The Effects of Weak Turbulence on Parametric Processes in an Unmagnetized Plasma (Decay Instability, Stimulated Scattering, Quasi-Linear Theory).

The object of this dissertation is to determine the effect of weak turbulence on parametric excitations in an unmagnetized plasma; Two Plasmon Decay (TPD), Raman Scattering (RS), Parametric Decay (PD) and Brillouin Scattering (BS). A system of equations describing the parametic process is derived from kinetic equations. The effect of the external radiation is represented as ponderomotive force in the electron kinetic equation. Langmuir and ion-acoustic turbulences are modeled as fluctuating background electric field and plasma density. Langmuir turbulence lowers the effective plasma frequency. The turbulence also may drive an Oscillating Two Stream Instability (OTSI), and may enhance the coupling process of longitudinal waves. Due to the lowering of the plasma frequency, the best matching pump frequency for TPD is lowered. The frequency of the reflected light of RS is increased. The thresholds of PD and RS abruptly drop when OTSI is driven. The threshold of PD is further reduced because of the enhanced coupling. Ion-acoustic turbulence does not interact strongly with ion-acoustic modes of parametric processes. The ion-acoustic turbulence, however, r and omly generates the spatial regions characterized by plasma frequencies ranging from slightly lower to higher than the average. Long wavelength turbulence creates the regions that are best tuned to different pump frequencies for PD. For the whole plasma, the best matching frequency now has a width rather than one value, as in the case of quiescent plasma. The short wavelength turbulence creates these regions within a typical wavelength of an ion-acoustic mode of parametric processes. For PD under the turbulence with strength parameter over the threshold value, the effective pump strength is reduced in half since the pump must excite two separate sideb and plasmons simultaneously. The effect on BS is not strong, since the turbulence does not interact with either the principal mode or the sideb and mode.PhDPlasma physicsUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/160430/1/8502938.pd