When two waves, ${\omega }_1$ and ${\omega }_1+\Delta \omega$, 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 ${\omega }_1\pm n\Delta \omega$ 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.

• Received 13 January 1972

DOI:https://doi.org/10.1103/PhysRevLett.28.662