Refractive index of [100] lithium fluoride under shock pressures up to 151 GPa

G Young,Haijun Huang,Chunwei Leng,Xun Liu,Jun Yang

doi:10.1063/1.5065543

Abstract

Single-crystal lithium fluoride(LiF) is a widely used window material in shock wave experiments to maintain the shock stress on the sample surface. It is transparent under shock compression up to ∼200 GPa, allowing the interferometer to record the movement of the monitored surface. However, the interferometer techniques require knowing the accurate refractive index of LiF under shock compression to deduce the true particle velocity. Although the refractive index of LiF under shock compression has long been studied, different experimental results diverge, especially in the high pressure region. Here, we used iron as the standard material to measure the refractive index of [100] LiF up to 151 GPa using two-stage light gas gun. Our results show that for the 1550 nm light, the true particle velocity (utrue) has a linear dependence on the apparent particle velocity (uapp), utrue = 12.26(2.07)m/s + 0.7759(0.0005)uapp, when the apparent particle velocity, uapp &gt; 0.38 km/s.

Highlights

Interferometers, VISAR(Velocity Interferometer System for Any Reflect),[1,2,3] DIASAR (Displacement Interferometer System for Any Reflector),[4,5,6] and DPS(Doppler Pin System),[7,8] are widely used to record the interface velocity history in dynamic compression experiments
Lithium fluoride as an optical window was first applied to velocity interferometry by Wise and Chhabildas,[10] and they found that the refractive index was non-linear with density up to 115 GPa
A series of shock wave experiments were performed to determine the refractive index of LiF in the pressure regime between 64 and 151 GPa using a DISAR at 1550 nm

Summary

Introduction

Interferometers, VISAR(Velocity Interferometer System for Any Reflect),[1,2,3] DIASAR (Displacement Interferometer System for Any Reflector),[4,5,6] and DPS(Doppler Pin System),[7,8] are widely used to record the interface velocity history in dynamic compression experiments. The attached window used in velocity interferometry requires optical transparency under stress.[9] Among these window materials, lithium fluoride is especially suitable for the application due to its ability to remain transparent for both 532 nm and 1550 nm under very high pressures and no structural phase transition under compression. Lithium fluoride as an optical window was first applied to velocity interferometry by Wise and Chhabildas,[10] and they found that the refractive index was non-linear with density up to 115 GPa. The refractive index of LiF at 1550 nm has been measured by Jensen et al.[7] and LaLone et al.[11] in the stress range below 17.5 GPa and by Zhou et al.[12] up to 124 GPa, but the obtained data were able to satisfy the linear relationship. The experimental data measured by Zhou et al are not consistent with that measured by Rigg et al, especially at high pressure

Methods

Results

Conclusion