Experimental and numerical evaluation of laser shinethrough in polystyrene at 3<i>ω</i> and 4<i>ω</i> laser frequencies

In laser-direct-drive inertial confinement fusion (ICF), <i>laser shinethrough</i> is the process associated with laser propagation through the target plastic ablator, polystyrene is often used in ICF experiments, and leads to energy deposition in the target until the critical density is established at the ablator front surface. This process causes material modifications that may change the shock propagation and lead to density and pressure perturbations at the interface between the polystyrene and cryogenic deuterium-tritium (DT) layer. This work reports on experimental observations and numerical investigations of laser shinethrough at the third and the fourth harmonic of ICF class laser systems (1<i>ω</i> corresponds to a wavelength of ∼1.06 <i>μm</i>). The experiments evidence that, at 4<i>ω</i>, the laser is absorbed over a distance of about a few microns and laser shinethrough is completely eliminated in contrast with the behavior at 3<i>ω</i>. These experimental observations are reproduced by numerical simulations. The laser fluence required to reach the critical density at the ablator front surface is also found to be more than one order of magnitude higher at the third harmonic. Finally, modeling suggests that the back surface of the ablator exhibits ion temperatures greater than the melting temperature of cryogenic DT and electron pressure larger than the bulk and shear moduli of cryogenic DT for the case of using a 3<i>ω</i> laser.