  The stability properties of Fourier-based multipass amplifier to
misalignments (tilts) of its optical components has been investigated. For this
purpose, a method to quantify the sensitivity to tilts based on the amplifier
small signal gain has been elaborated and compared with measurements. To
improve on the tilt stability by more than an order of magnitude a simple
auto-alignment system has been proposed and tested. This study, combined with
other investigations devoted to the stability of the output beam to variations
of aperture and thermal lens effects of the active medium, qualifies the
Fourier-based amplifier for the high-energy and the high-power sector.
