  We propose a marginally stable optical resonator suitable for atom
interferometry. The resonator geometry is based on two flat mirrors at the
focal planes of a lens that produces the large beam waist required to
coherently manipulate cold atomic ensembles. Optical gains of about 100 are
achievable using optics with part-per-thousand losses. The resulting power
build-up will allow for enhanced coherent manipulation of the atomic
wavepackets such as large separation beamsplitters. We study the effect of
longitudinal misalignments and assess the robustness of the resonator in terms
of intensity and phase profiles of the intra-cavity field. We also study how to
implement atom interferometry based on Large Momentum Transfer Bragg
diffraction in such a cavity.
