Effects of the frequency detuning in Raman backscattering of infinitely long laser pulses in plasmas

Raman backscattering (RBS) in an infinite homogeneous laser-plasma system was investigated with the three-wave fluid model and averaged particle-in-cell (aPIC) simulations in the nonrelativistic and low temperature regime. It was found that the periodic boundary condition for the electrostatic potential, which is commonly used in an infinite homogeneous plasma, induces a numerical frequency shift of the plasma wave. The initial frequency detuning between the three waves is modified by the frequency shift, leading to a significantly wrong result in the RBS system. An alternative boundary condition based on the Maxwell equation is presented. The aPIC simulations with the modified boundary condition show that the pump depletion level depends sensitively on the frequency mismatch between the three waves. This sensitivity is closely related with the erroneous RBS: the numerical frequency shift is very minor (a few percent of the plasma frequency or less than that) but RBS can be greatly affected even by such a small frequency change. Analytic formulas for the pump depletion time and level is derived and compared to the aPIC simulations with the modified boundary condition, showing an excellent agreement. (c) 2006 American Institute of Physics.