October 2001 saw the inauguration of the Swiss Light Source at the Paul Scherrer Institute. As Peter-Raymond Kettle reports, the source gives Switzerland a world-class synchrotron radiation facility.
The Swiss Light Source (SLS) marks a milestone in Swiss science policy, as well as in the development of multidisciplinary and complementary research facilities at the Paul Scherrer Institute (PSI). The combination of the SLS with PSI’s existing SINQ spallation neutron source and high-intensity muon beams from its proton cyclotron allows a diversity of probes to be used. It also makes a range of new applications available, from structural research in biology, physics, chemistry and materials sciences to nanotechnology and X-ray lithography.
Synchrotron radiation, originally viewed by machine designers and experimentalists as a troublesome by-product of high-energy accelerators, has developed over the years into a powerful, multidisciplinary tool that is now fully exploited in modern synchrotron radiation light sources. The first dedicated source came into operation 35 years ago and there are now about 44 in operation worldwide. The demand for high-quality synchrotron radiation is still increasing, with an estimated 6000 users in Europe alone.
At present, third-generation light sources fall into two main categories according to machine energy. Both categories are typically based on storage rings optimized for magnetic insertion devices – wigglers and undulators – that enhance the brilliance (a simultaneous measure of the intensity and collimation) of the photon beams.
Facilities with electron energies below 3 GeV are particularly suited to the generation of radiation in the ultraviolet and soft X-ray region. Examples are national facilities such as the US’s Advanced Light Source, France’s SuperACO, Italy’s ELETTRA and MAX-Lab in Sweden. Larger-scale international centres, based on machines with electron energies above 5 GeV, are optimized for the production of hard X-rays. There are now three such facilities in operation: the 6 GeV European Synchrotron Radiation Facility in France, the 7 GeV Advanced Photon Source in the US, and the 8 GeV SPring-8 in Japan.
The Swiss Light Source
The SLS was designed as an advanced third-generation light source capable of exceeding the performance of low-energy national sources and able to overlap with the hard X-ray spectral range of high-energy sources. Its performance is optimized for the production of light with a maximum brilliance in the vacuum ultraviolet to soft X-ray regions.
The SLS machine complex has three main components. There is a two-stage acceleration phase comprising a 100 MeV electron linear accelerator followed by a booster synchrotron to accelerate the electrons to their final energy of 2.4 GeV, and the storage ring itself. It takes about 3 min to reach the design current of 400 mA in the 288 m circumference ring. The radius of each ring is similar, allowing them to be in the same shielding tunnel. The storage ring is a polygon with 12 straight sections where wigglers and undulators are installed. Together with the ring’s bending magnets, these are the spectral sources of synchrotron radiation produced at the facility.
Wigglers and undulators consist of linear arrays of alternating magnetic poles placed above and below the beam axis. Depending on the strength of the magnetic field and the periodicity of the poles, the electrons either “wiggle” or “undulate” in the horizontal plane, greatly enhancing the emission of synchrotron radiation by multiple transverse acceleration. The higher magnetic fields and larger periodicity of the arrays in the case of wigglers leads to a wider spectral range of photons compared with undulators, where a much narrower cone of radiation is produced at each set of poles. This creates peak intensities at certain energies and high-brilliance beams that are tunable to experimental requirements.
Four beamlines are currently available to users at the SLS, spanning the spectral range of 10 eV – 40 keV. For the future, the facility has a projected capability of nine insertion devices and 24 bending magnet sources. Because of the high brilliance of the emitted radiation, several characteristics associated with this attribute, such as high flux density, a high degree of coherence, high energy resolution, good spatial resolution and good timing resolution can be simultaneously optimized in experiments. This opens up the possibility for novel imaging techniques such as holography to be exploited, or for time-dependent investigations of systems at the picosecond scale. The polarization of X-rays from linear to circular is also possible at the SLS, providing an important tool for investigating the magnetic properties of materials, for example, by imaging magnetic domains.
From initial ideas in 1990 to the start of the SLS project in 1997 was a process that involved around 30 votes and decisions. The project was first presented in 1993 and approved by the Swiss Government in 1996. Its budget of SwFr 159 m (Euro108 m) received near unanimous approval from both Houses of Parliament the following year, marking the official start of the project. The building and construction phase, which began in summer 1998, was followed by the start of machine installation just a year later. Commissioning of the machines proceeded rapidly, starting with the LINAC in February 2000 and ending with a Christmas present in the form of the first stored beam in the storage ring on 15 December 2000. By the following June, the design current of 400 mA was reached, and with the measurement of the first sample diffraction pattern in the protein crystallography beamline one month later, an exemplary commissioning phase was complete, bringing the new light source on stream in time and on budget.
The construction of the beamlines and experimental facilities benefited greatly from co-operation with sister synchrotron sources around the world, and by August 2001, 70% of available beamtime could be given over to a few selected users. A test phase running to the end of 2001 ensued before the facilities were made available to the full SLS user community of around 80 groups.
The SLS was officially inaugurated on 19 October 2001 at an event celebrated by more than 200 prominent guests from the worlds of politics, science and industry. It was headed by Ruth Dreifuss, Swiss federal minister for internal affairs. Among the scientific guests at the inauguration were Nobel Laureates Heinrich Rohrer and K Alexander Müller, who had been involved in the assessment process of the project.
Festivities began with an official welcome by PSI director Meinrad K Eberle and were followed by speeches from minister Dreifuss, Stephan Bieri, vice-president of council of the Swiss Federal Institutes of Technology, and Heinrich Rohrer. All stressed the importance of the SLS in the context of international scientific co-operation. Director Eberle reviewed the project’s history from conception to realization, while the technical aspects and the research prospects were covered in talks from project leader Albin Wrulich and research leader Frisco van der Veen.
The highlight of the inauguration festivities was a tour of the complex, the inside of the space-age building being bathed in a spectral extravaganza of light and sound. The inauguration was performed by Dreifuss and Eberle, who operated the power key to start up a symbolic “light source” that slowly appeared above the shielding wall, in the form of three chandeliers, and came to rest over a well decked buffet.
The whole event was, to the amusement of the guests, accompanied by film clips of historic scenes from previous ceremonies that were not executed quite so smoothly, while the musical ambience was provided by a modern jazz quartet. The festivities were rounded off with a series of congratulations from representatives of local government and sister laboratories, including Eberhard Jaeschke, technical director of the BESSY laboratory in Berlin, and Massimo Altarelli, scientific director of ELETTRA.