Observation of Stimulated Electron-Acoustic-Wave Scattering

Abstract

A diffraction-limited laser interacts with a plasma whose conditions are uniform on the scale of the focused laser spot. Two distinct, narrow waves are observed in the backscattered spectrum with phase velocities of ${v}_{\ensuremath{\varphi}}{/v}_{e}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.4\ifmmode\pm\else\textpm\fi{}0.08$ and $4.2\ifmmode\pm\else\textpm\fi{}0.1$, where ${v}_{e}$ is the electron thermal speed. The high-velocity wave is ordinary stimulated Raman scattering (SRS) from a Langmuir wave. The low-velocity wave corresponds to stimulated scattering from an electron-acoustic wave (SEAS), and implies strong electron trapping. Previous SRS data from low-density plasmas are reinterpreted in terms of SEAS.