The new PACRI (Plasma Accelerator Systems for Compact Research Infrastructure) project has recently been awarded €10 million in funding from the European Commission to develop highly important and ground-breaking plasma accelerator technologies, significantly reducing the size, energy consumption, environmental impact and cost of future facilities.
These developments will enable the production of unique particle and photon beams with a wide range of applications, from ultrafast science, high-precision medical imaging, materials diagnostics and medical treatment. The project aims to extend the reach and applications of these plasma technologies in domains ranging from manufacturing to healthcare, serving the needs of hospitals and universities as well as museums and industries. In the long run, they will also support developments towards future compact particle colliders.
PACRI is a collaborative effort involving 19 universities and international research laboratories, supported by seven industrial partners. It is due to launch in March 2025 and will run for four years until 2029.
The project has been designed specifically to complement two existing European research infrastructures, EuPRAXIA and the Extreme Light Infrastructure, ELI-ERIC, facilities. The goal of EuPRAXIA is to establish the world’s first high energy plasma-based accelerator that can provide industrial beam quality and user areas. ELI-ERIC is the world's largest and most advanced high-power laser infrastructure. Both these ambitious infrastructures still require innovative new technologies and advancements in order to maximise their potential – hence the creation of the PACRI project.
Plasma acceleration
Using plasma to generate an accelerating field helps overcome limitations due to damage issues associated with metallic or dielectric structures, which can occur due to breakdowns encountered in high-gradient operations. Plasma accelerators have been tested with active lengths ranging from the millimetre to the metre scale and accelerating gradients higher than 10 GV/m have been demonstrated in experiments. This technology, when fully exploited, will considerably reduce the size and cost of particle accelerators, thus opening a wider range of accelerator-based applications. With a smaller footprint and associated reduced need for concrete shielding afforded by the large acceleration gradient, plasma-based facilities show good promise to reach a higher sustainability level than any other acceleration schemes.
In this context, the EuPRAXIA project will implement a plasma-based accelerator infrastructure by the end of 2029. The project will serve users in ultra-fast science with X-rays for high-resolution medical imaging, with deeply penetrating positron annihilation spectroscopy for materials and with Europe's most southern Free Electron Laser in Frascati (Rome). EuPRAXIA will unite efforts from the accelerator, plasma, and laser research communities, in collaboration with European industry, to make plasma accelerators accessible to users and wider applications. In the short term EuPRAXIA is also expected to deliver betatron X-rays to users from the medical area.
A potential limitation of a widespread adoption of the EuPRAXIA concept is its relatively low repetition rate, which is currently restricted to 10 Hz (laser wakefield accelerators - LWFA) or 100 Hz (plasma wakefield accelerators - PWFA).
The work foreseen in PACRI plays a crucial role in complementing the EuPRAXIA construction project by further advancing the Plasma Accelerating Modules with high repetition rates and strengthening the technologies required to drive the plasma, such as RF linear accelerator (linac) and lasers, with higher repetition rates and improved efficiency.
The development of high-repetition-rate plasma targets has only recently begun. Although no fundamental limitations have been identified so far for achieving high repetition rates, potentially up to the kHz range, realizing this goal will require a more systematic approach and significant breakthroughs in the engineering of plasma targets. To this end, PACRI plans to approach this problem with an intense R&D programme that includes the development of new simulation tools, the design of plasma targets exploiting different materials, and the associated development of diagnostic tools.
The project intends to explore the different target systems common in LWFAs and PWFAs like capillary discharges, gas jets, gas cells, and plasma waveguides and address challenges for their development for high repetition rate and high average power operation.
A comprehensive suite of numerical instruments will also be developed to characterise plasma and beam dynamics, along with AI algorithms for real-time machine control and optimisation. By combining novel hardware and software solutions a superior type of virtual diagnostics will be produced, making it possible to infer beam properties such as 4D emittance, beam coupling and energy spread.
Normal conducting RF accelerator technology
The factor that greatly impacts on the energy gain of a linac, as well as its efficiency, energy consumption and cost, is the technology adopted for the accelerating structures and the RF power generator.
Most of the existing facilities use S-band (3 GHz) or C-band (6 GHz) structures, given the maturity of these technologies. However, although consolidated, these technologies are not optimal. A higher frequency accelerating structure, i.e., at X-band (12 GHz), can achieve much higher gradients with lower power requirements than those produced by lower frequency structures.
High-frequency X-band structures can also run at lower gradients and high repetition rate, up to the kHz regime, enabling a new set of operational scenarios for many applications ranging from synchrotron light, basic research, medicine, industry, etc.
One of the specific objectives of PACRI is to extend the repetition rate of normal conducting (NC) linacs to the kHz range, by developing and testing an RF power plant, based on a high efficiency klystron and a solid state modulator, capable of operating up to 1 kHz.
High-power laser technology
The use of lasers as power drivers for plasma accelerators has been emerging dramatically in the past decades, not only for the well-known effectiveness in exciting and driving plasma waves, but also for the fast-developing technologies that lead to the continuous improvement of lasers in terms of performance and specifications. Beyond that, entirely new technologies are also maturing, with specifications significantly more advanced than those of the lasers originally used in the context of pioneering laser-driven acceleration experiments, opening up the perspective for the delivery of viable drivers for industrial grade plasma accelerators.
Currently, available off-the-shelf technology is mainly based on flashlamp pumping and limited to a few tens of watts average power and Hz-level repetition rate. Remarkably, new approaches based on robust solid-state diode laser pumping technologies are already emerging as powerful alternative solutions to overcome such limitations.
Planned laser-based plasma accelerator infrastructures, like the laser-driven pillar of EuPRAXIA, rely on multi-kW average power, petawatt peak power, ultra-short pulse laser systems with ultra-short pulse duration. Scaling of existing systems to kW average power still requires innovative solutions, including the transition from a flashlamp pumping to the efficient, fully diode pumping, and sustainable thermal management in both the amplifier and the whole transport chain, from the compressor to the target plasma.
As shown in the figure 2, these specifications are beyond the current state of the art and require dedicated effort to overcome existing technological bottlenecks. In view of these required developments, PACRI is bringing together leading laboratories in high power laser development and industrial partners, all of which have a track record of successful collaboration. This new consortium will focus on five main building blocks of laser drivers for plasma acceleration, from high repetition rate titanium sapphire amplifiers to novel laser architecture for fully diode pumped kHz systems, through innovation in laser-diode based pump sources.
The accomplishment of these objectives will provide technical solutions for forthcoming laser-based infrastructures (e.g., EuPRAXIA) and strengthen the international competitiveness of existing laser-based infrastructures (ELI-ERIC) over the coming years. At the same time, the laser demonstrators are expected to solve crucial bottlenecks towards the development of next generation high average power, ultrashort pulse lasers. These hold great promise to finally unlock several important societal applications and provide unprecedented market perspectives.
The PACRI project has received funding from the European Union’s Horizon Europe Research and Innovation programme under Grant Agreement No 101188004.