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The Crossed Beam Energy Transfer (CBET) of two large laser beams is modeled using two approaches: (i) the time-independent Paraxial Complex Geometrical Optics (PCGO) for stochastically distributed Gaussian-shaped beamlets and (ii) the time-dependent conventional paraxial propagation of smoothed laser beams. Each description of the laser propagation is coupled to a hydrodynamics code. Both approaches are compared in a well-defined plasma configuration with density- and velocity- profiles corresponding to an inhomogeneous plasma, including a resonance zone in which the matching conditions for a resonant coupling between the two laser beams are fulfilled. The comparison is made for laser beams smoothed by random phase plates and for “regular beams” without speckles. The role of the laser speckles is also investigated for each approach. In general, a good agreement is found between the PCGO simulations and the fully time-dependent paraxial-type simulations, carried out with the code Harmony, past a transient period on the picosecond time scale. The PCGO-based CBET model is applied to the hydrodynamics simulations of a CBET experiment, the results of which reproduce essential features of the experimental data. Based on these comparisons, performed for interaction parameters up to 2×1014W cm−2μm2, the PCGO approach proves to be a reliable method to be implemented in the hydrodynamics codes to describe the CBET in mm-scale plasmas.

We investigate the nonlinear evolution of the backward stimulated Raman scattering (BSRS) in the regime where the nonlinear saturation mechanism is the Langmuir decay instability resulting from the coupling of the BSRS-generated Langmuir wave with the ion acoustic waves. We present numerical results obtained with a fluid-type code in one- and two-dimensional spatial dimensions, in the case of an inhomogeneous plasma. The plasma density is under quarter-critical and depends linearly on the longitudinal spatial coordinate, in the regime where the Rosenbluth gain factor for the amplitude, denoted as G(Ros), is in the range pi/2< or =G_(Ros)< or =6. We observe that the Langmuir decay instability is able to suppress the gradient stabilization and restore the absolute nature of BSRS, thus leading to a significantly increased BSRS reflectivity.

A study of the secondary instability of a finite-amplitude ion acoustic wave (IAW) affecting the saturation of stimulated Brillouin backscattering (SBS) of laser light in a plasma is presented. The secondary instability of the SBS IAW provides a nonlinear dissipation mechanism for the SBS IAW and can reduce the SBS reflectivity. To better understand the physics of the secondary instability and SBS, particle-in-cell kinetic simulations, analysis of dispersion relations, and integration of coupled mode equations have been undertaken and compared. Among the effects examined are the influences on the secondary instability of the frequency of the primary IAW, the second-harmonic IAW, and ion trapping in the primary SBS IAW which affects Landau damping and the IAW frequency.

The excitation of absolute instabilities in three-wave parametric couplin g processes in inhomogeneous plasmas has been studied analytically . A detailed examination of the full fourth-order set of differential equations describing such processes in infinite media confirms the WKB-criterion that absolute instabilities arise only if . A model calculation using the WKB-approximation has also been carried out to study the effects of finite plasma extent and pump depletion; conditions for the excitation of absolute instabilities in this case have also been obtained.

This paper is concerned with the role of self-consistency of the electric field in 1-0 plasma turbulence. We first show that in the non-self consistent electric field problem excellent agreement is found between numerical experiments and quasilinear theory whenever the imposed electric field Fourier components have random phase. A discrepancy is exhibited between quasilinear prediction and numerical simulations in the self-consistent electric field case. This discrepancy is explained by the creation of a long correlation time of the electric field resulting from a strong wave-particle interaction. A comparison is made between quasilinear and renormalized propagator theories, and the Dupree “Clump” theory. These three theories are found to be self-contradictory in the regime of strong wave-particle interaction because they make an a priori quasi-gaussian assumption for the electric field. The Direct Interaction Approximation is then shown to be a closure taking into account in a self-consistent way nongaussian effects.

Numerical simulations of the weak beam-plasma instability were done in the turbulent regime where small-scale trapping is a dominant feature of the instability, a regime with behavior not predicted by quasi-linear theory. The results of the simulations were compared with those of a specific model of the turbulence, the so-called 'turbulent trapping' model, which gives precise formulas for the particle correlation functions, and predicts a growth rate well enhanced over the quasi-linear value. It was found that the model gives accurate predictions for the correlation functions. On the other hand, while growth rates were enhanced over the quasi-linear values, the enhancements observed are smaller than expected from the quantitative predictions of the model.

It is shown, using weak turbulence theory, that Langmuir waves are stabilized by a nonresonant quasilinear scattering of Langmuir waves by ion‐sound waves in the presence of a background of developing ion‐sound turbulence. This effect, which prevents the conversion of ion‐sound turbulence into Langmuir turbulence in this transient regime, was ignored in previous theories. The destabilizing effect of the so‐called ‘‘induced bremmsstrahlung’’ of Langmuir waves as proposed by Nambu is furthermore shown to vanish exactly. The need for a renormalized theory in the case of steady‐state ion‐sound turbulence is discussed.

The weak coupling approximation of Kadomtsev is applied to Vlasov turbulence and shown to lead straightforwardly to the equations of the direct interaction approximation. This application of the weak coupling approximation was apparently incorrectly carried out in Kadomtsev’s book. The method has the advantage of simplicity and makes contact with other more conventional approximations.

Mode‐coupling terms yield non‐negligible corrections to the quasilinear terms in the evolution of one‐dimensional Langmuir turbulence generated by the weak warm beam instability.