In certain ionospheric and laboratory parametric-decay-instability experiments, pump-dependent refraction modifies the nature of the instabilities. In the ionosphere, electrostatic waves cna be trapped in the gradients of the high-frequency pump. In the laboratory the refraction, and hence the convective nature of the instability, is determined completely by the pump. These simple examples suggest that pump-energy absorption may be enhanced by tailoring the pump profile to create a nonconvective instability.

• Received 22 May 1972

DOI:https://doi.org/10.1103/PhysRevLett.30.597