Athos just got even better

An upgrade at the soft X-ray beamline of the free electron laser SwissFEL will open up new experimental capabilities. Using an external laser system to ‘seed’ the emission of X-ray photons, and thus imprint well-defined optical properties on the beam, the upgrade gives the Athos beamline unprecedented stability. With this, ultra-fast ‘attosecond’ timescales that probe the movements of electrons in chemical reactions become possible.

Free electron lasers (FELs) in the X-ray regime, such as the X-ray free electron laser SwissFEL, produce short pulses of brilliant light that give unique insights into the structure and dynamics of materials through so-called ‘molecular movies’. The Athos beamline of SwissFEL produces so-called ‘soft’ X-rays whose comparatively low photon energies are useful for studying the interactions between molecules.

A limitation for the Athos beamline, as for almost all FELs, is stability. The reason for this comes down to the process by which light is made: a process called self-amplified spontaneous emission (SASE). In a FEL, electrons, accelerated to close to the speed of the light, are wiggled by a series of magnets, called undulators. Once wiggling, they produce photons: at SwissFEL, in the form of X-rays. SASE describes the process by which these photons repeatedly interact with the electron beam and stimulate – or ‘seed’ – the emission of more photons in subsequent parts of the electron beam. The spontaneous emission of radiation in this way is a stochastic process. This means that the X-ray beam created is inherently unstable, characterised by variations in wavelength and pulse energy.

Thanks to funding from the European Research Council (ERC), a new upgrade of Athos tackles this fundamental challenge of X-ray FELs. The upgrade forms part of the HERO project, which in 2018 was awarded a prestigious Synergy grant of 14 million Euros and incorporates principal investigators from PSI, EPFL, ETHZ and Stockholm. Standing for ‘Hidden, Entangled and Resonating Orders’, the HERO project, which is coordinated by PSI, aims to uncover hidden quantum properties in materials that cannot be studied with existing methods. The HERO upgrade of the Athos beamline will enable such new insights.

“What is demonstrated here is the power of funding for blue-skies research that the ERC uniquely provides to associated countries,” states Gabriel Aeppli, head of the Photon Science Division at PSI, who is the coordinating principal investigator for the HERO project.

Bringing Athos in line

In a classroom, one particularly well-behaved child can serve as a role model for all the children. In a similar vein, at the Athos beamline, the upgrade uses an external laser to imprint its well-behaved properties on the FEL beam. Instead of relying on the stochastic, spontaneous emission of radiation, a ‘seed-laser’ interacts with the wiggling electron beam to amplify the emission of radiation. As this external, optical laser has a well-defined pulse and coherence properties, it can transfer these to the emitted X-rays.

There is a reason that an X-ray FEL has never before been externally seeded. “Although the principle of ‘seeding’ a FEL is not entirely new, seeding a FEL at an energy range as high as this is,” explains Alexandre Trisorio, head of the gun laser group, who developed the seed-laser system. “The trouble is that there are no external laser sources that operate in the right wavelength range”.

To get around this, the scientists – through some serious feats of electron bunch gymnastics and tricks of the light – are employing a technique known as echo-enabled high-harmonic generation (EEHG), whereby higher frequency resonances are created that seed the FEL. The full upgrade is a two phase project, the first of which has now been successfully completed.

Read more on the PSI website

Image: An X-ray FEL cannot be seeded with a simple optical laser: there is none that can deliver a short enough wavelength. So, more complicated techniques are required. In a dedicated room alongside the Athos beamline, an 11m optical bench will host two titanium sapphire seed laser systems. Martin Huppert fine tunes the first of these, installed in the first phase of the HERO upgrade.

Credit: Paul Scherrer Institute / Markus Fischer

New SwissFEL soft X-ray endstation welcomes first users

Maloja is go. On Wednesday, 23rd March 2022, first user experiments began at the Maloja endstation, which enables explorations into atomic, molecular and optical physics and chemical dynamics. These user experiments mark a double first, not only for Maloja but also for the second, soft X-ray beamline of the SwissFEL, Athos.

Following two years of tireless development, Maloja is beginning to yield its scientific fruits. Developed in parallel with soft X-ray beamline Athos, Maloja is the first endstation to be up and running, and takes advantage of advanced beam parameters, namely, very short pulses, two colour pulses and pump-probe experiments. A key feature of the Maloja endstation is its modular nature, enabling straightforward exchange of chambers and tailoring to individual experimental requirements.

“Because of its flexible design, a wide variety of investigations are possible at Maloja, such as time-resolved measurements of electronic structure changes, non-linear X-ray spectroscopy or research into gas-phase atoms or nanoparticles. I’m really excited to see the diverse science that future users will turn up with,” enthuses Kirsten Schnorr, lead scientist at the Maloja endstation.

Work began on the Maloja project in 2019, with the COVID pandemic striking a few months after first hardware deliveries. With staff working night shifts to create ‘time-dimensional social distancing’, in June 2020 first light entered the Maloja endstation. This heralded the beginning of commissioning experiments and a very close collaboration between the Maloja team and accelerator groups as, step-by-step, the teams developed not only the Maloja endstation, but also a whole new branch of the SwissFEL: Athos.

Read more on the PSI website

Image: Members of Nanostructures and Ultrafast X-ray Science Group, including Daniela Rupp and Mario Sauppe (3rd and 4th from L) together with the Maloja team and Christoph Bostedt (far R) during the beamtime

Credit: Alessandro Colombo

First light at Furka: The experiments can begin

It’s another milestone on the path to full operation of the X-ray free-electron laser SwissFEL with five experiment stations in all: “First light” at the experiment station Furka. It clears the way for experimental possibilities that are unique worldwide. Team leader Elia Razzoli explains what the Furka Group is planning to do.

Why is “first light” such an important occasion for your team?

Elia Razzoli: It means we’re in business. Or to be more specific: Now we can begin working on the first experiments.

The general public might imagine that you simply flip a switch, and then the light is there. But presumably it’s not that simple in your case . . .

No, it is a complex task. When we at SwissFEL talk about light, we do not mean visible light, but rather X-ray light with characteristics that are unique in the world. To generate that light, and for research to be able to use it, several teams at PSI have to work together. With the Furka experiment station we are, so to speak, at the end of the food chain. To generate the X-ray light of SwissFEL, electrons must be forced onto a sinuous track with the aid of magnets. In the process, they emit the X-ray light that we need to carry out the actual investigations. The magnets that redirect the electrons in this way are called undulators. And they are precisely what makes the whole thing so difficult, because they have to work exactly in sync; otherwise the X-ray light doesn’t have the quality that we need. The complexity of the system grows exponentially with the number and length of the undulators. That is why first light at Furka is already a masterful technical and organisational feat.

Read more on the PSI website

Image: Members of the team that achieved the milestone at the Furka station of SwissFEL: Eugenio Paris (left), Elia Razzoli, Cristian Svetina (right)

Credit: Paul Scherrer Institute/Mahir Dzambegovic