Europe targets a user facility for plasma acceleration

Energetic beams of particles are used to explore the fundamental forces of nature, produce known and unknown particles such as the Higgs boson at the LHC, and generate new forms of matter, for example at the future FAIR facility. Photon science also relies on particle beams: electron beams that emit pulses of intense synchrotron light, including soft and hard X-rays, in either circular or linear machines. Such light sources enable time-resolved measurements of biological, chemical and physical structures on the molecular down to the atomic scale, allowing a diverse global community of users to investigate systems ranging from viruses and bacteria to materials science, planetary science, environmental science, nanotechnology and archaeology. Last but not least, particle beams for industry and health support many societal applications ranging from the X-ray inspection of cargo containers to food sterilisation, and from chip manufacturing to cancer therapy. 

This scientific success story has been made possible through a continuous cycle of innovation in the physics and technology of particle accelerators, driven for many decades by exploratory research in nuclear and particle physics. The invention of radio-frequency (RF) technology in the 1920s opened the path to an energy gain of several tens of MeV per metre. Very-high-energy accelerators were constructed with RF technology, entering the GeV and finally the TeV energy scales at the Tevatron and the LHC. New collision schemes were developed, for example the mini “beta squeeze” in the 1970s, advancing luminosity and collision rates by orders of magnitudes. The invention of stochastic cooling at CERN enabled the discovery of the W and Z bosons 40 years ago. 

However, intrinsic technological and conceptual limits mean that the size and cost of RF-based particle accelerators are increasing as researchers seek higher beam energies. Colliders for particle physics have reached a circumference of 27 km at LEP/LHC and close to 100 km for next-generation facilities such as the proposed Future Circular Collider. Machines for photon science, operating in the GeV regime, occupy a footprint of up to several km and the approval of new facilities is becoming limited by physical and financial constraints. As a result, the exponential progress in maximum beam energy that has taken place during the past several decades has started to saturate (see “Levelling off” figure). For photon science, where beam-time on the most powerful facilities is heavily over-subscribed, progress in scientific research and capabilities threatens to become limited by access. It is therefore hoped that the development of innovative and compact accelerator technology will provide a practical path to more research facilities and ultimately to higher beam energies for the same investment. 

At present the most successful new technology relies on the concept of plasma acceleration. Proposed in 1979, this technique promises energy gains up to 100 GeV per metre of acceleration and therefore up to 1000 times higher than is possible in RF accelerators. In essence, the metallic walls of an RF cavity, with their intrinsic field limitations, are replaced by a dynamic and robust plasma structure with very high fields. First, the free electrons in a neutral plasma are used to convert the transverse ponderomotive force of a laser, or the transverse space charge force of a charged particle beam, into a longitudinal accelerating field. While the “light” electrons in the plasma column are expelled from the path of the driving force, the “heavy” plasma ions remain in place. The ions therefore establish a restoring force and re-attract the oscillating plasma electrons. A plasma cavity forms behind the drive pulse in which the main electron beam is placed and accelerated with up to 100 GV per metre. Difficulties in the plasma-acceleration scheme arise from the small scales involved (sub-mm transverse diameter), the required micrometre tolerances and stability. Different concepts include laser-driven plasma wakefield acceleration (LWFA), electron-driven plasma wakefield acceleration (PWFA) and proton-beam-driven plasma wakefield acceleration. Gains in electron energy have reached 8 GeV (BELLA, Berkeley), 42 GeV (FFTB, SLAC) and 2 GeV (AWAKE, CERN) in these three schemes, respectively. 

At the same time, the beam quality of plasma-acceleration schemes has advanced sufficiently to reach the quality required for free-electron lasers (FELs): linac-based facilities that produce extremely brilliant and short pulses of radiation for the study of ultrafast molecular and other processes. There have been several reports of free-electron lasing in plasma-based accelerators in recent years, one relying on LWFA by a team in China and one on PWFA by the EuPRAXIA team in Frascati, Italy. Another publication by a French and German team has recently demonstrated seeding of the FEL process in a LWFA plasma accelerator. 

Scientific and technical progress in plasma accelerators is driven by several dozen groups and a number of major test facilities worldwide, including internationally leading programmes at CERN, STFC, CNRS, DESY, various centres and institutes in the Helmholtz Association, INFN, LBNL, RAL, Shanghai XFEL, SCAPA, SLAC, SPRING-8, Tsinghua University and others. In Europe, the 2020 update of the European strategy for particle physics included plasma accelerators as one of five major themes, and a strategic analysis towards a possible plasma-based collider was published in a 2022 CERN Yellow Report on future accelerator R&D. 

Enter EuPRAXIA

In 2014 researchers in Europe agreed that a combined, coordinated R&D effort should be set up to realise a larger plasma-based accelerator facility that serves as a demonstrator. The project should aim to produce high-quality 5 GeV electron beams via innovative laser- and electron-driven plasma wakefield acceleration, achieving a significant reduction in size and possible savings in cost over state-of-the-art RF accelerators. This project was named the European Plasma Research Accelerator with Excellence in Applications (EuPRAXIA) and it was agreed that it should deliver pulses of X rays, photons, electrons and positrons to users from several disciplines. EuPRAXIA’s beams will mainly serve the fields of structural biology, chemistry, material science, medical imaging, particle-physics detectors and archaeology. It is not a dedicated particle-physics facility but will be an important stepping stone towards any plasma-based collider. 

