Synchrotron radiation sources in Brazil

Abstract

The development of synchrotron radiation sources in Brazil is described from a brief historical point of view followed by a description of the Sirius project, a new 3 GeV fourth-generation synchrotron light source with 518 m circumference and 0.25 nm.rad emittance, in final construction stage at the Brazilian Synchrotron Light Laboratory campus, in Campinas. As one of the pioneer fourth-generation machines, many accelerator engineering challenges were studied in depth and resulted in quite a few innovative developments. In this paper, we review some of these developments.

This article is part of the theme issue ‘Fifty years of synchrotron science: achievements and opportunities’.

1. Introduction

In celebration of the 50 years of synchrotron radiation in the UK, we would like to contribute this text about the development of synchrotron radiation sources in Brazil. We will briefly review the history and present status of synchrotron light sources in Brazil from a mostly personal perspective.

In an amazing coincidence, the year of 2018 also marks the 50th anniversary of Queen Elizabeth II's visit to Brazil. Among other places, figure 1 shows her visit to Estância Eudoxia, a ranch in Campinas within 1 km of the Brazilian Synchrotron Light Laboratory (LNLS) campus today, where the development of synchrotron light science and technology in Brazil started more than 30 years ago.

Figure 1.

Queen Elizabeth II is pictured horse riding at the Estancia Eudoxia ranch near Campinas. She is accompanied by the British Ambassador, Sir John Russel, on the left of picture and the Duke of Edinburgh in the background (8 November 1968). Photo from the ‘Estadão’ newspaper collection (https://acervo.estadao.com.br/noticias/acervo,galeria-de-fotos-rainha-elizabeth-ii-no-brasil,9364,0.htm).

In this campus, a second-generation light source, named UVX, has been in operation for users since 1997. After more than 20 years, the UVX experimental hall today is completely filled with 17 beamlines that serve about 1500 users per year with more than 97% availability. UVX will be shut down in 2019, when a new state-of-the-art storage ring, Sirius, will go into operation.

2. Historical overview of synchrotron radiation sources in Brazil

The first discussions about a synchrotron light source facility in Brazil date back to the early 1980s. The discussions eventually resulted in a decision in 1985 to fund a preliminary design effort and send a small team of young physicists for a short period to SSRL, at Stanford, USA. There was no expertise in Brazil at that time for such a task. Under the supervision of Prof. Helmut Wiedemann, a first draft of a conceptual design report was prepared for a 2–3 GeV storage ring with full energy injection. One of us (A.R.D.R.) was the team leader. Another one of us (L.L.) was responsible for accelerator physics calculations. The resulting design report (later known as Project 1) was the basis for cost estimates and further discussions within the Brazilian government and scientific community. The ‘Laboratório Nacional de Luz Síncrotron’ (LNLS) was created by the newly established Ministry of Science and Technology by the end of 1986, with a threefold aim, as Cylon G. da Silva, the first LNLS director, used to point out: to develop in Brazil accelerator technologies and experimental scientific research, and also a new model for scientific organization management, a National Laboratory, with open access facilities.

The economic circumstances in Brazil in the late 1980s and early 1990s led the LNLS board of directors to decide for a smaller storage ring design, which evolved into the present UVX machine, a second-generation 1.37 GeV storage ring with 93 m circumference. UVX was opened for users in July 1997 with seven beamlines.

The mostly in-house design and construction of the UVX machine, as well as its later upgrades, served as a grand school for an initially young and inexperienced team of physicists and engineers. This continued human resources training has no doubt paved the way for the development of a much more daring endeavour in the 2010s, the construction of Sirius, a state-of-the-art 3 GeV low emittance machine that will be the second fourth-generation storage ring to start operation in the world. Sirius is based on a 3 GeV, 518 m circumference storage ring with bare lattice emittance of 0.25 nm.rad.

After more than 20 years of operation, the UVX experimental hall today is completely filled with 17 beamlines that serve about 1500 users per year with more than 97% availability. UVX will be shut down in 2019, when Sirius will go into operation.

