PLASMA WAKEFIELD GENERATION AND ELECTRON ACCELERATION IN A SELF-MODULATED LASER WAKEFIELD ACCELERATOR EXPERIMENT

A self-modulated laser wakefield accelerator (SM-LWFA) experiment was performed at the Naval Research Laboratory. Large amplitude plasma wak efields produced by a sub-picosecond, high intensity laser pulse (7 x 10(18) W/cm(2)) in an underdense plasma (n(e) approximate to 10(19) cm (-3)) were measured with a pump-probe coherent Thomson scattering (CTS ) technique to last for less than 5 ps, consistent with the decay of l arge amplitude plasma waves due to the modulational instability. A pla sma channel was observed to form in the wake of the pump laser pulse, and its evolution was measured with the pump-probe CTS diagnostic. The trailing probe laser pulse was observed to be guided by this channel for about 20 Rayleigh lengths. High energy electrons (up to 30 MeV) ha ve been measured using an electro-magnetic spectrometer, with the ener gy spectra and divergence of lower energy (up to 4 MeV) electrons obta ined using photographic films. Highly nonlinear plasma waves were also detected using forward Raman scattering diagnostics and were observed to correlate with the electron signals. Simulations of self-trapping of plasma electrons from the interaction of the laser wakefield with t he slow plasma wave generated by Raman backscattering are also present ed. (C) 1997 American Institute of Physics.