Dependence of stimulated Brillouin scattering on target material and f number

Stimulated Brillouin scattering of CO2 laser radiation from a pre-ionized gas target is observed and studied with the following parameter changes: (1) f/7.5 focusing versus f/2 focusing, (2) nitrogen plasma versus hydrogen plasma, (3) pure nitrogen versus nitrogen with a hydrogen impurity, and (4) single line operation of the laser versus multiline operation. The f-number dependence is found to be weak because the increased intensity with the smaller f number is offset by the shorter interaction length for the Brillouin instability. The instability growth in nitrogen is found to be more severe than in hydrogen because the nitrogen plasma has a higher ZTe/Ti resulting in a lower damping rate for the ion-acoustic wave. In all the above cases the Brillouin reflectivity reaches saturation and the inferred saturation amplitudes of the ion-acoustic waves are consistent with ion trapping being the saturation mechanism. Addition of a small amount (≃6%) of a light ion impurity (hydrogen) to a heavy ion plasma (nitrogen) virtually extinguishes the Brillouin instability. Finally, multiline operation of the CO2 laser pump, with the line separation Δω and the instability growth rate γ such that Δω/γ≫1, is seen to reduce the Brillouin reflectivity as if each line were acting independently, as expected.