FEATURE ARTICLE OPENA ACCESS Institut d’Estudis Catalans, Barcelona, Catalonia www.cat-science.cat CONTRIB SCI 12(1):13-21(2016) doi:10.2436/20.7010.01.239 RESEARCH CENTRES IN CATALONIA The ALBA Synchrotron Light Source Ana Belén Martínez, Caterina Biscari, Gastón García ALBA Synchrotron, Cerdanyola del Vallès (Barcelona), Spain Summary. ALBA is the Spanish third-generation synchrotron light source. It is lo- cated in Cerdanyola del Vallès (Barcelona) and constitutes the largest scientific infra- structure in Spain. The facility consists of an accelerator complex providing 3 GeV electron beam and several experimental beamlines, with photon energies currently ranging from IR up to hard X-rays of tens of KeV. Different synchrotron radiation tech- *Correspondence: Ana Belén Martínez niques are available including diffraction, spectroscopies and imaging. [Contrib Sci [email protected] 12(1):13-21 (2016)] Synchrotron light precision of the measurement is also many orders of magni- tude better, being the essential reason why synchrotron light Synchrotron light is electromagnetic radiation that is sources are today absolutely necessary for competitive fun- produced when, within an accelerator, the circulating damental or applied research. And why is synchrotron light bunches of charged particles (typically electrons) are so bright? In a synchrotron facility, as the particles are travel- accelerated by the magnetic fields that are used to curve ling at speeds close to that of light, the light they produce is their trajectory in order to keep them inside a circular orbit. confined within a cone in the direction of propagation of the What we call “synchrotron light” is not something new. It has particles that can be contained within fractions of a millira- always existed in our universe. In a star, electrons travelling dian. Also, synchrotron light can provide a very broad range at almost the speed of light emit synchrotron radiation when of wavelengths: from infrared to hard X-rays, containing also they are under electromagnetic forces. However, in the last soft X-rays and UV. In addition, the size of the light source is 75 years, humankind has been able to produce synchrotron related to the size of the electron bunch. In a modern accel- light by building synchrotron facilities. erator the latter can have a cross-section of only a few tens of In the last 50 years, synchrotron light facilities have be- micrometers. come a major research tool to observe the properties of mat- The combination of a small source and a small angle of ter thanks to their powerful properties. As the brilliance of emission implies an extremely high brilliance (it should be synchrotron light is so much greater than that of more con- noted that brilliance is a measure of the flux of photons emit- ventional sources, such as rotating anode X-ray tubes, the ted per unit area and unit solid angle within a certain wave- Keywords: synchrotron · beamlines · biosciences · materials science · condensed matter ISSN (print): 1575-6343 e-ISSN: 2013-410X CONTRIBUTIONS to SCIENCE 12(1):13-21 (2016) ALBA Synchrotron length bandpass) and the very broad range of wavelengths rent of 250 mA. There is a large number of straight sections available means that this very high brilliance extends over a (24) available, whose essential role will be explained below, large range of the electromagnetic spectrum. In practice, the despite the relatively short circumference, thanks to the very brilliance of synchrotron light is trillions of times greater than compact lattice design, which incorporates a quadrupolar that of other conventional sources of light over most of the field component in the dipoles. The vacuum chamber has range of the electromagnetic spectrum. Moreover, not only more than 20 windows for the light extraction. Twelve of the brilliance is very high but synchrotron light is also polar- them are presently used (2 for accelerator diagnostics and 10 ized in the plane of the orbit, something exceptionally useful for beamlines, both operational and under construction), for the study of magnetic properties of materials, and, as and the others witness the large potentiality of ALBA for the consequence of the electrons travelling in short regular future. bunches, the light is emitted in very short pulses lasting ALBA is a 3rd generation synchrotron facility. That means around a few tens of picoseconds (a millionth of a millionth that its design incorporates long straight sections in between of a second, or 10–12 seconds). This latter property makes syn- the cells containing the electron optics. In these straight sec- chrotron light sources highly suited for the study of short- tions the electrons fly freely (i.e., no synchrotron light is emit- lived phenomena. ted as a baseline). However, these straight sections are used to house ad-hoc multipolar magnetic structures, named in- sertion