Effect of counterstreaming ion flow on ion acoustic modes and stimulated Brillouin scattered light

The stimulated Brillouin scattering spectrum from a plasma is determined by the ion acoustic normal mode frequencies, which are dependent on the ion temperature and flow velocity. Thus these quantities are indirectly measured by the scattered light. The frequencies of ion acoustic normal modes and stimulated Brillouin scattered light are calculated for a model plasma formed by collisionlessly interpenetrating ion beams by assuming the ion–ion two‐stream instability is marginally stable. The spectrum differs measurably from the spectrum expected from the hot, stationary plasma created by collisional stagnation. Comparison of the scattered light spectra for these two limiting cases suggests that careful observation of the scattered light near the laser frequency is a potentially useful diagnostic of the collisionality of interpenetrating ion beams.