Experimental evaluation of the W.K.B.-described spatial extent of Backscattered Stimulated Raman Scattering spectrum from a plasma density ramp.

We present experimental observations, simulations, and theoretical comparison to the W.K.B.-described regions of amplification of the Stimulated Raman Backscattering (SRBS) instability in an underdense plasma from a prescribed density gradient. This instability is driven by a 2 ps FWHM CO₂ laser with a₀ ranging from 0.4 to 1.2. A plasma density profile with scale lengths (L_n: 100λ_o – 1000λ_o) was formed by tunnel ionization a known neutral density gas profile—produced by a supersonic gas jet—by the leading edge of the laser, generating a plasma with a prescribed spatial density profile, n_e(z), spanning the SRBS-relevant range 0 ≤ n_e(z)/n_cr ≤ 0.2.

Contrary to the widely held assumption that the scattered spectrum in such ramps is continuous, due to local satisfaction of the three-wave resonance conditions over the Raman range of plasma densities, we consistently observed (via a direct imaging spectrometer) discrete spectral peaks, within our detection band of the Raman region of 0.92 – 0.58 k_s/k_o. If we assume conservation of energy requires the observed peaks to satisfy the frequency matching condition, ω_pe ≈ ω_EPW = ω_o – ω_s, we can determine the resonant plasma densities. We find that these discrete spectral SRBS events exhibited density differences corresponding to the Rosenbluth W.K.B. described turning-point lengths z_T = 2γ_o/κ'√(ν_g,EPWν_g,s). In other words, no more than one SRBS event occurs in a given amplification length. These findings experimentally validate the spatial extent associated with the W.K.B. determined region of amplification (l_a=2z_T) in the medium-intensity, above-threshold regime; experimentally validating Rosenbluth’s theory of SRBS in an inhomogeneous density plasma.

[1] M. N. Rosenbluth, Phys. Rev. Lett. 29, 565 (1972).