  Using the slab plasma as a paradigm model, we have derived analytically
equations for the nonlinear generation of zero-frequency zonal flows by
electron drift waves including, on the same footing, both the beat-driven and
spontaneous excitations. It is found that the beat-driven zonal flow tends to
reduce the frequency mismatch between the electron drift waves and, thereby,
contributes to a significant O(1) enhancement of the modulational instability
drive and lowering its threshold. Implications to tokamaks plasmas as well as
drift-wave soliton formation are also discussed.
