Résumé

L’avenir de FAIR se dessine

Une cérémonie a eu lieu le 4 juillet pour marquer le début des travaux de construction de l’installation européenne de recherche sur les antiprotons et les ions (FAIR) auprès du laboratoire GSI de Darmstadt (Allemagne). FAIR fournira à plusieurs expériences différentes des faisceaux d’une vaste gamme d’intensités et d’énergies constitués de particules allant des protons aux ions uranium. L’installation fournira aussi des faisceaux secondaires d’antiprotons et d’isotopes rares. Le programme scientifique de FAIR comprend la physique des hadrons, la physique atomique et la physique des plasmas, la recherche sur les matériaux et la biophysique des rayonnements. Ce programme, qui s’appuie sur les fondamentaux du laboratoire GSI, devrait débuter en 2025.

Supernova explosions, neutron-star mergers and rare radioactive ions might not seem to have much connection to terrestrial matters. Yet, while the lightest elements were synthesised immediately after the Big Bang, and elements up to iron were created in stellar cores, all of the heavy elements beyond gold and platinum were produced via complex production paths during extreme astrophysical events. Experiments with intense heavy-ion beams produced at the international Facility for Antiproton and Ion Research (FAIR), which is under construction at Darmstadt in Germany, promise new and detailed insights into the nuclear reactions and rare radioactive ion species that underpin the synthesis of heavy elements in the universe.

FAIR is a multipurpose accelerator facility that will provide beams, from protons up to uranium ions, with a wide range of intensities and energies, in addition to secondary beams of antiprotons and rare isotopes. Complementary to CERN’s Large Hadron Collider or Super Proton Synchrotron, FAIR is pushing the intensity rather than the energy frontier for hadron beams. It will enable scientists to produce and study reactions involving rare exotic hadronic states or rare, very short-lived radioactive nuclei. It will enable the investigation of processes under the extreme temperatures and pressures that prevail in large planets, stars and stellar explosions. FAIR will also allow physicists to produce and study dense hadronic matter and its transition to quark matter, and permit tests of quantum electrodynamics in the regime of very strong electromagnetic fields, to name but a few goals.

Overall, FAIR’s scientific programme comprises hadron physics, nuclear structure and astrophysics, atomic physics, plasma physics, materials research, and radiation biophysics and its applications in cancer therapy and space research. Its science is divided between four main pillars (see panel "FAIR's four scientific pillars" below), including experiments similar in design to those in high-energy physics. After a lengthy and complex phase of development, a groundbreaking ceremony held on 4 July 2017 marked the start of construction of the FAIR facility.

Project evolution

FAIR was developed by the international science community and the GSI laboratory (the Helmholtz Centre for Heavy Ion Research) around the turn of the millennium. GSI, founded in 1969, has a long tradition in nuclear and atomic physics and, more generally, heavy-ion research, and was therefore a natural site on which to develop the next generation of accelerators and experiments for these fields. The initial start date of FAIR was 7 October 2010, when nine partner countries (Finland, France, Germany, India, Poland, Romania, Russia, Slovenia and Sweden) signed an intergovernmental agreement for its construction and operation. The UK joined FAIR as an associate member in 2013.

During late 2014, the then FAIR management reported difficulties surrounding new construction requirements. Although not unusual for a complex, one-of-a-kind facility such as FAIR, this caused major modifications of the civil-construction design and resulted in a delay and cost increase of the overall project. In September 2015 the FAIR Council, representing the nine shareholders, unanimously agreed to adapt the FAIR construction budget and timeline according to the necessary design modifications.

Following this key decision, FAIR was completely reorganised and consolidated: the FAIR and GSI GmbH companies aligned their managerial and administrative structures and processes, and a joint management team was installed in a stepwise process, with former spokesperson for the ALICE experiment at CERN, Paolo Giubellino, appointed as scientific managing director and spokesperson for FAIR-GSI in January 2017. Thanks to these and other changes, civil-construction work for the tunnel that will house FAIR’s main accelerator began on schedule this summer, with the goal to finish all FAIR buildings by the end of 2022. In parallel, procurement of the FAIR accelerator systems and construction of the FAIR detector instrumentation is progressing well. Following the installation and commissioning of the accelerators and experiments starting in 2020/2021, the FAIR science programme is expected to start operation in 2025.

