  The generation of high power coherent soft X-ray pulses of sub-100 as
duration and 10 nm wavelength using beams from a GeV energy plasma wakefield
accelerator has been recently investigated in Ref. [arXiv:2011.07163]. As a
future upgrade to this concept, this contribution investigates scaling to
shorter X-ray wavelengths by cascading undulators tuned to higher harmonics of
the fundamental. We present two simulation studies for plasma-driven attosecond
harmonic generation schemes with final photon wavelengths of 2 nm and 0.40 nm.
We demonstrate in these schemes that using undulators with retuned fundamental
frequencies can produce GW-scale pulses of sub-nm radiation with tens of
attosecond-scale pulse lengths, an order of magnitude shorter than current
state-of-the-art attosecond XFELs. This multi-pulse multi-color operation will
be broadly applicable to attosecond pump-probe experiments.