The preliminary discussions about the need of a new light source in Brazil date back to 2006. Three years later, in 2009, the development of the new source was given highest priority by the new LNLS Director, Antonio José Roque da Silva, who was successful in convincing the Ministry of Science and Technology of its importance for the country's development. In the same year, two prospective workshops were organized to discuss the scientific cases and machine specifications. Following these workshops, LNLS focused in designing a new light source based on a third-generation storage ring. The initial funding for the conceptual design phase was spent to improve the in-house engineering and technical infrastructure in the campus, in preparation for the R&D needed for the new machine components. In 2012, the design of a 1.7 nm.rad emittance machine was mature enough to be presented to an International Machine Advisory Committee (MAC). The MAC recommendation to pursue a much lower emittance in view of the Swedish MAX-IV plans gave the final incentive for the team to start over a new project design despite having already invested 3 years in the old one. In a period of a few months, after intense design and evaluation work, the official decision to change the project completely to a much more challenging—and therefore exciting—design based on a 5BA lattice was taken on July 2012. This kind of lattice has later been classified as of a new generation, the fourth-generation storage rings. The new challenges initiated an intense phase of lattice optimization and R&D for the accelerator components to achieve the required beam stability and reliability, including studies on the building, specially the floor, magnets, supports, diagnostics, vacuum system, etc. The ground work on the Sirius site, a land next to LNLS, newly incorporated to the campus, started late in 2012 and the building construction started in 2015. In parallel, the beamlines definition and design also evolved and reached the present outline with 13 beamlines in Phase I (six presently under construction and seven under design).

At the moment of this writing, that is December 2018, the Linac is already commissioned, the booster magnets, diagnostics, RF and vacuum systems are in place and the storage ring girders with magnets installation started two months ago. Commissioning of the storage ring is programmed to start in June next year.

Different from the first machine, many of the Sirius components were contracted from the local Brazilian industry. The nationalization index is larger than 86%. In the following sections, the Sirius project will be described in more detail.

3. The fourth-generation storage rings

The key idea to the fourth-generation storage rings is the implementation of the Multiple Bend Achromat (MBA) lattice to reach very low emittance, up to 2 orders of magnitude smaller than previous-generation machines. MAX-IV in Sweden was the first machine to explore this kind of lattice and its successful commissioning [1] late in 2015 marks the beginning of the new fourth generation of storage ring synchrotron light sources. MAX-IV will soon be followed by Sirius, in Brazil, with commissioning scheduled for 2019. Both are green-field projects. The great improvement in brightness promised by the new approach triggered upgrade programmes and studies in most of the existing third-generation machines, such as ESRF-EBS [2] in France, APS-U [3] and ALS-U [4] in the USA. The area of storage ring based synchrotron light sources is experiencing a very exciting period with new developments worldwide in various fields: tools to design, model and optimize accelerators; new accelerator technologies to cope with very tight lattices, higher demands and requirements on tolerances and stability, smaller apertures for beam injection; new beamline optics and experimental techniques, etc. New ingredients seldom used in third-generation machines are now being used: transverse and longitudinal field gradient in the dipoles, reverse bend dipoles, on-axis swap out injection, use of octupoles, etc. In parallel, continuous improvement of design and accelerator modelling tools together with modern computational resources have contributed a lot to the performance optimization of these highly nonlinear machines.

The effect of MBA lattices on emittance reduction can be clearly seen in figure 2, where the normalized emittance is displayed as a function of ring circumference for various machines. The fourth-generation rings made it possible to reduce the emittance while keeping a limited ring circumference, a feature with a big impact on the total cost of the facility.

Figure 2.

Normalized emittance as a function of ring circumference for various storage ring light sources. The trend lines clearly show the effect of MBA lattices on emittance reduction for the new fourth-generation machines. (Online version in colour.)

Although emittance is a key parameter, the proper matching of the electrons and photons phase-space is also important to maximize the brightness of undulator sources. This matching requires low betatron function values in both planes, horizontal and vertical, in the insertion device straight sections. This is a demanding requirement, especially for the horizontal plane, that nevertheless has interesting side effects, such as the reduction of impedance and ID effects as well as the extraordinary possibility of installing low horizontal as well as vertical gap insertion devices such as Delta undulators.

In the following section, we describe the Brazilian Sirius project.

