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.

We report the demonstration of a diode-pumped chirped pulse amplification Yb:YAG laser that produces λ=1.03  μm pulses of up to 1.5 J energy compressible to sub-5 ps duration at a repetition rate of 500 Hz (750 W average power). Amplification to high energy takes place in cryogenically cooled Yb:YAG active mirrors designed for kilowatt average power laser operation. This compact laser system will enable new advances in high-average-power ultrashort-pulse lasers and high-repetition-rate tabletop soft x-ray lasers. As a first application, the laser was used to pump a 400 Hz λ=18.9  nm laser.

We demonstrate a significant extension of the high-order harmonic cutoff by using a fully-ionized capillary discharge plasma as the generation medium. The preionized plasma dramatically reduces ionization-induced defocusing and energy loss of the driving laser due to ionization. This allows for significantly higher photon energies, up to 150 eV, to be generated from xenon ions, compared with the 70 eV observed previously. We also demonstrate enhancement of the harmonic flux of nearly 2 orders of magnitude at photon energies around 90 eV when the capillary discharge is used to ionize xenon, compared with harmonic generation in a hollow waveguide. The use of a plasma as a medium for highorder harmonic generation shows great promise for extending efficient harmonic generation to much shorter wavelengths using ions. High-order harmonic generation (HHG) coherently upconverts laser light from the visible and infrared into the extreme-ultraviolet region of the spectrum. Over the past decade, HHG has been demonstrated as a useful light source for a wide range of applications, such as investigating surface dynamics [1], holographic imaging [2], and more recently for probing static molecular structure [3,4], or internal molecular dynamics. In HHG, the nonlinear interaction between a material, typically a gas [5,6], and an intense laser field produces high-order harmonics of the fundamental laser. The laser field first ionizes the atom or molecule, then accelerates the liberated electron away from the ion, finally generating high-order harmonic photons when the laser field reverses and the oscillating electron recollides with its parent ion. The highest photon energy that can be produced via this interaction is predicted [7,8] by the cutoff rule to be h max I p 3:17U p , where I p is the ionization potential of the atom and U p / I L 2 is the ponderomotive energy of the electron in the laser field. Here I L is the peak laser intensity and is the wavelength of the driving laser field.From the cutoff rule, the range of photon energies that are generated in HHG is determined by the laser intensity. However, in most experiments to date, the maximum observed HHG photon energy has been limited not by the available laser intensity, but by the intensity at which the target atoms are nearly completely ( 98%) ionized -or the ''saturation intensity'' I s (< I L ) of the medium. This is because at near full ionization in a medium, ionizationinduced refraction [9,10] of the laser beam reduces the effective laser intensity compared with what could be obtained in a vacuum. Moreover, reduced coherence lengths also limit the number of atoms contributing to the harmonic emission. To obtain the highest possible photon energy, I s can be increased either by using a shorter duration laser pulse, or by using atoms with a higherionization potential, both of which allow neutral atoms to survive to a higher laser intensity. Partial phase matching of the harmonic emission in a plasma can also be implemented to increase the harmonic output. Recently...

We demonstrate the operation of a gain-saturated table-top soft x-ray laser at 100 Hz repetition rate. The laser generates an average power of 0.15 mW at λ=18.9  nm, the highest laser power reported to date from a sub-20-nm wavelength compact source. Picosecond laser pulses of 1.5 μJ energy were produced at λ=18.9  nm by amplification in a Mo plasma created by tailoring the temporal intensity profile of single pump pulses with 1 J energy produced by a diode-pumped chirped pulse amplification Yb:YAG laser. Lasing was also obtained in the 13.9 nm line of Ni-like Ag. These results increase by an order of magnitude the repetition rate of plasma-based soft x-ray lasers opening the path to milliwatt average power table-top lasers at sub-20 nm wavelengths.

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