Study On The Equation Of State Of High-density Carbon In Laser Fusion

In laser fusion,laser or X-ray first irradiates the ablator which is at the outermost layer of the capsule.The irradiated ablator material is heated and then expands outwards.At the same time,the shell is pushed inward to compress the fuel inside the capsule.Therefore,the material properties of the ablator will directly affect important physical processes in fusion ignition such as the absorption of laser or X-ray energy by the capsule,the preheating of the inner deuterium-tritium(DT)cold fuel,or the growth of hydrodynamic instability.At present,the main ablator materials used in laser fusion are hydrocarbon(CH),beryllium(Be)and high-density carbon(HDC).Although HDC was used much later than CH or Be,it has been widely used in recent years due to its high initial density,high energy absorption efficiency,short ignition pulse design and other advantages.The implosion experiment with HDC ablator have been constantly refreshing the records of neutron yield in recent years,and have reached another milestone in the path to ignition of"burning plasma".As polycrystalline diamond,HDC has a lower density and smaller grain size than single-crystal diamond.Mid-Z or high-Z element is doped in HDC to prevent the preheat of inner cold DT fuel.Due to these factors,the equation of state(EOS)of HDC could be different with that of single-crystal diamond.The Hugoniot is very sensitive to the variation of the initial state of the material.And the Hugoniot of the ablator material is closely related to the first shock compression in the multi-shock design of laser fusion.Therefore,the study of the equation of state especially the Hugoniot of HDC and doped HDC in the pressure range related to laser fusion will help to optimize the design of ignition.Through preliminary investigation,we found that in the pressure range of 1～5 TPa related to laser fusion,the study of Hugoniot of HDC is less,and the study of Hugoniot of doped HDC is completely blank.Therefore,we decided to study the equation of state of HDC and doped HDC,and the research route is mainly based on precision experimental research,supplemented by the development of practical and convenient theoretical simulation method.The study focuses on the Hugoniot of pure HDC and doped HDC at pressure of 1～2.5 TPa which is related to the first shock strength in laser fusion.The main work of the thesis includes:(1)The first accurate shock Hugoniot experimental diagnostic platform for high-density carbon in large laser facility in China has been established.Through the optimization of experimental design,the accurate positioning of PSBO diagnosis based on the measurement of target before experiment and the fine processing of the experimental data,etc.,we established the high-precision shock Hugoniot diagnostic platform for HDC at TPa pressure.The relative diagnostic uncertainty of shock wave velocity was in 1.2%～1.5%,which was comparable to the international diagnostic level.The experimental data of HDC shock Hugoniot obtained from the verification experiment show good consistency with the data reported by different platforms abroad,which verifies the reliability of the precision experiment.(2)We carried out shock experiment of HDC with different initial densities which was first to cover the density range of HDC used in laser fusion based on the developed shock Hugoniot diagnostic platform.With the experimental data,the EOS of HDC at high pressure were extended,and the density effects were further studied.Through the shock experiment of HDC with lower initial density(3.23 g/cm3),we extended the Hugoniot of HDC at high pressure.Through the analysis of Hugoniot data of HDC with different initial densities and the results of EOS models,it is found that the existing SESAME models underestimate the low-density effect of HDC,while the porous models can more reliably describe this behavior of low-density HDC.The results provide more reliable EOS constraint for the design of implosion of HDC with different densities.(3)Based on the Hugoniot data of HDC,the Grüneisen parameter of liquid carbon under high pressure of TPa was studied.Grüneisen parameter is an important material parameter in EOS research,which is often used to construct isentropic curves,release curves,off-Hugoniot curves,etc.,of materials.We calculated the Grüneisen parameter of liquid carbon at TPa pressure from the Hugoniot experiment of HDC with different initial densities.Through the analysis of the results from Hugoniot experiment and EOS models,it is found that under the TPa pressure,the Gruneisen parameter of liquid carbon is strongly related to the temperature,while the classical treatment of the Grüneisen parameter ignoring the temperature effect is not suitable at TPa pressure.This will provide guidance for subsequent theoretical corrections of the Gruneisen parameter.(4)A convenient and practical method for constructing the equation of state of doped HDC is proposed.For materials at TPa pressure of transition pressure region,the EOS theory developed at low pressure is often not suitable due to the complexity of the atomic shell structure caused by the ionization of the inner shell electrons,and the first-principles calculations are expensive and can only obtain limited data points.QEOS model is a model that can be used to construct the global EOS for a variety of materials with a certain accuracy,but it requires the basic parameters of materials as input.To obtain the basic parameters of various materials through experimental measurements faces heavy workload.We propose a method to construct the EOS of doped HDC.First,the doped HDC is simulated by molecular dynamics code to obtain the basic material parameters,and then these parameters are input into QEOS model to calculate the EOS of doped HDC.This method is more convenient and has certain physical rationality.(5)We developed the precision shock experiment platform of tungsten-doped HDC,and the effectiveness of the experimental technical route was successfully verified.The Hugoniot data of tungsten-doped HDC play an important role in the construction and verification of doped EOS models.From the perspective of identifying doping effect in EOS models,we evaluated and designed the shock experiment of tungsten-doped HDC,proposed the requirements for tungsten-doped HDC target,and promoted the development of related target fabrication technology.The first shock experiment of tungsten-doped HDC was accomplished.The diagnostic uncertainty of shock wave velocity(1.2%～1.4%)basically meets the requirements of precision experiment analysis,which verifies the effectiveness of the shock Hugoniot experimental technical route of tungsten-doped HDC.