The modulational instability of copropagating light waves was studied recently by McKinstrie and Bingham [Phys. Fluids B 1, 230 (1989)], who applied their general theory to the study of the relativistic modulational instability (RMI) of light waves in the beat‐wave accelerator. However, in rarefied plasma, the RMI merges with stimulated Raman forward scattering. The longitudinal RMI is suppressed over most of its expected range and the study of secondary instabilities in the beat‐wave accelerator must be amended accordingly. A preliminary analysis indicates that stimulated Raman backward scattering is likely to be important in current beat‐wave experiments, while near‐resonant stimulated Raman forward scattering and near‐forward stimulated Raman scattering could be important in proposed beat‐wave experiments.
- Raman forward scattering
- relativistic modulational instability
- rarefied plasma