Subattosecond keV beats of the high-harmonic x-ray field produced with few-cycle mid-IR laser pulses: Magnetic-field effects

Using the theoretical description beyond the dipole approximation, we examine the impact of the electron magnetic drift caused by a strong midinfrared laser field on the feasibility and ultimate limitations of the method proposed recently [C. Hernandez-Garcia et al., Phys. Rev. Lett. 111, 033002 (2013)] as a route to the generation of zeptosecond x-ray waveforms; this method relies on the interference of high-harmonic emission from multiple reencounters of the electron wave packet with the ion. We show that the electron magnetic drift serves as the spectral filter changing the relative weights of the contributions to the high-harmonic signal from different rescattering events. For a range of driving wavelengths in the midinfrared, the use of the control of the carrier-envelope phase, occasionally in combination with the spectral filtering, to cope with the magnetic drift effect is shown to facilitate the production of intense high-contrast keV beats of durations shorter than 0.8 attosecond. The limitations on the laser wavelengths usable for implementing this approach are determined by the growing unamendable imbalance between the contributions of interfering paths and by an overall decline in the efficiency of high-harmonic generation at longer driving wavelengths.