Temporal Characterization of a Self-Modulated Laser Wakefield

The temporal envelope of plasma density oscillations in the wake of an intense ( $I\sim 4\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$, $\lambda =1\mathrm{}\mu \mathrm{m}$) laser pulse (400 fs) is measured using forward Thomson scattering from a copropagating, frequency-doubled probe pulse. The wakefield oscillations in a fully ionized helium plasma ( $n_e=3\mathrm{}\times {10}^{19}{\mathrm{cm}}^{-3\mathrm{}}$) are observed to reach maximum amplitude ( $\delta n_e{/n}_e\sim 0.1$) 300 fs after the pump pulse. The wakefield growth ( $3.5{\mathrm{ps}}^{-1\mathrm{}}$) and decay ( $1.9{\mathrm{ps}}^{-1\mathrm{}}$) rates are consistent with the forward Raman scattering instability and Landau damping, respectively.