Lithium Fluoride optical properties at 825 GPa and the propagation of uncertainty to windowed equation of state measurements

Lithium fluoride (LiF) is an optically transparent material that tamps reverberations and preserves targets from residual gas in dynamic compression equation of state (EOS) measurements. However, LiF's non-trivial refractive index distorts interferometry measurements of target-window interface velocity. An accurate optical correction to this distortion is crucial in the determination of isentropes in other experiments. We present a measurement of LiF refractive index for stress up to 825 GPa from a shock-ramp experiment at the National Ignition Facility. We argue that a relationship between true and apparent velocity of the target-window interface is more valuable than refractive index to the EOS community. Finally, we present simulated data of dynamically compressed tin and LiF to demonstrate the propagation of the optical uncertainty from this work to EOS measurements. Simulations in which the tin-LiF interface reaches a peak stress of 825 GPa show that the tin isentrope can be determined up to a peak stress of 1.5 TPa with a 0.2% uncertainty in density due to the optical response of LiF.