The Channeling Effect in the Underdense Plasma

In the fast-ignition approach to laser fusion, the nonlinear effects could greatly weaken the ignition laser in the underdense plasma. One way to overcome this is to send a channeling pulse to open a clear path for the ignition pulse. The PIC code \textit{OSIRIS} is used to simulate the laser--plasma interaction in 2-D space for the channeling process. For a typical run, the laser intensity is 1 $\times $ 10$^{19}$ W/cm$^{2 }$(\textit{$\lambda $} = 1.06 \textit{$\mu $}m) with a spot size of 14 \textit{$\mu $}m, and the density profile is taken from \textit{LILAC} simulations.$_{ }$Two-step simulations are carried out. In the first step, the density changes from 0.1 to 0.3 $n_{c}$ within 477 \textit{$\mu $}m $\times $ direction length. In the second step, the density changes from 0.3 to 1.0 $n_{c}$ within 523 \textit{$\mu $}m $\times $ direction length. Hosing, filaments, and self-focusing are observed. The channel created from the ponderomotive force, however, displays a remarkable regularity, showing that the channeling process has a self-healing characteristic. The channeling speed is also measured. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-92SF19460 and DE-FC02-04ER54789.