The upgrade project SLS 2.0

The Swiss Light Source SLS delivers special beams of light for research. It is extremely bright X-ray light, suitable for many different types of investigations in physics, materials science, biology, chemistry, the environmental sciences, and cultural heritage, and the list does not stop there.

SLS today

The facility has world-leading instruments at its research stations, currently 20 in all. At some of these so-called beamlines, the structure of proteins is being deciphered; at others, researchers can look inside materials with nanometre or even higher precision and in 3D. At others still, it is possible to explore how the electrons in solid matter behave and how, as a result, magnetism or superconductivity arises.

How synchrotron light is produced

The special light of SLS is generated with the help of electrons accelerated to very high speed: The SLS building houses a so-called electron storage ring. Here, well protected behind thick concrete walls, a metal tube runs in a circle. The tube is connected to strong pumps that suck all the air out of it and thus create a vacuum. Electrons – unimaginably tiny, negatively charged elementary particles – are first brought up to a high speed in an accelerator and then guided into this tube. There they race, at 99.999998 percent of the speed of light, around a circle with a circumference of 288 metres. In other words: Per second, every electron flies a million laps around the storage ring.

The electrons are held on this circular path by special magnets. These components – which vary from the size of a shoe box up to that of a packing box – surround the tube in many places along the storage ring. These magnets impart a change of direction to the trajectory of the electrons. The characteristics of the magnets determine, in each case, how strongly the electrons are redirected. Strictly speaking, the electrons do not fly a perfectly circular path, but rather are deflected so often that they fly along a polygon.

Some of the magnets also have a further task: Here the X-ray light, which is automatically generated by electrons at every change of their direction and thus in every curve, is guided from the electron storage ring to the experimental stations. This X-ray light is also called synchrotron light.

The goal

The quality of the synchrotron light depends to a large extent on the details of the electrons’ path in the storage ring. In practice, although tight curves create beams that are useful for research, a large number of more modest direction changes, which collectively yield smoother curves, generate qualitatively better beams for the researchers – that is, above all, brighter beams. In other words, the multi-sided polygon of the electron storage ring should become an "even-more-sided" polygon.

This change is the heart of the SLS 2.0 upgrade: It should incorporate significantly more magnets than before, each of which modifies the previous electron path with its relatively large angles into one with many small angles.

The challenge – and the solution

Accommodating still more magnets along the storage ring, which in addition is equipped with around 100,000 control instruments – for temperature, magnet currents, vacuum pressure, and the like – is the big challenge of the SLS 2.0 project.

Solving this problem requires a whole chain of factors:

To make room for more magnets, each magnet must first be smaller. However, for a given separation between the north and south poles, smaller magnets provide a weaker effect on the electron beam. To still achieve the necessary field strength, the new magnet components must therefore be placed closer to the path of the electrons. For that, the diameter of the existing storage ring tube is too large. This vacuum tube must thus be exchanged for a narrower one. But it is much more difficult to suck all the air out of a narrower tube to create the necessary vacuum quality. To address this problem, in turn, the inner surface of the new tube will be coated with a special material, a so-called non-evaporable getter coating. This absorbs gas atoms permanently and thereby achieves the crucial step in improving the quality of the vacuum.

SLS after the upgrade

All of these measures are expected to lead to a synchrotron beam which finally arrives at the SLS experimental stations having 30 to 35 times better values than at present. This means that the cross-section of the beam shrinks, so the beam becomes even finer with the same intensity. It will also be even more collimated and scarcely broaden even after several metres. The examination of very small samples, among other things, will benefit greatly from this.