The growth and saturation of stimulated brillouin scattering in a CO2 laser-produced plasma

• Main Author: Bernard, John Edward
• Language: English
• Published: University of British Columbia 2010
• Subjects: Brillouin scattering
• Online Access: http://hdl.handle.net/2429/25551

The growth and saturation characteristics of stimulated Brillouin scattering (SBS) in the interaction of an intense (Ｉ≤ 10¹³ W/cm²) CO₂ laser beam with an underdense plasma are investigated experimentally. The plasma is produced by focussing a short (2 ns FWHM), CO₂ laser pulse onto a stabilized nitrogen gas jet which flows from a Laval nozzle into low pressure helium. The resulting SBS interaction is studied through observations of the intensity and spectral behavior of the backscattered light as well as through temporally resolved ruby laser Thomson scattering measurements of the spatial and spectral behavior of the SBS generated ion acoustic waves. SBS occurs primarily in the long scale length, low density plasma located in the background gas in front of the jet. Initially, the instability grows absolutely at a rate within a factor of two of the predicted temporal growth rate. The SBS reflectivity is observed to saturate at less than 10%. This low reflectivity is a result of two processes. First, the SBS interaction region and the associated ion acoustic waves are broken up into several smaller regions, hence limiting the coherence length of the waves, and second, the ion acoustic fluctuation amplitude saturates at less than 20%. The latter saturation is attributed to trapping of ions within the potential troughs of the ion acoustic waves. The observed occurrence of the first harmonic in the ion acoustic wave spectrum as well as temporal modulations in the wave amplitude and sidebands in the spectrum of the backscattered light can be explained as consequences of the ion trapping. === Science, Faculty of === Physics and Astronomy, Department of === Graduate