A journey through FAIR

The FAIR accelerator complex is optimised to deliver intense and energetic beams of particles to different production targets. The resulting beams will then be steered to various fixed-target experiments or injected into storage-cooler rings for novel in-ring experiments with beams of secondary antiprotons or radioactive ions at the highest beam qualities. The central machines of FAIR are: the fast-ramping SIS100 synchrotron, which provides intense primary beams; the large-aperture Super Fragment Separator (Super-FRS), which filters out the exotic ion beams; and the cooler storage rings CR and HESR (see image). The SIS100 is the heart of FAIR. With a circumference of 1.1 km and a maximum magnetic bending power of 100 Tm, the machine will accelerate ion beams with maximum intensities ranging from 4 × 1013 protons at 29 GeV to 5 × 1011 uranium (28+) ions at 2.7 GeV/u. The existing GSI accelerators UNILAC and SIS18 will serve as injectors and pre-accelerators for SIS100, while a new proton linac will be installed for high-intensity injection into the SIS18/SIS100 synchrotron chain.

To maximise the luminosity of the SIS100, fast-ramped superconducting superferric magnets with a maximum field of 1.9 T and ramp rates up to 4 T per second have been developed to enable cycle times of the same order as the cooling rates in the storage rings (see image). Together with the upgraded SIS18 pre-accelerator, the SIS100 will provide uranium ion beams 10 times more intense than previously available beams at GSI. The cold machine design has a further advantage: the SIS100 beam pipe enables heavy residual gas components to be pumped, potentially stabilising the dynamic pressure. Due to the tight beam-loss budget, the iron yoke of the superconducting magnets must be built with the highest precision and reproducibility. Production has already started for the SIS100 dipole magnets, and the first beams from SIS100 are foreseen for 2025. Three test facilities at GSI, JINR/Dubna and CERN have been established to assess the different types of superconducting magnets.

Two production targets for rare isotope and antiproton beams will be served by the SIS100. A primary ion beam can either be slowly extracted to the Super-FRS over a period of many seconds to produce radioactive secondary beams for fixed-target experiments, or it can be extracted quickly in the form of a single, compressed, short bunch to produce a secondary beam of antiprotons or exotic ions. The in-flight-generated rare isotopes, produced via projectile fragmentation of all primary beams up to uranium-238 or alternatively via fission of uranium-238 beams, are efficiently separated in the large aperture of the Super-FRS. Due to the large acceptance of this machine, the gain in primary-beam intensities for uranium ions in the SIS100 translates into a factor of more than 1000 for secondary-beam intensities of rare, radioactive isotopes.

After production and separation, the hot secondary ion beams drive three experimental scenarios: they can be stopped to allow studies of their ground-state properties; used in in-flight and secondary reactions to produce even more exotic species; or stored and pre-cooled in the collector ring (CR). The fast stochastic cooling process in the CR relies on a fast de-bunching of the injected short bunch. Pre-cooled secondaries will then be transferred from the CR to the high-energy storage ring (HESR), where they can be accumulated and accelerated up to an energy of 15 GeV for antiprotons and about 5–6 GeV/u for very heavy ions. The HESR can also store and cool stable high-charge-state heavy-ion beams, directly injected from the SIS100 via the CR, for precision studies in atomic, nuclear and fundamental physics, such as tests of quantum electrodynamics (QED) in strong fields or tests of special relativity.

