Abstract
A stimulated Raman scattering (SRS) is a great concern for laser fusion, causing an energy and symmetry loss and target preheating. In particular, for dominating backward- SRS (BRS), a complexity of nonlinear saturation was revealed. Recently, to develop lasers at multi-exawatts and beyond, relevant to high-energy physics, a proposed Raman amplification based on BRS met a restrictive operation window. Here, we stress BRS generics that due to a nonlinear frequency shift (relativistic/trapping effect), it nonlinearly saturates with intermittent pulsations. A “break up” of Manley-Rowe invariants explains a non-steady saturation. Further, a coherent pulsation BRS regime is proposed for femto-sec pulse generation in a thin exploding foil plasma, with parameter scalings investigated by analytics and particle simulations.
Highlights
Nonlinear laser plasma instabilites are useful paradigms of complex phenomena
In a standard 3-coupled mode model of backward stimulated Raman scattering (BRS) we outline the condition for an absolute instability [2, 6], that is readily satisfied in a uniform plasma which amplifies large Raman signals from a background noise
Large coherent pulsation regime is proposed for efficient femto-sec optical pulse generation by BRS in thin foil plasmas [9], with scalings studied by analytics and particle simulations
Summary
Nonlinear laser plasma instabilites are useful paradigms of complex phenomena. In laser fusion, backward stimulated Raman scattering (BRS) in an underdense plasma results in a reduced coupling of laser energy to the target [1,2,3]. In a standard 3-coupled mode model of BRS we outline the condition for an absolute instability [2, 6], that is readily satisfied in a uniform plasma which amplifies large Raman signals from a background noise. Due to a nonlinear frequency shift (in EPW) [7,8,9], instead to a steady-state via pump depletion, BRS saturates through intermittent pulsations with reflectivity bursts and spectral broadening [2, 6].
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