Interaction of ultraintense laser pulses with an underdense, preformed plasma channel

We report on experimental results regarding the propagation of ultraintense laser pulses in a preformed plasma channel. In this experiment, the long (4-mm) fully ionized plasma channel created by the amplified spontaneous emission (ASE) was measured by interferometry before and after the propagation of the short laser pulse. Forward spectra show a cascade of Raman satellites, which merge with one another when the laser power was increased up to critical power for relativistic self-focusing P/sub c/. The number of filaments measured by interferometry increases when the laser power increases. High conversion efficiency (/spl ap/10%) of second harmonic generation was observed in the interaction.