Laser-driven high-harmonic generation provides the only demonstrated route to generating stable, tabletop attosecond
x-ray pulses but has low flux compared to other x-ray technologies. We show that high-harmonic generation can produce
higher photon energies and flux by using higher laser intensities than are typical, strongly ionizing the medium and
creating plasma that reshapes the driving laser field. We obtain high harmonics capable of supporting attosecond
pulses up to photon energies of 600 eV and a photon flux inside the water window (284 to 540 eV) 10 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 longerwavelength driving fields.