FAIR science ahead

About 3000 scientists including more than 500 PhD students from around the world will carry out experiments at FAIR to understand the fundamental structure of matter, explore its exotic forms, and to understand how the universe evolved from its primordial state. FAIR’s science programme is structured into four pillars and organised in four large collaborations with several hundred members each: APPA, serving communities in atomic, plasma physics and applications; CBM, the Compressed Baryonic Matter experiment; NUSTAR, the NUclear STructure, Astrophysics and Reactions programme; and PANDA (antiProton ANihilation in DArmstadt), which aims to study hadrons using antiproton beams. APPA and NUSTAR consist of several sub-collaborations, while CBM and PANDA are rather monolithic experiments involving large detectors (see panel on previous page).

Well before the start of the SIS100 operation in 2025, an upgrade of the GSI accelerators due for completion this year will allow extensive testing of FAIR components. This upgrade will also allow researchers to trial novel FAIR instrumentation for an attractive intermediate research programme, named FAIR phase 0. For instance, the NUSTAR “R3B” spectrometer, the CRYRING and the HITRAP facility will be available and will enable, in combination with the intensity-upgraded SIS18 synchrotron and GSI’s fragment separator, novel experiments in nuclear structure and reactions in so far unexplored areas of the nuclear chart.

The CRYRING and the HITRAP facility will enable physicists to further increase the precision of both atomic-physics measurements of QED effects in highly charged heavy ions and of measurements of fundamental constants. Moreover, the hadronic-matter experimental programme of HADES (High Acceptance Di-Electron Spectrometer) will benefit from the higher intensities from the SIS18. HADES is a versatile detector for the study of dielectron (e+e) and hadron production in heavy-ion collisions, as well as in proton- and pion-induced reactions in the energy range of 1–4 GeV. These are just a few examples from the intermediate research programme, which will start in 2018 and offer about three months of beam time per year, thereby bridging the gap until the commissioning of the SIS100.

The FAIR phase 0 programme intends to maintain and further establish the FAIR-GSI community by offering attractive science before the full complex is up and running. It will also educate and train the next generation of scientists and engineers for FAIR and, last but not least, maintain and extend the technical skills required to operate such a large accelerator complex. While FAIR phase 0 is an important and necessary step offering new and excellent research opportunities for users, full exploitation of the unique science potential opened up by FAIR has to await the start of SIS100 operation in 2025.

Depending on how rich the scientific harvest from FAIR will be and in which specific directions it will be most prominent, one can conceive of several upgrade options. One is a further increase of intensities by up to two orders of magnitude for nuclear structure, reactions and astrophysics, which will also benefit dense-plasma research. Another option is a further increase of beam energy by a factor of 3–6 for hadron- and quark-matter research. Other upgrade possibilities include strengthening the antiproton research programme, via cooled low-energy antiproton beams, for the study of fundamental interactions and symmetries. FAIR is expected to be the flagship facility for hadron, nuclear and atomic physics – as well as related science fields exploiting intense beams of antiprotons and heavy ions – until around 2040.

FAIR’s four scientific pillars

APPA

Atomic and Plasma Physics, and Applied sciences (APPA)
With about 700 participants, APPA is an umbrella for several sub-collaborations working across atomic physics, plasma physics and applied sciences, with specific programmes in biophysics, medical physics and materials science. Several experimental stations, in addition to the CRYRING and HESR storage rings and the trapping facility HITRAP, will allow the APPA community to tackle a variety of challenges. In atomic physics, for example, high-precision tests of bound-state QED in the non-perturbative regime become possible. A precise determination of fundamental constants such as the fine-structure constant is also a target, which involves very precise measurements of the bound-state g-factors in medium to high-Z hydrogen-like ions confined in a trap. Plasma physicists will be able to create and probe dense plasmas to test models of planetary and stellar structure. By means of FAIR beams, the high-energy component of galactic cosmic radiation can also be simulated to assess the risk of space missions for astronauts and electronic equipment by dedicated irradiation experiments. Finally, the material science and geoscience communities will be able to test how materials respond to the simultaneous application of irradiation and pressure, which is of interest for the synthesis of new materials from highly non-equilibrium conditions and for understanding processes in the Earth’s mantle.

