Study Of Simulation And Experiment On Metal-nonmetal Transition Of Warm Dense Gold Fluids

Warm dense matter is widespread in planetary core and in the implosion path of the inertial confinement fusion.The study of material properties in warm dense matter is of great significance for understanding the generation of magnetic field and the target design of inertial confinement fusion.The metal-nonmetal transition in warm dense matter is an important scientific problem in the study of material properties under extreme conditions.The direct current(DC)conductivity is the most direct evidence for judging whether the metal-nonmetal transition occurs in the warm dense matter.The DC conductivity in warm dense matter is often obtained by measuring the optical reflectivity combined with Drude model.but in the metal-nonmetal transition regime,the Drude model is no longer applicable due to the localization of conduction electrons.The method obtaining DC conductivity by Drude model fails.The metal-nonmetal transition can also be judged by the optical reflectivity.The theoretical calculation of the optical reflectivity and conductivity could become the criterion for judging the metal-nonmetal transition.This thesis studied the metal-nonmetal transition during expansion in warm dense gold fluid with both theoretical and experimental methods.The physical parameters of warm dense matter are calculated by quantum molecular dynamics based on first-principles.In the experiments.warm dense gold fluid is generated by the hard X-rays in hohlraum and heated based on short pulse proton beam.The time-resolved expansion velocity and reflectivity are obtained by using VISAR respectively.The DC conductivity and optical reflectivity along the expanding path of the gold fluid are calculated by using one-dimensional radiation hydrodynamic simulation combined with the first-principles calculation.and they are compared with the experimental results.The research contents are as follows:1.The first-principles molecular dynamics based on density functional theory is used to calculate the optical and electrical parameters in the thermodynamic range of 5000 K-20000K from solid density to 1/6 solid density of the expanding warm dense gold fluid.It is found that with the decrease of density.the dynamic conductivity changes from metallic to nonmetallic.The DC conductivity is fitted by Drude model or Drude-smith model according to the calculated dynamic conductivity.Based on the shift of the central peak,the change of the temperature slope of the DC conductivity and the distribution of charge density,the metal-nonmetallic transition of the expanded gold fluid occurs at one third of the solid density.2.The metal-nonmetal transition in the process of expansion of gold fluid is studied by using the L band X-ray in the hohlraum to produce warm dense gold fluid.The average density and temperature of the reflective layer were obtained by the expansion velocity of the reflection layer combined with one-dimensional radiation hydrodynamic simulation.The dielectric function of the warm dense gold fluid at different average densities is calculated by using first-principles method.The reflectivity of the reflection layer during evolution was calculated by solving the Helmholtz equation and compared with the reflectivity measured experimentally.According to the time-resolved optical reflectivity,the average DC conductivity of the reflective layer is calculated,and the calculated result is less than 2000 S/cm.According to this evidence,the metal-nonmetal transition occurs at the abrupt point of reflectivity.3.The warm dense gold fluid is generated by heating the gold sample with collimated proton beam produced by the interaction of picosecond laser with cylinder target on the XG-? laser facility.The reflectivity evolution of warm dense gold fluid during expansion is calculated according to Helmholtz equation.The dielectric functions are calculated by first-principles and the calculated reflectivity is compared with the experimental measurement.It is found that the calculated reflectivity is significantly lower than the experimental measurement.The difference of reflectivity is mainly due to the energy shift of dielectric function calculated by first-principles.