Discreteness of the backscattered Stimulated Raman Scattering spectrum from a plasma density ramp.*

We are investigating the fundamental aspects of the light scattered by the Stimulated Raman BackScatter (SRBS) instability occurring in a plasma density ramp driven by a picosecond, long-wavelength laser.  

To generate this instability, we employed a 2 ps FWHM (~60 laser cycles) CO₂ laser with a range of laser normalized vector potentials (a_o) varying from 0.4 to 1.2. We used an imaging spectrometer to observe the plasma source emitting region, while also spectrally dispersing the daughter electromagnetic wave between (10 – 16 µm) 0.92 – 0.58 k_s/k_o. The SRBS instability was excited in a plasma density ramps of L_n: (100 – 1000)*λ_o over a density range of 0 – 0.2 n_cr. These experiments were conducted with a picosecond laser because hydrodynamic expansion of the plasma is small during the passage of the laser pulse, but the relatively high normalized vector potential amplifies SRBS over spatially long distances,±z_T = 2γ_o/κ'√(v_g,sv_g,EPW).

It is normally assumed that because ω & k matching conditions are satisfied at all densities below quarter critical, the backscattered SRBS instability spectrum emanating from a density ramp should be continuous. In our experiments this was not found to be the case. The observed spectrum had multiple discrete spectral peaks separated from each other by a density increment deduced from the sum of the interaction lengths of two adjacent peaks. The interaction lengths are consistent with Rosenbluth’s analytical equation of amplification that is solved using the WKB approximation. This analysis conducts an experimental evaluation of Rosenbluth’s work for parameters well above threshold and provides a new view to our understanding of how SRBS is amplified.