Transient enhancement and detuning of laser-driven parametric instabilities by particle trapping

Kinetic simulations of backward stimulated Raman scattering (BSRS), where t he Langmuir wave coherence time is greater than the bounce time for trapped electrons, yield transient reflectivity levels far above those predicted b y fluidlike models. Electron trapping reduces the Langmuir wave damping and lowers the Langmuir wave frequency, and leads to a secular phase shift bet ween the Langmuir wave and the BSRS beat ponderomotive force. This phase sh ift detunes and saturates BSRS and a similar effect, due to ion trapping, i s the saturation mechanism for backward stimulated Brillouin scattering. Co mpetition with forward SRS is discussed.