devices (ID), which force the electrons to undergo The ALBA accelerators more or less exotic trajectories, the simplest being a sinusoi- dal one. Depending on the dimensions of the excursions im- The ALBA accelerator system consists of a linear accelerator posed on the electron beams, insertion devices are concep- (Linac) (where electrons reach 100 MeV), a low-emittance, tually sub-divided between wigglers―so named when the full-energy Booster (where electrons are accelerated to excursions imposed on the electrons are large relative to the 3GeV), and the Storage Ring (where electrons are injected beam angular divergence―and undulators―when the excur- and stored for the synchrotron light emission) (Fig. 1). The sions are comparable to the beam angular divergence. Booster (250 m of circumference) and the Storage Ring (269 The light emitted by the ID is even brighter than that gen- m) are both hosted in the same tunnel (Fig. 2). The lattice is erated at the bending magnets. This can amount to an en- optimized for high photon flux density, with a nominal cur- hancement of several orders of magnitude and, furthermore, Fig. 1. Scheme of the Accelerators complex and the structure of a beamline. www.cat-science.cat 14 CONTRIBUTIONS to SCIENCE 12(1):13-21 (2016) Martínez et al. Fig. 2. View of the tunnel of accelerators. Storage Ring (left). Booster (right). insertion devices can be tailored to the specific requirements into operation, stabilising the photon beam at the source lo- of a given experiment and may be substituted easily without cation at frequencies up to 100 Hz. The immediate conclu- changing all the magnetic lattice of the Storage Ring. This sig- sion of this development is that now the orbit in the Storage nificantly increases the useful life of the facility. Ring is much more stable, better than 600 nm (rms) in the The ALBA Accelerators run on a 24 h a day, 7 days a week horizontal plane and 100 nm (rms) in the vertical plane; in basis, for periods that usually are 4 to 5 weeks long. In 2015, other words, the electron beam is stable to less than 1% of its more than 4300 h were devoted for users with a beam avail- beam size (at the medium straight, typical reference point ability of 97.3%. About 1400 h were dedicated to the optimi- where light is generated to be fed to the experimental beam- zation of the accelerators for the users as well as to testing lines), placing ALBA at the frontier of the beam stability new developments. among the synchrotron facilities worldwide. In 2015, the injection in top-up mode has been consoli- An additional development, a bunch-by-bunch transverse dated. In the top-up mode injection, the current in the Stor- feedback system, has also started operating in 2015. This sys- age Ring is kept constant, injecting almost continuously a tem fights electron beam instabilities on a bunch-by-bunch very small fraction of current to cope with the beam losses basis, acting directly on the individual bunches by damping due to the finite lifetime of the beam. In this situation, the high frequency oscillations (up to 250 MHz) that may affect front ends (the tube connecting the sources of light to the the brilliance of the photon beam. beamlines), interfaces between the accelerator and the beamlines wherein experiments with synchrotron light are performed, remain open during injection. This injection The beamlines mode ensures a constant thermal load on the accelerators and on the optical components of the beamlines, which in- Once synchrotron light is emitted by the magnet systems, it is creases greatly the position stability of the photon beam at redirected through the front ends to the beamlines, where the sample. This, together with the fact that a constant pho- experiments are performed by the users. ALBA started opera- ton flux at the sample means a constant signal level at the tion in May 2012 with seven beamlines dedicated to different detectors, sensibly improves the data quality of the experi- scientific fields, mainly physics, chemistry, life sciences, ma- ments performed at ALBA. terials science, cultural heritage, biology and nanotechnolo- In May 2015, a fast orbit feedback system (FOFB) came gy. Two new beamlines were initiated in 2014, one of them in www.cat-science.cat 15 CONTRIBUTIONS to SCIENCE 12(1):13-21 (2016) ALBA Synchrotron operation already in 2016 and the second one to become vestigating surface chemical reactions and surfaces of operational in 2018 and one additional beamline has been liquid samples. started in 2016. So, the current portfolio of ALBA is of 10 BOREAS. X-ray magnetic circular dichroism (XMCD) and X-ray beamlines: 8 operational, and 2 under construction. As men- magnetic linear dichroism (XMLD) techniques for the tioned in the previous section, the ALBA Synchrotron is de- study of advanced magnetic materials. With a second ex- signed to host more than 20 beamlines (Table 1).