Channeling dynamics of relativistic-intensity laser pulses

Two-dimensional particle-in-cell simulations were performed to study the channeling in long ( > 500 μ m) underdense plasmas of long duration ( > 10 ps), relativistic-intensity ( I = 10 18 - 20 W/cm 2 ) laser pulses. We describe five different types of channeling behaviors, and the corresponding ranges of plasmas and laser parameters are given. In all of these cases, self-corrective mechanisms come into play, which help straighten the channel provided that the laser pulse is long enough to push the plasma ahead. High-quality channels are observed when ξ = ( n n c ( 1 + a 0 2 / 2 ) - 0 . 5 ) 1 . 2 2 π W 0 λ a 0 < 0 . 2 , where n c is the critical density, a 0 is the vacuum vector potential, W 0 is the waist of the laser pulse, and λ is its wavelength. We also define a method to measure the channeling velocity without ambiguity, and we establish scaling laws. It is then possible to use them to predict the channel front position in an inhomogeneous plasma, such as the coronal plasma of a fast ignition target, and to deduce the energy needed to reach the critical density. Our scaling laws indicate that the required laser energy is 50 times higher when using a laser with I = 10 20 W/cm 2 than with I = 10 18 W/cm 2 . Our predictions are compared with a simulation of the laser propagation through a mm-long exponential plasma.