High-order wave mixing in beam--plasma instabilities

When two waves, ω₁ and ω₁+Δω, are externally driven in an unstable beam-plasma system, they are found to couple in the nonlinear regime to produce up to ten waves, with frequencies given by ω₁±nΔω and amplitudes comparable to the driver amplitudes. This situation is described in terms of third- and higher-order wave-interaction processes. It is suggested that this phenomenon is responsible for nonlinear broadening of the amplified noise spectrum.