Accelerator Laboratory), told Nature Photonics that one more hard X-ray line is “on its way to being finished within four years…with a total budget of US$40 million”. There are also plans to add attosecond XFEL capability to the soft X-ray line. Kang emphasized that such short pulses are the key to acquiring damage-free data from samples.

Much is also happening for XFELs in Europe. Gabriel Aeppli, professor of physics at ETHZ and EPFL and deputy director of the Paul Scherrer Institut, told Nature Photonics about the major upgrade, called Porthos, planned for SwissFEL. Aeppli said this involves a third undulator branch of SwissFEL, which currently has hard X-ray (Aramis) and soft X-ray (Athos) branches. “The main performance changes envisioned are to go towards full coherence as well as polarization control, realizing the vision of the arbitrary waveform X-ray generator,” Aeppli explained. “This will allow hard X-ray quantum optics to take advantage of both electronic and nuclear resonances, with applications extending from fundamental physics to molecular biology.”

Speaking from Italy, Luca Giannessi, ‘head of machine’ at FERMI at Elettra Sincrotrone Trieste and senior scientist at Istituto Nazionale di Fisica Nucleare (INFN) at the LaboratorI Nazionali di Frascati (LNF), told Nature Photonics that the emergence of high-repetition-rate FELs, made possible by superconducting linacs, will increase the signal-to-noise ratio in all experimental conditions that can handle the associated high average power. Giannessi highlighted the desire for fully coherent output, noting that seeded FEL techniques, some of which the team pioneered, are providing pathways to full coherence. According to Giannessi, the Italian XFEL, FERMI, has an ultimate upgrade goal of extending the spectral range of the facility to cover the water window and above, eventually up to 1 keV, and to reduce the minimum pulse duration below the characteristic lifetime of the core hole electrons of light elements. The strategy will involve introducing an echo-enabled harmonic generation (EEHG) scheme. Giannessi stated that the first phase of the upgrade of FERMI, consisting of the EEHG upgrade of FEL-1, has just been completed and commissioning is progressing extremely well.

Moving over to Germany, Thomas Feurer, management board chairman and managing director of the European XFEL — which features a 17.5-GeV superconducting accelerator, three undulator lines, and seven experimental stations — told Nature Photonics that they are “laying the groundwork for a significant upgrade beyond 2030” but that the “ultimate design of this major upgrade is still in development”.

Circling back to Asia, we also spoke to Zhentang Zhao, an accelerator scientist and the project director of SHINE (Shanghai HIgh repetitioN XFEL and Extreme light facility). The facility already has a soft X-ray free-electron laser facility (SXFEL) in operation. According to Zhao, upgrades focus on “testing seeding techniques to enhance and control the temporal coherence of XFELs and developing ultrafast FEL technologies to generate pulses with attosecond-level time resolution”. The SHINE team is currently constructing a hard X-ray free-electron laser based on an 8-GeV superconducting RF linac.

Zhao says that civil engineering of tunnels and construction of facility utilities are close to completion, and that components of the accelerator and beamlines are in fabrication and integration. The goal is to commence experiments in 2027. In terms of future directions, Zhao thinks that cavity-based XFELs (a concept discussed in the Q&A with Dunne) are interesting, especially for high-repetition-rate facilities. SHINE has a cavity-based upgrade named MING in the works.