Temporal Characterization of a Self-Modulated Laser Wakefield

Abstract

The temporal envelope of plasma density oscillations in the wake of an intense ( $I\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{18}\mathrm{W}/{\mathrm{cm}}^{2}$, $\ensuremath{\lambda}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1\ensuremath{\mu}\mathrm{m}$) 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 ( ${n}_{e}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{cm}}^{\ensuremath{-}3}$) are observed to reach maximum amplitude ( $\ensuremath{\delta}{n}_{e}{/n}_{e}\ensuremath{\sim}0.1$) 300 fs after the pump pulse. The wakefield growth ( $3.5{\mathrm{ps}}^{\ensuremath{-}1}$) and decay ( $1.9{\mathrm{ps}}^{\ensuremath{-}1}$) rates are consistent with the forward Raman scattering instability and Landau damping, respectively.