Particle beams are used to explore the fundamental forces of nature, produce known and unknown particles, and generate new forms of matter.

The ESFRI project EuPRAXIA, which aims to create the first dedicated research infrastructure based on cutting-edge plasma-acceleration ideas, is described by Ralph Assmann, Massimo Ferrario, and Carsten Welsch.

Particle beams are used to explore the fundamental forces of nature, produce known and unknown particles, and generate new forms of matter. Photon science also relies on particle beams, which emit pulses of intense synchrotron light.

These 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.

Particle beams for industry and health support many societal applications, such as X-ray inspection of cargo containers, food sterilisation, chip manufacturing, and cancer therapy.

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 TeV energy scales at the Tevatron and the LHC.

New collision schemes were developed, such as the mini “beta squeeze” in the 1970s, which advanced 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.

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.

It is 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.

Plasma acceleration is a new technology proposed in 1979 that promises energy gains up to 100 GeV per metre of acceleration and 1000 times higher than RF accelerators.

The free electrons in a neutral plasma are used to convert the transverse ponderomotive force of a laser or charged particle beam into a longitudinal accelerating field, while the “light” electrons are expelled from the path of the driving force.

The plasma-acceleration scheme is difficult due to the small scales involved, 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 (AWAKE). Gains in electron energy have reached 8 GeV (BELLA, Berkeley), 42 GeV (FFTB, SLAC) and 2 GeV (AWAKE, CERN).

The beam quality of plasma-acceleration schemes has advanced sufficiently to reach the quality required for free-electron lasers (FELs). There have been several reports of free-electron lasing in plasma-based accelerators in recent years.

Scientific and technical progress in plasma accelerators is driven by several dozen groups and a number of major test facilities worldwide.

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.

In 2014, researchers in Europe agreed to set up a combined, coordinated R&D effort to realise a larger plasma-based accelerator facility that serves as a demonstrator.

This project was named the European Plasma Research Accelerator with Excellence in Applications (EuPRAXIA) and it should deliver pulses of X rays, photons, electrons and positrons to users from several disciplines. 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 and culminated in 2019 with the publication of the world’s first conceptual design report for a plasma-accelerator facility. The design included realistic constraints on transfer lines, facility infrastructure, laser-lab space, undulator technologies, user areas and radiation shielding.

Innovative solutions were developed, including the use of magnetic chicanes for high quality, multi-stage plasma accelerators. The 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. This concept will ensure international competitiveness and leverage existing investments in Europe.

The consortium applied to the European Strategy Forum on Research Infrastructures (ESFRI) in 2020 and was included in the 2021 ESFRI roadmap. EuPRAXIA is the first plasma-accelerator project on the ESFRI roadmap and the first accelerator project since the 2016 placement of the High-Luminosity LHC.

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.

The EU, Switzerland and the UK have awarded €3.69 million to the EuPRAXIA preparatory phase, which includes 34 participating institutes from Italy, the Czech Republic, France, Germany, Greece, Hungary, Israel, Portugal, Spain, Switzerland, the UK, the US and CERN.

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.

It will connect research institutions and industry from the above countries plus China, Japan, Poland and Sweden, and define the full implementation of the €569 million EuPRAXIA facility.

The EU and UK have funded a Marie Skodowska-Curie doctoral network to offer 12 high-level fellowships between 10 universities, six research centres and seven industry partners. Italy is supporting the EuPRAXIA advanced photon sources project (EuAPS) with €22 million.

This project will build and commission a distributed user facility providing users with advanced photon sources, including a plasma-based betatron source, a mid-power, high-repetition-rate laser and a high-power laser. R&D activities for the beam-driven facility are currently being performed at the INFN SPARC_LAB laboratory.

The INFN Frascati National Laboratory’s user facility of the future is called EuPRAXIA. In 2024, the location for the second, laser-driven leg of EuPRAXIA will be chosen.

With applications in a variety of fields, including free-electron lasers, compact sources for positron generation and medical imaging, tabletop test beams for particle detectors, and deeply penetrating X-ray and gamma-ray sources for materials testing, it will have an electron energy range of 1–5 GeV. Numerous physicists, engineers, students, and support personnel are responsible for the project’s excellence, inventiveness, and hard work.