A roadmap for strategic pilot experiments at the first plasma-driven x-ray FEL

Abstract Plasma-acceleration technology offers a route to drastically reduce the size and cost of x-ray free-electron lasers (FELs), potentially enabling a broad diffusion of FEL facilities worldwide and a scientific impact comparable to, or even exceeding, that associated with the advent of third-generation synchrotron radiation sources three decades ago. However, in order to compete with conventional FELs based on high-quality relativistic electron bunches with low energy spread and high repetition rates, substantial advances in plasma-acceleration technique are still required. This Perspective analyzes the classes of experiments that could best exploit the characteristics of the forthcoming EuPRAXIA@SPARC_LAB facility, set to be the first plasma wakefield acceleration soft x-ray FEL, during its initial operational phase. The transformative potential of this new light source as well as its role in the photon science community are strictly related to the adopted experimental strategy.