We present half-cycle cutoff features in the water window using high-order-harmonic generation (HHG) with two-cycle, carrier-envelope-phase-controlled pulses at 1.85 μm in a high-pressure neon target. A relative carrier-envelope-phase (CEP) shift of the acquired spectra at different pressures is related to an in situ slip of the CEP in the HHG target. A simple model based on the pressure-dependent dispersion of the target gas reveals the importance of the free electrons for this effect. Our analysis also reveals that the effect is relaxed at high target pressures due to plasma defocusing of the driving pulse. We show that as a consequence of the CEP slip mediated by free electrons, moderate-intensity fluctuations can have a detrimental influence on the generation and application of attosecond pulses. Our findings emphasize the potential of combining high target pressures and long driver wavelengths to reveal subtle details of the complex light-matter interaction of HHG.

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