Laser channeling in mm-scale underdense plasmas of fast ignition targets

In the fast ignition approach to laser fusion, non-linear laser-plasma interactions could cause significant energy loss for an ignition laser in an underdense plasma. One way to avoid this is to use a channeling pulse to create a low-density channel for the ignition pulse. Two dimensional Particle-in-cell simulations show that laser channeling in mm-scale underdense plasmas has many new phenomena that are not present in previous short-scale experiments and simulations, including plasma buildup to $n_c$ in front of the laser, laser hosing/refraction, channel bifurcation, and self-correction and electron heating to relativistic temperatures. The channeling speed is much less than the linear group velocity of the laser. The simulations find that low- intensity channeling pulses are preferred to minimize the required laser energy. The channel is also shown to significantly increase the transmission of an ignition pulse.