Validity of the weak-coupling approximation in strong plasma turbulence

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Abstract
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 ≪Tt 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 εNL =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 γNL, provided Δv≫vth.

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