Enhanced Beat Wave Saturation Amplitude in an Ionizing Plasma

The excitation of a plasma wave by two laser beams, whose frequency difference is near the plasma frequency, is studied in a plasma with a density that is slowly increasing with time due to ongoing ionization as appropriate for experiments done in laser breakdown plasmas. Numerical integration of the relativistic equation for the evolution of the wave amplitude reveals that for a rate of increase of the plasma density of approximately 1017 cm-3/ns at a laser intensity I = 1014 W/cm2, the wave amplitude can rise considerably above the relativistic saturation limit of Rosenbluth and Liu which was obtained for a plasma of constant density. This increase in plasma density compensates the reduction in plasma frequency caused by the relativistic electron mass increase when the wave amplitude is large. The frequency and phase excursions of the plasma wave are reduced for an optimum time increasing density. We find that moderate damping can stabilize both the amplitude and the phase of the plasma wave with respect to the pump.