Investigation of stimulated brillouin scattering under well-defined interaction conditions

Abstract

The spatial and temporal development of the SBS ion wave was investigated extensively by ruby-laser light scattering techniques using a picosecond streak-camera for recording. The measurements performed for various levels of peak backscattering provide the ion wave energy density as a function of space, time and backscatter level, i.e. peak power density of CO₂ laser radiation focussed into an underdense and homogeneous target plasma of large extent. In an attempt to understand the various experimental aspects, numerical solutions of respective theories were compared with observations. Whilst for backscatter levels below 5% the three-wave description of Forslund et al. [11] does suffice, it took an extensive review of nonlinear mechanisms to pin down harmonic production of the ion wave according to Karttunen and Salomaa [23] as the process governing SBS behaviour above 5% up to the Manley-Rowe limit. The corresponding system of four-wave equations is capable to explain reasonably well all the aspects observed; in particular, it shows, how it comes about that the dangerous Manley-Rowe limit is reached already at moderate power densities below 10¹³ W/cm² such as in [1]. From this description, it is also evident that — by contrast to many other aspects of laser fusion — this is an effect which becomes the more serious theshorter the laser wavelength is.