  Coherent soft x-ray (SXR) sources enable fundamental studies in the important
water window spectral region. Until now, such sources have been limited to
repetition rates of 1 kHz or less, which limits count rates and signal-to-noise
ratio for a variety of experiments. SXR generation at high repetition rate has
remained challenging because of the missing high-power mid-infrared (mid-IR)
laser sources to drive the high-harmonic generation (HHG) process. Here we
present a mid-IR optical parametric chirped pulse amplifier (OPCPA) centered at
a wavelength of 2.2 {\mu}m and generating 16.5-fs pulses (2.2 oscillation
cycles of the carrier wave) with 25 W of average power and a peak power
exceeding 14 GW at 100-kHz pulse repetition rate. This corresponds to the
highest reported peak power for high-repetition-rate mid-IR laser systems. The
output of this 2.2-{\mu}m OPCPA system was used to generate a SXR continuum
extending beyond 0.6 keV through HHG in a high-pressure gas cell.
