Direct observation of the two-plasmon-decay common plasma wave using ultraviolet Thomson scattering

A 263-nm Thomson-scattering beam was used to directly probe two-plasmon-decay (TPD) excited electron plasma waves (EPWs) driven by between two and five 351-nm beams on the OMEGA Laser System. The amplitude of these waves was nearly independent of the number of drive beams at constant overlapped intensity, showing that the observed EPWs are common to the multiple beams. In an experimental configuration where the Thomson-scattering diagnostic was not wave matched to the common TPD EPWs, a broad spectrum of TPD-driven EPWs was observed, indicative of nonlinear effects associated with TPD saturation. Electron plasma waves corresponding to Langmuir decay of TPD EPWs were observed in both Thomson-scattering spectra, suggesting the Langmuir decay instability as a TPD saturation mechanism. Simulated Thomson-scattering spectra from three-dimensional numerical solutions of the extended Zakharov equations of TPD are in excellent agreement with the experimental spectra and verify the presence of the Langmuir decay instability.

Figure 1

(a) The normalized five-beam common-wave growth rate (color scale) in the Thomson-scattering plane [defined by $\widehat{y}\left|\right|({\widehat{k}}_{4\omega }\times {\widehat{k}}_{\text{s}})$ with the target normal in the $-\widehat{z}$ direction] showing that the five-beam common-wave matching conditions are satisfied only along the $z$ axis. The dashed curves show the linear TPD theory maximum growth for each drive beam. The range of wave vectors probed in the two Thomson diagnostic configurations, common-wave geometry (white box) and non-common-wave geometry (red box). (b) Wave-matching conditions for Thomson scattering (${\mathbf{k}}_{\text{c}}={\mathbf{k}}_{4\omega }-{\mathbf{k}}_{\text{s}}$) from common TPD EPWs, and (c) daughter EPWs from Langmuir decay of backscattered TPD EPWs (${\mathbf{k}}_2^{\prime }={\mathbf{k}}_2-{\mathbf{k}}_{\text{iaw}}={\mathbf{k}}_{4\omega }-{\mathbf{k}}_{\text{s}}$).

Figure 2

Thomson-scattering spectra for scattering from EPWs with dashed lines at wavelengths corresponding to the quarter ($n_{\text{c}}/4$) and fifth ($n_{\text{c}}/5$) critical surfaces. (a) Scattering from thermal EPWs (150 $\mu \mathrm{m}$ from the target surface) generates a broad spectrum corresponding to the range of densities within the Thomson-scattering volume. (b) Scattering spectra from common EPWs (100 $\mu \mathrm{m}$ from the target surface) shows narrow peaks corresponding to locally driven TPD EPWs. (c) Off-hyperbola scattering (100 $\mu \mathrm{m}$ from the target surface) results in a broad spectrum of TPD-driven EPWs. The dip in scattering amplitude at 0.9 ns in all three spectra is caused by a shock [32], reflected from the molybdenum layer, traveling through the Thomson-scattering volume.

Figure 3

Thomson-scattering spectrum measured at $\sim 1$ ns (solid) and simulated (dashed) in the (a) common-wave and (b) non-common-wave Thomson-scattering configurations.

Figure 4

Simulated electron plasma wave spectra for five drive beams (a) during linear TPD growth (1 ps) and (b) after saturation (2 ps), where $\phi$ is the high-frequency (enveloped) potential.