This report presents the conceptual design of a new European research infrastructure EuPRAXIA. The concept has been established over the last four years in a unique collaboration of 41 laboratories within a Horizon 2020 design study funded by the European Union. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma acceleration concepts and laser technology. It focuses on the development of electron accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in Europe. EuPRAXIA has involved, amongst others, the international laser community and industry to build links and bridges with accelerator science — through realising synergies, identifying disruptive ideas, innovating, and fostering knowledge exchange. The Eu-PRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency-based accelerators. The foreseen electron energy range of one to five gigaelectronvolts (GeV) and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, table-top test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing. EuPRAXIA is designed to be the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier. Consistent with a high-confidence approach, the project includes measures to retire risk by establishing scaled technology demonstrators. This report includes preliminary models for project implementation, cost and schedule that would allow operation of the full Eu-PRAXIA facility within 8—10 years.

Plasma-based technology promises a tremendous reduction in size of accelerators used for research, medical, and industrial applications, making it possible to develop tabletop machines accessible for a broader scientific community. By overcoming current limits of conventional accelerators and pushing particles to larger and larger energies, the availability of strong and tunable focusing optics is mandatory also because plasma-accelerated beams usually have large angular divergences. In this regard, activeplasma lenses represent a compact and affordable tool to generate radially symmetric magnetic fields several orders of magnitude larger than conventional quadrupoles and solenoids. However, it has been recently proved that the focusing can be highly nonlinear and induce a dramatic emittance growth. Here, we present experimental results showing how these nonlinearities can be minimized and lensing improved. These achievements represent a major breakthrough toward the miniaturization of next-generation focusing devices.

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The development of compact accelerator facilities providing high-brightness beams is one of the most challenging tasks in field of next-generation compact and cost affordable particle accelerators, to be used in many fields for industrial, medical and research applications. The ability to shape the beam longitudinal phase-space, in particular, plays a key role to achieve high-peak brightness. Here we present a new approach that allows to tune the longitudinal phase-space of a high-brightness beam by means of a plasma wakefields. The electron beam passing through the plasma drives large wakefields that are used to manipulate the time-energy correlation of particles along the beam itself. We experimentally demonstrate that such solution is highly tunable by simply adjusting the density of the plasma and can be used to imprint or remove any correlation onto the beam. This is a fundamental requirement when dealing with largely time-energy correlated beams coming from future plasma accelerators.High-brightness electron beams are nowadays used for many applications like, for instance, Inverse Compton Scattering [1,2], the generation of THz [3,4], Free Electron Laser (FEL) radiation [5-8] and for new plasmabased acceleration techniques [9][10][11][12]. The generation of such beams always require manipulations of their longitudinal phase-space (LPS) in order to achieve peak currents as large as required by the specific task. The ability to shape the energy and temporal profiles is thus of paramount importance. In FEL facilities, for instance, peak currents of several kA are produced by longitudinally compressing a time-energy correlated (i.e. chirped ) beam in a dispersive magnetic chicane, where the path length is energy dependent [7,13]. The manipulation of the LPS is also a fundamental step in view of the development of new compact machines that exploit advanced acceleration techniques based on plasma wakefields. In this case accelerating fields up to tens of GV/m, ∼ 2−3 orders of magnitude larger than conventional radio-frequency (RF) structures, have been demonstrated allowing to produce GeV level beams in few centimeters [12,[14][15][16]. However, due to the shortness of the accelerating field wavelength a large correlated energy spread is imprinted on the accelerated beam, making difficult to transport the beam using conventional magnetic optics (like solenoids and quadrupoles), due to chromatic effects. In this case, a technique able to remove such an energy-chirp must be foreseen.In this Letter we discuss a new approach that allows to tune the beam LPS by using the wakefields excited in a plasma channel. Other techniques based on the use of metallic [17,18] or dielectric structures [19][20][21] have been also demonstrated. However, in the first case the imprinted energy-chirps cannot exceed few MeV/m while in the second one the tunability is rather limited, depending on the aperture and size of the employed devices. -250 -200 -150 -100 -50 0 50 100 150 z (um) 97.5 98 98.5 99 99.5 100 100.5 Energy (MeV) -60 -50 -...

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