  Attosecond pulses created by high-order harmonic generation in gases often
exhibit strong chromatic aberrations, arising from the broad bandwidth and
wavelength-dependent nonlinear light-matter interaction. When the driving laser
intensity varies spatially, as for Gaussian driving beams, the apparent source
position of the harmonics differs significantly from one order to the next,
thus affecting the achievable intensity and duration of the attosecond pulses
when they are focused on a target. We show that these chromatic aberrations can
be reduced by spatially shaping the fundamental beam to generate high-order
harmonics with a driver having a flat-top profile inside the gas medium. By
measuring both the intensity profile and wavefront for each harmonic in a
plane, we access the extreme ultra-violet (XUV) beam properties and investigate
these properties near focus. We observe that controlling chromatic aberrations
by flat-top spatial shaping strongly reduces the variation of the XUV spectrum
on the beam axis during propagation and, in return, the longitudinal
sensitivity of both the temporal profiles and the temporal shifts of the
focused attosecond pulses.
