  We analyze the outcoupling of a matter wave into a guide by a time-dependent
spilling of the atoms from an initially trapped Bose-Einstein condensate. This
process yields intrinsically a breakdown of the adiabatic condition that
triggers the outcoupling of the wave function. Our analysis of the
time-dependent engineering and manipulation of condensates in momentum space in
this context enables to work out the limits due to interactions in the mode
quality of a guided atom laser. This study is consistent with recent
experimental observations of low transverse excitations of guided atom lasers
and suggests (i) an optimal strategy to realize such quasi-monomode guided atom
lasers with, in addition, the lowest possible longitudinal velocity dispersion,
or alternatively (ii) a strategy for engineering the atomic flux of the atom
laser.
