  Electron beam quality is paramount for X-ray pulse production in
free-electron-lasers (FELs). State-of-the-art linear accelerators (linacs) can
deliver multi-GeV electron beams with sufficient quality for hard X-ray-FELs,
albeit requiring km-scale setups, whereas plasma-based accelerators can produce
multi-GeV electron beams on metre-scale distances, and begin to reach beam
qualities sufficient for EUV FELs. We show, that electron beams from plasma
photocathodes many orders of magnitude brighter than state-of-the-art can be
generated in plasma wakefield accelerators (PWFA), and then extracted,
captured, transported and injected into undulators without quality loss. These
ultrabright, sub-femtosecond electron beams can drive hard X-FELs near the cold
beam limit to generate coherent X-ray pulses of attosecond-Angstrom class,
reaching saturation after only 10 metres of undulator. This plasma-X-FEL opens
pathways for novel photon science capabilities, such as unperturbed observation
of electronic motion inside atoms at their natural time and length scale, and
towards higher photon energies.