4. Sirius light source design

Sirius is a fourth-generation synchrotron radiation facility being constructed at the LNLS campus in Campinas, Brazil. It will be the second fourth-generation machine to start operation worldwide. The 518 m circumference electron storage ring is based on a 5BA lattice with a bare lattice horizontal emittance of 250 pm.rad for a 3 GeV beam. The lattice is designed such that a further reduction to 150 pm.rad can be achieved with the extra damping provided by the envisaged insertion devices. Figure 3 shows an aerial view of the Sirius building construction and figure 4 shows the main accelerator tunnel with the booster attached to the inner wall. Table 1 lists Sirius main parameters.

Figure 3.

Aerial view of the Sirius building construction. (Online version in colour.)

Figure 4.

Booster installed in the same tunnel as the storage ring, attached to the inner wall. (Online version in colour.)

Table 1.

Sirius main parameters.

parameter	value	unit
beam energy	3.0	GeV
circumference	518.4	m
lattice	20 × 5 BA
natural emittancea	0.25 → 0.15	nm.rad
nominal current	350	mA
straight sections	5 × 7 m; 15 × 6 m
energy spread	0.085	%

^abare machine → with IDs.

5. Sirius lattice design and optics

The Sirius magnet lattice is based on a modified 5BA cell, reaching a horizontal emittance of 250 pm.rad with 20 cells in a 518 m circumference and 3 GeV ring. There are two kinds of dipoles, with transverse and longitudinal field gradients. Four of the five dipoles in the achromat are of the transverse field gradient type and are electromagnets, excited by current in coils. The centre dipole, BC, has a longitudinal field gradient and is based on permanent magnet (NdFeB) technology. BC is also used as a hard X-ray source; there is a sharp peak field in the centre that reaches 3.2 T, creating a collimated hard X-ray dipole source with critical photon energy of 19 keV and low contribution to the total energy loss per turn. A schematic picture of the Sirius 5BA cell with the magnetic elements is shown in figure 5.

Figure 5.

One Sirius modified 5BA cell with electromagnetic low field dipoles B1 and B2, and a permanent magnet dipole BC with a 3.2 tesla high field peak in the middle. The matching sections consist of quadrupole doublets in the high betax sections and quadrupole triplets in low betax sections. (Online version in colour.)

The lattice has two types of straight sections for insertion devices, 7 m long high horizontal beta sections and 6 m long low horizontal beta sections. The vertical beta is always low in both types of section. A quadrupole doublet is used to match the arcs to the high beta sections whereas a quadrupole triplet is used for the low beta ones. In the low beta sections, we have simultaneously focused both the x and y beta functions to 1.5 m at the centre of the straight section to improve the phase-space matching of the electron beam to the photon beam from undulators [5]. The optics is fivefold symmetric with 15 low and 5 high beta sections.

In addition to improving the electron–photon phase-space matching, the low beta sections have also a reduced horizontal beam-stay-clear, allowing for the installation of small horizontal as well as vertical gap insertion devices. With this capability, Sirius will be one of the first storage rings to extensively use Delta type undulators.

The optical functions in the arc have mirror symmetry about the arc centre and either a low or a high beta straight section can be matched to the arc depending on whether a quadrupole triplet or doublet is used. The Sirius lattice functions are shown in figure 6.

Figure 6.

The lattice functions for one Sirius 5BA cell with 1/2 high beta straight to the left and 1/2 low beta straight to the right. The optical functions in the arc are mirror symmetric. One machine period consists of one high beta and three low beta straights. (Online version in colour.)

Out of a total of 20 straight sections, 17 will be fully available for insertion devices, two will be used for machine installations and one will be shared between a small ID and machine installations. There will also be 20 superbend dipole sources available for beamlines.

6. Low beta sections: phase-space matching

Extracting the highest possible coherence fraction from the emitted radiation is one of the main goals in fourth-generation storage rings. Although a higher brightness and coherence is usually associated with the reduction of the electron beam emittance, this is not the only optimization parameter. The phase-space distribution of the photons emitted by a single electron and the phase-distribution of the electron beam in the storage ring must be matched to minimize the effective emittance, resulting from the convolution between both phase-spaces. The highest brightness from an undulator of length L is achieved when the beta function β = L/π [6]. For typical undulator lengths, this requires both, x and y, betatron functions simultaneously low.

