A kinetic model is proposed to investigate how application of a uniform external electric field can appreciably increase the 'tail' of the electron distribution function (EDF) in sustained particle-beam-generated plasmas. Such an increase is of interest in laser plasmas for the enhancement of the laser effect directly related to the 'tail' of the EDF through the branching ratios for energy deposition on excited states. Application is made to argon and we have obtained that, for the amplitude of the electric field commonly generated, the enhancement of the laser effect can be of the order of 15 per cent.