Application of Thomson scattering to identify ion acoustic waves stimulated by the Langmuir decay instability

In this article, we present how ion acoustic waves can be unambiguously ide ntified and can be used to definitely demonstrate the existence of the Lang muir decay instability (LDI) of electron plasma waves associated with stimu lated Raman scattering. This observation is very important in the context o f inertial confinement fusion because the LDI is a mechanism that may influ ence the stimulated Raman scattering saturation and growth. The experimenta l identification of those waves relies on Thomson scattering of a short wav elength probe beam. The Thomson scattered light was analyzed by two sets of spectrometer and streak camera providing information on the density fluctu ations associated with the electron plasma waves and ion acoustic waves wit h temporal and spectral resolutions. Selection of a narrow range of angles (4 degrees of aperture) inside the probe and scattered light apertures prov ided necessary resolution in wave vector. The method used for the design of the Thomson scattering diagnostic is described as well as the technique us ed to clearly identify the observed ion acoustic waves as one of the Langmu ir decay instability products. (C) 2000 American Institute of Physics. [S00 34-6748(00)04809-7].