The low beta sections also allow for simultaneous horizontal and vertical small gap insertion devices to be placed in the centre of the section. Besides that, low betatron functions are also beneficial from the ID and impedance perturbations point of view.

7. Beam stability and sirius subsystems

The small beam sizes that will be provided by the fourth-generation light sources make stability for users one of the most important source quality parameters. Achieving the desired stability in beam position and angle around 5% of the beam size and divergence requires integrated state-of-the-art design of most subsystems, including the machine floor, the magnet alignment and support system, the beam position monitor mechanics and instrumentation, the stabilizing feedback systems, among others.

In this way, a stability task force to design a better approach for Sirius has been set. Starting with the slab, two different types of prototypes were built, measured and modelled, leading to a final slab configuration shown in figure 7.

Figure 7.

Final slab configuration for Sirius building. (Online version in colour.)

The accelerators and the main tunnel sit on a 90 cm thick concrete floor placed on top of a 3 m thick compacted soil–cement layered structure to attenuate vibrations. Underneath the accelerators, 1500 piles, from 10 to 15 m deep, will provide long-term stability to the soil due to the cut and fill operation that was required for the Sirius site.

To achieve very high stiffness from floor to magnets in order to minimize the transmission of vibrations, an integrated optimization approach has been adopted leading to smaller magnets with lower centre of mass that are positioned by reference surfaces on the relatively short girders. The welded steel girders are aligned through especially designed high rigidity levelling units, placed on high compressive strength concrete blocks fixed to the ground by 5 GPa elastic modulus resin. As a result, the modelled very high first horizontal and vertical resonant modes of 150 Hz and 260 Hz, respectively, were confirmed by measurements. Figure 8 shows the final configuration for the magnet support and alignment system.

Figure 8.

Integrated solution for Sirius girders with magnets aligned by construction, high rigidity levelling units and concrete blocks. The lowest vibration modes for the whole system reached 152 Hz and 268 Hz in the horizontal and vertical planes, respectively, confirmed with measurements. (Online version in colour.)

8. Vacuum system

The strong field gradients and very compact lattices required by MBA low emittance machines imply using small aperture magnets and, as a consequence, small vacuum chambers, making proper pumping more difficult than previous-generation machines. In the case of Sirius, the initially planned stainless steel chambers for the old design were replaced by copper chambers in the new version due to the higher heat and electrical conductivity of copper. The vacuum system was perhaps the subsystem requiring more drastic changes with the Sirius design upgrade from a third- to a fourth-generation machine.

A distributed pumping approach based on non-evaporable getter (NEG) coating was adopted after the in-house development of the NEG coating technique, developed at CERN and licensed to LNLS. About 99% of the 518 m storage ring circumference is NEG coated with in-house coating. A great effort was dedicated to R&D on all phases of this technique, from copper cleaning procedures, to coating of complex geometry chambers. NEG activation is also an issue. In order to be activated the coated chambers have to be heated up to a temperature close to 200°C for around 24 h. Since the chambers cannot be vented after activation, this procedure clearly represents a challenge due to the small free space between the chamber and the magnet poles. An in situ bake-out approach for NEG activation was developed at LNLS by using a thin polyimide heater jacket with 0.4 mm thickness.

Besides the pumping issue, the vacuum chambers and components contribute to the machine impedance and thus can affect the machine performance by limiting the beam intensity. It is important that the vacuum components (e. g. flanges, bellows, BPMs, pumping ports) be designed to minimize their contribution to the machine's impedance. Each vacuum component in Sirius was carefully studied to minimize machine impedance. Examples range from the zero-impedance flange to the BPM button optimization to reduce the beam loss factor.

Data accessibility

This article has no additional data.

Authors' contributions

L.L. drafted the text and coordinated Sirius lattice design and optics calculations; A.R.D.R. revised the text and coordinated Sirius engineering activities; R.T.N. presented the work at UK50SR, revised the text and co-coordinated Sirius engineering activities.

Competing interests

We declare we have no competing interests.

Funding

The Sirius Project is funded by the Brazilian Ministry of Science, Technology, Innovation and Communication (MCTIC).