Coherent attosecond x-ray pulses from beam-driven relativistic plasma mirrors

We propose a compact source of coherent attosecond x-ray pulses based on laser reflection from nonlinear plasma waves driven by relativistic charged particle beams. Analytical theory and particle-in-cell simulations show that such beams can excite strongly nonlinear plasma waves acting as relativistic mirrors. The reflection of millijoule-level laser pulses from such mirrors in micron-scale plasmas can produce coherent few-attosecond x-ray pulses with pulse duration, peak intensity, and spectral brightness potentially surpassing those of state-of-the-art coherent x-ray sources, all within a significantly reduced footprint. Laser intensification by up to 2 orders of magnitude is demonstrated. The resulting coherent attosecond pulses open avenues for probing ultrafast dynamics of atoms, molecules, and even electrons, enabling transformative applications across atomic physics, chemistry, materials science, and structural biology.