Stimulated Raman Side Scattering in Laser Wakefield Acceleration

Stimulated Raman side scattering of an ultrashort high power laser pulse is studied in experiments on laser wakefield acceleration. Experiments and simulations reveal that stimulated Raman side scattering occurs at the beginning of the interaction, that it contributes to the evolution of the pulse prior to wakefield formation, and also that it affects the quality of electron beams generated. The relativistic shift of the plasma frequency is measured.

Figure 1

(a) and (d) Interferogram, (c) and (f) electron spectrum, (b), (e), and (g) laser mode images. (h) Vector diagram of scattering. Sets of images (a)–(c) and (d)–(f) are obtained for a single shot of laser power at 26 TW and 95 TW, respectively. The plasma density is 2.5 and $2.9\times {10}^{19}{\mathrm{cm}}^{-3}$. The spatial scale in (a) is 0.5 mm per division. In (d), the laser propagation axis ($k_0$: laser wave vector) is shown by the black solid arrow, and the ionized region due to side scattered light showing maximum and minimum scattering angles ($\varphi$ measured from $k_0$) is indicated by $k_{s1}$ and $k_{s2}$, respectively. The intensity scale of laser mode images is normalized to peak intensity for each image. (b) and (e) show mode images at the exit of the gas jet, while (g) shows the focal spot in vacuum. The intensity of image (c) is multiplied by 10.

Figure 2

Plasma density dependence of scattered light angle, $\varphi$, from (diamonds) 30 TW shots and (squares) 100 TW shots. The lines are from Eq. (1) at (solid) $a_0=0$, (dash-dotted), $a_0=1.2$, and (dashed) $a_0=2.1$. Nonzero $a_0$ values are consistent with vacuum focus diameter for laser power of 30 and 100 TW. The error bar corresponds to variation of $k_0$ including focusing cone angle, i.e., 3.8°.

Figure 3

Spatially resolved spectrum of side scattering light. (a) $a_0=4.8$, $\tau =35\mathrm{fs}$, $E=2.3\mathrm{J}$. (b) $a_0=1.3$, $\tau =170\mathrm{fs}$, $E=0.96\mathrm{J}$. Here $E$ is laser energy. The vertical axis is the spatial axis. The laser pulse propagates from top to bottom. SRSS satellites’ lines are overlaid on images as indicated. Emission around 800 nm in (b) is due to the probe pulse and is too weak to measure in (a). (c) Frequency shift as a function of laser intensity. Open (solid) squares are experimental (simulation) data points. The solid curve is ${\omega }_{\mathrm{pr}}$. Error bars are due to density gradients within the observation window. Each data point is an average of 1–6 shots.

Figure 4

Electron density profiles from OSIRIS simulation of a 42 fs pulse with $a_0=1.5$. (a) $t=330\mathrm{fs}$, (b) $t=660\mathrm{fs}$, (c) $t=1000\mathrm{fs}$. The dashed line and the arrow show wave front and $k_p$ of the plasma wave. Insets are magnified by twice. Two dashed lines in (a) and (b) have the same $\theta =63°$, which is consistent with the value obtained from Eq. (1).