  Plasma accelerators can generate ultra high brightness electron beams which
open the door to light sources with smaller physical footprint and properties
un-achievable with conventional accelerator technology. In this paper we show
that electron beams from Plasma WakeField Accelerators (PWFAs) can generate
few-cycle coherent tunable soft X-ray pulses with TW peak power and a duration
of tens of attoseconds, an order of magnitude more powerful, shorter and with
better stability than state-of-the-art X-ray Free Electron Lasers (XFELs). Such
a light source would significantly enhance the ability to experimentally
investigate electron dynamics on ultrafast timescales, having broad-ranging
impact across multiple scientific fields. Rather than starting from noise as in
typical XFELs, the X-rays emission in this approach is driven by coherent
radiation from a pre-bunched, near Mega Ampere (MA) current electron beam of
attosecond duration. This relaxes the restrictive tolerances which have
hindered progress towards utilizing plasma accelerators as coherent X-ray
drivers thus far, presenting a new paradigm for advanced-accelerator light
source applications.