The EuPRAXIA project started in 2015 with a design study, which was funded under the European Union (EU) Horizon 2020 programme and culminated at the end of 2019 with the publication of the worldwide first conceptual design report for a plasma-accelerator facility. The targets set out in 2014 could all be achieved in the EuPRAXIA conceptual design. In particular, it was shown that sufficiently competitive performances could be reached and that an initial reduction in facility size by a factor of two-to-three is indeed achievable for a 5 GeV plasma-based FEL facility. The published design includes realistic constraints on transfer lines, facility infrastructure, laser-lab space, undulator technologies, user areas and radiation shielding. Several innovative solutions were developed, including the use of magnetic chicanes for high quality, multi-stage plasma accelerators. The EuPRAXIA conceptual design report was submitted to peer review and published in 2020. 

The EuPRAXIA implementation plan proposes a distributed research infrastructure with two construction and user sites and several centres of excellence. The presently foreseen centres, in the Czech Republic, France, Germany, Hungary, Portugal and the UK, will support R&D, prototyping and the construction of machine components for the two user sites. This distributed concept will ensure international competitiveness and leverage existing investments in Europe in an optimal way. Having received official government support from Italy, Portugal, the Czech Republic, Hungary and UK, the consortium applied in 2020 to the European Strategy Forum on Research Infrastructures (ESFRI). The proposed facility for a free-electron laser was then included in the 2021 ESFRI roadmap, which identifies those research facilities of pan-European importance that correspond to the long-term needs of European research communities. EuPRAXIA is the first ever plasma-accelerator project on the ESFRI roadmap and the first accelerator project since the 2016 placement of the High-Luminosity LHC. 

Stepping stones to a user facility 

In 2023 the European plasma-accelerator community received a major impulse for the development of a user-ready plasma-accelerator facility with the funding of several multi-million euro initiatives under the umbrella of the EuPRAXIA project. These are the EuPRAXIA preparatory phase, EuPRAXIA doctoral network and EuPRAXIA advanced photon sources, as well as funding for the construction of one of the EuPRAXIA sites in Frascati, near Rome (see “Frascati future” image). 

The EU, Switzerland and the UK have awarded €3.69 million to the EuPRAXIA preparatory phase, which comprises 34 participating institutes from Italy, the Czech Republic, France, Germany, Greece, Hungary, Israel, Portugal, Spain, Switzerland, the UK, the US and CERN as an international organisation. The new grant will give the consortium a unique chance to prepare the full implementation of EuPRAXIA over the next four years. The project will fund 548 person-months, including additional funding from the UK and Switzerland, and will be supported by an additional 1010 person-months in-kind. The preparatory-phase project will connect research institutions and industry from the above countries plus China, Japan, Poland and Sweden, which signed the EuPRAXIA ESFRI consortium agreement, and define the full implementation of the €569 million EuPRAXIA facility as a new, distributed research infrastructure for Europe. 

Alongside the EuPRAXIA preparatory phase, a new Marie Skłodowska-Curie doctoral network, coordinated by INFN, has also been funded by the EU and the UK. The network, which started in January 2023 and benefits from more than €3.2 million over its four-year duration, will offer 12 high-level fellowships between 10 universities, six research centres and seven industry partners that will carry out an interdisciplinary and cross-sector plasma-accelerator research and training programme. The project’s focus is on scientific and technical innovations, and on boosting the career prospects of its fellows.

EuPRAXIA at Frascati

Italy is supporting the EuPRAXIA advanced photon sources project (EuAPS) with €22 million. This project has been promoted by INFN, CNR and Tor Vergata University of Rome. EuAPS will fulfil some of the scientific goals defined in the EuPRAXIA conceptual design report by building and commissioning a distributed user facility providing users with advanced photon sources; these consist of a plasma-based betatron source delivering soft X-rays, a mid-power, high-repetition-rate laser and a high-power laser. The funding comes in addition to about €120 million for construction of the beam-driven facility and the FEL facility of EuPRAXIA at Frascati. R&D activities for the beam-driven facility are currently being performed at the INFN SPARC_LAB laboratory. 

EuPRAXIA is the first ever plasma-accelerator project on the ESFRI roadmap 

EuPRAXIA will be the user facility of the future for the INFN Frascati National Laboratory. The European site for the second, laser-driven leg of EuPRAXIA will be decided in 2024 as part of the preparatory-phase project. Present candidate sites include ELI-Beamlines in the Czech Republic, the future EPAC facility in the UK and CNR in Italy. With its foreseen electron energy range of 1–5 GeV, the facility will enable applications in diverse domains, for instance, as a compact free-electron laser, compact sources for medical imaging and positron generation, tabletop test beams for particle detectors, and deeply penetrating X-ray and gamma-ray sources for materials testing. The first parts of EuPRAXIA are foreseen to enter into operation in 2028 at Frascati and are designed to be a stepping stone for possible future plasma-based facilities, such as linear colliders at the energy frontier. The project is driven by the excellence, ingenuity and hard work of several hundred physicists, engineers, students and support staff who have worked on EuPRAXIA since 2015, connecting, at present, 54 institutes and industries from 18 countries in Europe, Asia and the US.