Physicists from across Europe have outlined plans to build a new type of accelerator facility based on “plasma wakefields”. Named the European Plasma Research Accelerator with eXcellence In Applications (EuPRAXIA), the project envisages building high-energy plasma devices on one or possibly two sites In Europe. Its aim is to show how the size and cost of accelerator facilities – particularly those relevant to industry – could be radically reduced compared to ones employing conventional radiofrequency technology.

Plasma wakefield accelerators exploit the extremely high electric-field gradients that are created when laser pulses or charged particles travel at high speed through a plasma. Those gradients, generated by a sharp separation of electrons and ions in a pulse’s wake, can be used to accelerate electrons travelling behind the pulses to very high energies over very short distances.

Researchers have already demonstrated the principle of wakefield acceleration in the lab, having achieved gradients as high as 100 GeVm-1 – compared to the roughly 0.1 GeVm-1 possible with radiofrequency cavities. However, the EuPRAXIA consortium, which consists of researchers from 16 institutes in five European countries, aims to show that the technology can be used by industry and academia to carry out specific tasks.

Direct gains

In a conceptual design report released at the end of October but not yet formally announced, the consortium says that it intends to build plasma accelerators fed by both high-powered lasers and conventional electron accelerators in order to generate high-quality beams of electrons with energies between 1 and 5 GeV. Those beams, it says, could be used in a range of applications including medical imaging, positron generation, material testing and, above all, to power compact free-electron lasers (FELs).

Existing FELs use either copper or superconducting cavities to accelerate electrons to high energies, with magnets then forcing the particles around sharp bends so that they emit exceptionally bright flashes of X-rays. But the world’s leading devices – located in Europe, the US and Asia – are hundreds of metres if not several kilometers long. The much higher gradients possible with plasma accelerators could potentially make FELs small enough to fit in the grounds of a hospital, for example.

Physicist Carsten Welsch from the University of Liverpool, who is EuPRAXIA’s head of communications, adds that existing FELs are heavily over-subscribed. “If you had a facility that could produce similar quality laser beams but with a smaller footprint there would be a direct gain,” he says.

In its report, the consortium puts forward several scenarios for the new facility along with their associated costs. The most expensive option, at €320m, involves building laser-driven and electron-driven accelerators on separate sites, complete with a number of applications as well as an extensive programme of laser R&D. The construction of a single type of accelerator combined with a FEL, however, comes in at €68m if fed by electrons and €75m if a laser is used.

Duel sites

The consortium explains that it originally intended to propose a design for a plasma accelerator facility independent of any site. In the end, however, it endorsed a specific location for the electron-driven accelerator – the Frascati National Laboratory outside Rome. Researchers there have already embarked on a programme to build a plasma device that doubles the energy provided by advanced radiofrequency cavities. Read more Consortium sets out to build European laser plasma accelerator

In contrast, the report lists four possible sites for a laser-driven accelerator. Two of these are in Italy – the Frascati lab and a branch of the National Optics Institute in Pisa – while the others are the ELI Beamlines Lasers centre near Prague in the Czech Republic and the Rutherford Appleton Laboratory in Oxfordshire, UK.

According to Welsch, if EuPRAXIA gets the go-ahead it should be providing beams to users within about 10 years. Given how long it will take to build and commission the accelerators, he says that a site decision “would need to be taken definitely in the next five years, and probably in the next two to three”.

As for using the facility to accelerate electrons for particle-physics experiments, according to Welsch the energies involved will be too low to make significant discoveries. “At least for the initial ten-year timeframe these applications wouldn’t be possible,” he says.