Coherent Thomson scattering of a picosecond probe laser was used to measure the time evolution of plasma wakefields produced by a high intensity laser pulse $(7\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2)$ in an underdense plasma $\left(n_e\approx {10}^{19}{\mathrm{cm}}^{-3\mathrm{}}\right)$ in the self-modulated laser wakefield accelerator configuration. Large amplitude plasma wakefields which lasted less than 5 ps were observed to decay into ion acoustic waves. The time scales associated with these measurements were consistent with the effects of the modulational instability and the enhancement of scattered signal from plasma channel formation.

• Received 20 June 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.5377