Validity of the weak-coupling approximation in strong plasma turbulence

The weak-coupling approximation takes stochastic perturbations of orbits into account, but neglects a part of mode coupling. This approximation, which is non-trivial, is discussed here. Its domain of validity appears to be T _E ^T _t provided the turbulence is strong enough so that γNL- γL ñ γL. If γL a stronger condition is required. It is shown that the growth rate γNL can be calculated either from the nonlinear dispersion equation ɛNL =0, or from the energy equation, provided thepropagator conserves particles. Dupree's propagator U does not do this; but it gives a correct approximation of γ_NL, provided Δ_vv_th.