Analysis of effects of laser profiles on fast electron generation by two-dimensional Particle-In-Cell simulations

Complete text of publication follows. A cone-guided target is used in the Fast Ignition Realization Experiment project phase-I (FIREX-I) and optimization of its design is performed. However a laser profile is not optimized much, because the laser profile that is the best for core heating is not known well. To find that, it is useful to investigate characteristics of generated fast electrons in each condition of different laser profiles. In this research, effects of laser profiles on fast electron generation are investigated on somewhat simple conditions by two-dimensional Particle-In-Cell simulations. In these simulations, a target is made up of Au pre-plasma and Au plasma. The Au pre-plasma has the exponential profile in the x direction with the scale length L = 4.0 μm and the density from 0.10 ncr to 20 ncr. The Au plasma has the flat profile in the x direction with 10 μm width and 20 ncr. Plasma profiles are uniform in the y direction. The ionization degree and the mass number of plasmas are 40 and 197, where the ionization degree is determined by PINOCO simulations. PINOCO is a two-dimensional radiation hydrodynamics simulation code, which simulates formation of the high-density plasma during the compression phase in the fast ignition. A laser is assumed to propagate as plane wave from the negative x direction to the positive x direction. Laser profiles are supposed to be uniform in the y direction. Three different laser profiles, namely flat one with tflat = 100 fs, Gaussian one with trise/fall = 47.0 fs and flat + Gaussian one with trise/fall = 23.5 fs and tflat = 50 fs are used. The energy and the peak intensity are constant with E = 107 J/cm2 and IL = 1020 W/cm2 in all cases of different laser profiles. We compare results in each condition of three different laser profiles and investigate effects of laser profiles on fast electron generation. Time-integrated energy spectra are similar in all cases of three different laser profiles. In the case of the short pulse, effects of laser profiles are small. We will discuss details in this case, results in the case of the long pulse of which the duration is grater than 500 fs and optimization of the laser profile.