Convective threshold of stimulated Brillouin scattering in a two-ion plasma

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Abstract
The convective threshold of stimulated Brillouin backscatter (SBS) in a laser-irradiated plasma containing two distinct ion species is investigated over a wide range of plasma conditions and laser intensities. This investigation, carried out in the context of kinetic theory, is a generalization of a previous investigation of ion acoustic waves in a two-ion plasma, i.e., the unpumped system. The totality of SBS modes in the two-ion system is obtained using a numerical sweep through the complex-ω plane. The coupling of the electromagnetic pump wave to the plasma produces only one mode, in addition to the ion acoustic modes. Because this additional mode is not a normal mode of the plasma in the absence of the pump wave, it is called a quasimode. The SBS modes fall into three classes: (a) at least one and, at most, two modes that are weakly damped for sufficiently low laser intensity, reducing to the weakly damped ion acoustic modes as the pump strength vanishes; (b) an infinity of critically damped modes, most of which are relatively insensitive to the laser intensity; and (c) a single mode (quasimode), which may exhibit either growth or damping as laser intensity increases. The threshold laser intensity for convective SBS growth, the smallest intensity, where one of the modes ceases to be damped, ωI=0, may be associated with modes of classes (a), (b), or (c). The present study shows that the growing mode is not necessarily associated with the SBS modes obtained from fluid theory. A practical numerical procedure is presented for quickly determining the threshold intensity for SBS in a two-ion system for a given set of plasma parameters. The method is quite efficient and may be readily incorporated into large-scale hydrodynamic simulation codes to provide an assessment of SBS growth over the numerical mesh of the computation.