The SLS 2.0 storage ring is a so-called "fourth generation synchrotron", or "diffraction-limited storage ring". The second-generation facilities were designed to use the synchrotron light produced in the bending magnets. The corresponding spectrum of these electromagnetic waves (also called "photons") is continuous, and a user has to select the desired wavelength for his experiment with a filter and discard the remaining part of the spectrum. The third generation facilities, like the original SLS, make use of so-called undulators. This clever device was invented in the 1970's by Klaus Halbach from the Lawrence Berkeley Laboratory in California. With permanent magnets, arranged in a periodic array, the electrons are forced to move in a slalom course. This trick concentrates the synchrotron light into discrete lines in the spectrum. The distance between each slalom turn and the strength of the magnetic field determine the desired wavelength. The effect on the character of the synchrotron light is dramatic:
- The light is emitted in a very narrow cone, similar to a laser beam; even 30m after the undulator the light spot has a diameter of only a few mm. With special mirrors the spotsize can even be reduced to less than a micron. This enables applications in microscopy and allows to examine very small samples.
- The intensity is about a factor of 1000 higher than the light from a bending magnet, with a corresponding reduction in the time needed to perform an experiment.
The SLS storage ring has a circumference of 288 m and was designed to have several straight sections of different lengths (from 4 to 11 m). One could thus incorporate a variety of undulators, covering a wide spectrum of synchrotron light from ultra-violet to hard X-rays. For the longest wavelength the magnetic field in the undulator is produced with electromagnets instead of permanent magnets.
The SLS was designed as a "state of the art" facility, producing synchrotron light of a very high quality. This required the installation of a large number of magnets - a total of 330 - to keep the diameter of the electron beam as small as possible. Special care was taken for the performance of the high- vacuum system, to guarantee a long lifetime (in the order of 10 h) of the circulating beam, ensuring very small beam losses. The synchrotron light emitted by the electrons is very intense, corresponding to an average power of 200 kW. This power has to be refurnished to the electrons by four RF transmitters, operating at a frequency of 500 MHz.
The SLS was upgraded, starting in October 2023, to a DLSR defined by its implementation of 7-bend achromats. The emittance has imporved by a factor of almost 40, resulting in a horizontal electron emittance of 157 pm.rad.
