  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 $20\mu$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 $E_m\simeq 10$MV/cm are shown to exist
inside the cell during several hundred femtoseconds after the ionizing
two-color laser pulse has gone.
