We study the excitation of electron currents in a transparent cell of sub-millimeter size filled by an atomic gas and illuminated by an intense two-color femtosecond laser pulse. The pulse consists of a strong fundamental component and its second harmonic of low intensity, both circularly polarized. We show that for sufficiently small $20mu$m cells the plasma oscillation excited by asymmetric ionization is almost spatially homogeneous within the interaction volume. This coherent dipole plasma oscillation results in a remarkably efficient conversion of the electron energy into that of radiation emitted in the terahertz frequency domain. Simultaneously, strong quasi-static electric fields of maximal strength $E_msimeq 10$MV/cm are shown to exist inside the cell during several hundred femtoseconds after the ionizing two-color laser pulse has gone.
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