ALBA : une source de rayonnement synchrotron

ALBA, une nouvelle source de rayonnement synchrotron de troisième génération, est en construction près de Barcelone. Elle comprend principalement un linac de 100 MeV fonctionnant à une fréquence de 3 GHz, un synchrotron injecteur de 3 GeV, et un anneau de stockage de 268,8 m de circonférence situé dans le même tunnel que l’injecteur. La phase de mise en service de l’injecteur commencera au second semestre de 2009 et sera suivie par la mise en service de l’anneau de stockage et des lignes de faisceaux. Les utilisateurs devraient pouvoir disposer des premières lignes de faisceaux d’ALBA d’ici au second semestre de 2010.

With the exception of the European Synchrotron Radiation Facility, which serves an international community, all existing synchrotron light sources in Europe are located to the north of an imaginary straight line going from Paris to Trieste. To the south, Spain has only a few accelerators and these have been “turnkey” products, mainly in the medical sector; none belongs to a “big” laboratory. For these reasons, in the early 1990s the Autonomous Government of Catalonia began to consider the construction of a new, third-generation synchrotron light source.

The Generalitat de Catalunya and the Spanish Government signed a first collaboration agreement in 1995 and then took a final decision in March 2002 to create CELLS, a consortium for the construction, equipping and operation of a synchrotron light laboratory. The aim was to establish a third-generation synchrotron light source in the municipality of Cerdanyola del Vallès, in a technology area to be built next to the campus of the Autonomous University of Barcelona, some 20 km from the city. CELLS came into being at the end of 2003 with the objective of constructing the ALBA light source. The initial plan, based on an existing preliminary design study, was for a storage ring of 3 GeV with five beamlines in the first phase. This was scheduled to start up at the end of 2008, for a total cost of €164 m shared between the two administrations on a 50:50 basis.

Studies of underground characteristics, which took more than a year, led to final agreement on the 60,000 m2 site for ALBA. This was followed by project approval for the building and conventional installations, thereby guaranteeing mechanical, electrical and thermal stability. Then in 2006 the governing board of CELLS decided to extend the construction phase to the beginning of 2010, to accept two more beamlines (bringing the total to seven) and to increase the total budget to the end of 2009 to €201 m.

The facility occupies a main building of some 18,500 m2. This will host the accelerators and the experimental stations, so it must respect restrictive vibration conditions. It is built on a hard floor floating on a bed of gravel 2 m deep. There will also be peripheral auxiliary laboratories, a technical services building of 7600 m2 (including an auxiliary storage building) and an administrative building of 4000 m2.

A high-quality 12 MW power supply will be provided by a natural gas power plant that provides thermal and electrical energy, backed up by a dedicated transformer connected to a 220 kV supply line. A system of static and dynamic uninterruptible power supplies will guarantee the supply to the most critical parts of the facility.

The main elements of ALBA will be a 100 MeV linac working at a frequency of 3 GHz with a repetition rate of 3 Hz; a booster synchrotron of four-fold symmetry with an energy of 3 GeV, a circumference of 249.6 m and less than 20 nm rad emittance; and a storage ring of 268.8 m circumference located in the same tunnel as the booster. The design of the accelerators is based on the latest, but well proven, technologies. To provide as much space as possible for the installation of insertion devices and diagnostics, the design includes an extremely compact double-bend achromatic lattice of 16 cells with four-fold symmetry. It will consist of 16 pairs of combined dipole and quadrupole magnets with a central field of 1.42 T and a central gradient of 5.9 T/m, located on large girders. The lattice will be complemented by 128 quadrupole magnets and 120 sextupole magnets with more than 100 correctors installed in the sextupoles. The light source will have an emittance of 4.3 nm rad, a current of 400 mA and a critical energy of about 4.8 keV.

ALBA’s ultra-high vacuum system is made of stainless steel. To cope with the synchrotron radiation, there are antechambers all around, where water-cooled copper and Glip Cop absorbers stop the synchrotron radiation so as to avoid heating the vacuum chamber.

The radiofrequency (RF) system is based on inductive output tube (IOT) amplifiers, running at 80 kW with 67% efficiency. There will be 13: one in the booster to feed a five-cell PETRA-type cavity; and 12 in the storage ring to feed six higher-order, mode-free cavities. A new cavity combiner designed for ALBA will combine the power of the IOTs.

To obtain submicron beam stability there will be a diagnostic system based on 120 beam-position monitors and digital electronics distributed around the storage ring. In addition, synchrotron radiation monitors, current transformers, fluorescent and optical-transition-radiation screens, strip lines and annular electrodes will determine the characteristics of the electron beam. In general, standardization, modularity and robustness have been the main concerns in achieving an accelerator with high reliability and easy maintenance.

The ring will have a capacity for more than 30 beamlines. There will be 16 of these in bending magnets, three in insertion devices in long (8 m) straight sections, 12 in medium (4.4 m) straight sections and two in short (2.6 m) straight sections. The remaining straight sections will be dedicated to injection and RF.

The Asociación de Usuarios de Sincrotrón de España proposed the first phase of beam lines, seven of which CELLS accepted after consultation with the Scientific Advisory Committee. These beamlines, which are now under construction, will be dedicated as follows: one to non-crystalline diffraction and one to macromolecular crystallography, both based on in-vacuum undulators; one to photoemission spectroscopy and microscopy and one to X-ray circular magnetic dichroism, both based on normal undulators; one to X-ray absorption spectroscopy based on a multipole wiggler; one for high-resolution powder diffraction based on a superconducting wiggler; and one for X-ray microscopy based on a bending magnet. A call for a second phase of beamlines is now under way and is scheduled for approval by the end of 2009.

At present, the ALBA project is developing roughly according to budget and schedule, with a delay of only a few months, mainly owing to problems related to civil engineering and conventional services. The linac has been installed and its commissioning has finished. The civil engineering and the conventional installations will be completed in December this year. Almost all of the accelerator components have been designed, produced, tested and delivered, and installation will start soon in the main tunnel. The commissioning phase of the booster will start at the beginning of the second half of 2009 and the storage ring and beamlines will start commissioning progressively in spring 2010. By the second half of the same year the first beam lines of ALBA should be open to users.