Synchrotron or X-ray laser: how is the Swiss Light Source different from SwissFEL?

What the two machines have in common

Both machines turn the energy of fast electrons into light. An electron that is forced to change direction gives off electromagnetic radiation, and the faster it moves, the shorter the wavelength it can emit. At the energies used at PSI that means X-rays, with wavelengths comparable to the distances between atoms.

Both also use the same key component: the undulator, a long row of magnets with alternating poles that pushes the electrons into a slalom. What differs is what happens to the electrons before and after the undulator, and that difference decides what kind of light comes out.

The ring: the same electrons, again and again

In the Swiss Light Source electrons are first accelerated in a linear accelerator and a booster, then stored in a ring with a circumference of 288 metres, where they travel at almost the speed of light. Magnets keep them on their track. Every bend and every undulator along the ring produces light, which leaves the ring tangentially and is carried along beamlines to experimental stations.

The same electrons go round again and again, and new ones are topped up every few minutes to replace those that are lost. The light is therefore practically continuous, and it is shared: PSI has described more than 20 experimental stations around the ring, each choosing the wavelength it needs, from infrared to hard X-rays. According to PSI, the SLS produces X-rays billions of times brighter than a hospital X-ray machine.

That makes a synchrotron the workhorse of structural research. Over the lifetime of the first SLS, from 2001 to 2023, PSI counts about 22,500 experiments and more than 10,000 solved protein structures. The rebuilt ring, SLS 2.0, was inaugurated in August 2025; at some beamlines its light is up to 1,000 times brighter.

The straight line: each electron used once

SwissFEL has no ring. Electrons knocked out of a metal plate by a laser are accelerated along a straight line to an energy equivalent to crossing a voltage of 6 billion volts, then sent once through a chain of undulators, 60 metres in total on the hard X-ray line Aramis.

The decisive step happens inside those undulators. The magnets force the electrons onto a wavy path, and over a long enough distance the X-rays they emit build up in an avalanche-like amplification. In this process the radiation acquires the properties of laser light – the key difference from a synchrotron. PSI describes the resulting flashes as lasting between 1 and 60 femtoseconds – a femtosecond is a millionth of a billionth of a second. On Aramis a new flash is produced every 10 milliseconds, 100 times a second.

Because each bunch of electrons is used only once, an X-ray laser serves far fewer experiments at a time than a synchrotron. SwissFEL has two beamlines, Aramis for hard and Athos for soft X-rays, with five experimental stations between them.

Photograph or film: which machine for which question

The choice follows from the question. If researchers want to know where the atoms are in a protein, a battery material or a catalyst, a steady, intense beam is ideal: they can collect data for minutes or hours and many groups can work in parallel. That is the domain of the SLS.

If they want to know how the atoms move – the intermediate steps of a chemical reaction, or how the electronic and atomic structure of a material change together – they need a shutter faster than the motion. SwissFEL provides it. A reaction is started with a laser and then imaged at different moments, frame by frame, to assemble a film of the process. The two machines are therefore not competitors but complements: one serves many experiments at once with a steady beam, the other is built to follow processes as they happen.

One of a small family of X-ray lasers

Synchrotrons are found in many countries; X-ray free-electron lasers are rare. The European XFEL in Hamburg lists the large X-ray free-electron lasers in a comparison table: LCLS in the United States began commissioning in 2009, SACLA in Japan in 2011, and SwissFEL, PAL-XFEL in South Korea and the European XFEL in 2016. SwissFEL is one of the most compact, at 0.74 kilometres, with 100 flashes a second. The European XFEL is 3.4 kilometres long and, using a superconducting accelerator cooled to minus 271 °C, produces 27,000 flashes per second.

And in Poland?

Poland has its own synchrotron. SOLARIS, the National Synchrotron Radiation Centre at the Jagiellonian University in Kraków, was opened on 21 September 2015. Its storage ring, a replica of one of the two storage rings built at the synchrotron centre in Lund, Sweden, has a circumference of 96 metres and an electron energy of 1.5 GeV. According to the centre, seven beamlines are in operation in 2026 and three more are under construction; access is free of charge and awarded in two calls a year (status as of 3 November 2026).

Poland has no X-ray laser of its own, but it is one of the shareholders of the European XFEL, represented by the National Centre for Nuclear Research (NCBJ). A page on Poland’s participation run by the Institute of Physics of the Polish Academy of Sciences gives the Polish share as 2.13 per cent. Polish researchers can also apply for beamtime at PSI through its open, peer-reviewed calls, described on PSI. PSI’s third kind of probe, neutrons, comes from neither machine; where they come from is explained in Does PSI have a nuclear reactor?