CBM

The Compressed Baryonic Matter experiment (CBM) 
The CBM experiment, which has more than 500 participants and is organised similarly to the LHC experiments at CERN, will use high-energy nucleus–nucleus collisions to investigate highly compressed nuclear matter. The fixed-target experiment is 10 m long and comprises a large-aperture superconducting dipole magnet and seven subsequent detector systems providing tracking and particle identification. CBM collisions will recreate the matter densities found in supernova explosions, the cores of neutron stars and neutron-star mergers. In contrast to the very high temperatures and low net-baryon densities reached at the Relativistic Heavy Ion Collider in Brookhaven and the LHC at CERN (conditions that are similar to the conditions that prevailed microseconds after the Big Bang), the energies of the FAIR beams are perfectly suited to study the QCD phase diagram of strongly interacting matter at large net baryon densities and low temperatures. Here, it is expected that the QCD phase diagram exhibits a rich structure such as a critical point, a first-order phase transition between hadronic and partonic matter, or new phases such as quarkyonic matter. Discovering these landmarks would be a breakthrough in our understanding of the strong interaction. The CBM experiment is designed to run at interaction rates of up to 10 MHz, which is 3–4 orders of magnitude higher than the rates reached in other high-energy heavy-ion experiments. It has very fast and radiation-hard detectors, a novel data read-out and analysis concept, and a high-performance computing cluster for online event reconstruction and selection.

PANDA

The PANDA experiment 
The antiProton ANihilation in DArmstadt (PANDA) collaboration is a co-operation of more than 400 scientists from 19 countries, similar to but smaller than the LHC experiments at CERN. Its goal is to understand hadrons using the power of an antiproton beam on fixed hydrogen or other nuclear targets. Antiproton–proton annihilations have enormous advantages compared to proton–proton collisions, such as small momentum-transfer at maximum released energy with well-defined initial states and high-precision mass scanning. The vast difference in mass between the proton and its individual quark constituents is a result of the binding among quarks in the confinement regime, and exotic hadrons such as tetra- and pentaquarks, hybrids and glueballs will reveal uncharted properties of this binding. PANDA will use proton form-factor measurements, deep virtual Compton scattering and quark dynamics, as well as the behaviour of hadrons inside nuclear media, as highly complementary tools with which to understand the very nature of hadrons. Strange quarks in hyperons, for instance, can be used as tags to trace quark dynamics with very high cross-sections and spin degrees of freedom. The PANDA experiment features a modern multipurpose detector with excellent tracking, calorimetry and particle-identification capabilities. Together with the high-quality antiproton beam at FAIR’s high-energy storage ring (HESR), an unprecedented annihilation rate and sophisticated event filtering, it will be ideally suited to address important questions in all aspects of this field.

NUSTAR

NUclear STructure, Astrophysics and Reactions (NUSTAR) 
The NUSTAR collaboration at FAIR has more than 800 participants from 180 institutes located in 38 countries. Similar to APPA, NUSTAR does not represent a single monolithic experiment but is structured in several sub-collaborations across different experimental set-ups tailored to various aspects of secondary radioactive ions, such as mass and lifetime measurements. A major goal of NUSTAR is to improve our knowledge of the synthesis and abundance of chemical elements, for which the collaboration will explore the structure and reaction properties of very rare radioactive ions produced for the first time by FAIR. Although much has been learnt about the behaviour of stable and unstable nuclei in past decades, we are still far from understanding how the very heavy elements are formed through reactions involving rare nuclei at the limit of stability. FAIR will allow scientists to artificially produce the nuclei that occur as radioactive intermediate products in the formation of stable isotopes, measuring directly in the laboratory the different processes involved. FAIR offers unique tools for such studies. The Super-FRS will make very efficient use of the highly intense beams at high energies to separate beams of the heaviest and most neutron-rich nuclei, while FAIR’s complex network of storage rings will allow mass and lifetime measurements. This will place NUSTAR at the forefront of this branch of science. Many of NUSTAR’s experimental set-ups are already complete, and the collaboration plans to transfer them into the new buildings starting from 2023.