Nonlinear Interaction of Helicons (Whistlers) in Inhomogeneous Media

The nonlinear interaction of helicons (whistlers) is found to depend radically on whether the plasma has diffuse or sharp boundaries. The conventional picture of three-wave interaction obtained by perturbing the normal-mode amplitudes is valid for the diffuse case. However, for a plasma with sharp boundaries, e.g., a solid-state plasma including collisions, surface currents play an important role in the limit of large Ω_τ, where Ω is the electron gyrofrequency and τ is the relaxation time. It is shown that the normal-mode method breaks down even at very small amplitudes. A new strong-coupling method is developed which predicts an interaction rate in the large-Ω_τ limit that never exceeds the order of the wave linear damping, but should be experimentally observable.