Laser driven high harmonic generation provides the only demonstrated route to generate
stable, tabletop attosecond X-ray pulses, but with low flux compared to other X-ray tech-
nologies. Here we show that higher photon energies and flux can be obtained from high
harmonic generation by using higher laser intensities than are typical, strongly ionizing the
medium and creating plasma which reshapes the driving laser field. We obtain high harmon-
ics capable of supporting attosecond pulses out to photon energies of 600 eV, and a photon
flux inside the water window (284 eV to 540 eV) ten times higher than previous attosecond
sources. We demonstrate that operating in this regime is key for attosecond pulse generation
in the X-ray range, and will become increasingly important as harmonic generation moves
to even longer wavelength driving fields.