Optical transparency of transparent window LiF in laser-driven quasi-isentropic compression experiment

Abstract

LiF is often used as a window in laser-driven shock experiments, which can transmit and reflect visible probe laser. Researches of LiF transparency almost focus on its optical reflectivity compressed by strong shock, but there is almost no research on its optical transmissivity compressed by weak shock. In order to study the optical transmissivity of LiF, the quasi-isentropic compression experiment is carried out on the ShenGuang-III prototype laser facility, in which the velocity interferometer system for any reflector is used to diagnose the optical reflectivity of the quasi-isentropic compression sample CH/Al/LiF. The experimental results indicate that the velocity interferometer fringes are missing in the late stage of this experiment. The probe laser should penetrate LiF before it hits the rear surface of aluminum and the laser reflected by aluminum should penetrate LiF before it is collected by the velocity interferometer system for any reflector. Therefore, the reflectivity diagnosed by the velocity interferometer system for any reflector is the product of the optical reflectivity of aluminum and the optical transmissivity of LiF under the experimental condition. However, there is no research about the optical transmissivity model of thick LiF compressed by laser-driven shock. In this paper, we develop a transmissivity model for transparent window LiF and simulate the optical reflectivity of sample CH/Al/LiF. Firstly, we simulate the temperature and density of the sample by the code for one-dimensional multigroup radiation hydrodynamics (MULTI-1D). Then, based on the resulting temperature and density, we simulate the optical reflectivity of the sample by using the optical reflectivity model of aluminum and the optical transmissivity model of LiF. Without considering the transparency of LiF, the simulated result indicates that there is no signal missing in the late stage, which is different from the experimental result. By considering the transparency of LiF, the simulated result is in good agreement with the experimental result. The simulated result indicates that the formation of the strong shock, because of the later shock's catching up with the early one, obviously reduces the optical transparency of LiF and finally causes the velocity interferometer fringes to disappear. The simulated result also indicates that the energy gap of LiF calculated from density-functional theory is 1-2 eV. In this experiment, when LiF becomes opaque, its temperature is 1 eV and its pressure is 2-3 Mbar.