  Extending the photon energy range of bright high-harmonic generation to cover
the entire soft X-ray region is important for many applications in science and
technology. The concept of critical ionization fraction has been essential,
because it dictates the maximum driving laser intensity that can be used while
preserving bright harmonic emission. In this work, we reveal a second,
nonadiabatic critical ionization fraction that substantially extends the
maximum phase-matched high-harmonic photon energy, that arises due to strong
reshaping of the intense driving laser field. We validate this understanding
through a systematic comparison between experiment and theory, for a wide range
of pulse durations and driving laser wavelengths. In particular, high harmonics
driven by intense few-cycle pulses experience the most pronounced spectral
reshaping, significantly extending the bright photon energy range. We also
present an analytical model that predicts the spectral extension that can be
achieved for different driving lasers. This reveals an increasing role of
nonadiabatic critical phase matching when driven by few-cycle mid-infrared
lasers. These findings are important for the development of high-brightness
soft X-ray sources for applications in spectroscopy and imaging.
