Peculiar melting of diamond on the Hugoniot*

Laser shock experiments using the SG III proto-type facility were performed to study the melting of diamond over a range of shock pressure. We used the standard techniques for laser shock experiments of line-VISAR to measure shock velocity and SOP to observe the brightness. For a reference material quartz was set together with diamond sample to monitor the shock velocity and brightness during the decay shock. The Hugoniot of diamond has been determined through the impedance match method and the results were compared with those from the previous studies to be very similar. However, the temperature, calculated by the SOP data and the reflectivity from the VISAR measurement, is much lower than the previous one. The measured reflectivity appears above a shock velocity of ~20 km/s and changes with the optical variations of the shock front with increasing shock velocity. These changes of temperature and pressure on the Hugoniot are compared with available theoretical simulations. We discuss the diamond melting along the Hugonuot to be under equilibrium or not. Furthermore, the observed melting behaviors for shocked diamond provides new critical constrain on theoretical calculation and will affect prospect of understanding carbon in the interiors of large ice giants and design of first shock drive for ICF ignition process.