Collisional regimes of radiation-driven Langmuir turbulence

Numerical solutions of the Zakharov equations for a plasma driven above the electron plasma frequency by a long-wavelength radiation pump can be applied to both ionospheric modification experiments and laboratory laser-plasma interactions. A key difference between these two environments is the much larger collisional damping of Langmuir waves near the critical density in laser plasmas. Zakharov equation simulations in one and two dimensions reveal a significant change in the character of the saturated turbulence state of the electromagnetic ion-acoustic decay instability for pump strengths near threshold as the collisional damping is increased to values appropriate to certain (low-intensity) laboratory laser-plasma experiments [Phys. Fluids B 3, 1983 (1991)]. The linear-instability is then characterized by the coupling of the up- and downshifted Langmuir decay modes. A new turbulence regime differing from existing models of both weak and strong turbulence is found, which is characterized by a sequence of narrow peaks in the Langmuir frequency spectrum and a nonlinearly broadened wave-vector spectrum centered near the linearly most unstable modes. Results in this regime may be relevant to second harmonic emission experiments [Phys. Fluids B 3, 1983 (1991)]