Validation Experiment Research Of 304 Stainless Steel High Pressure Unloading Model

The thermodynamics properties of low-Z gas,such as hydrogen and its isotopes,and helium,at ultrahigh temperatures and ultrahigh pressures attract most interests in inertial confinement fusion(ICF)and astrophysics.The multiple shock reverberation compression technique is one of the effective methods for precompressed low-Z gas material to achieve the high-energy density state,aiming studying the characteristic of material under extreme conditions.In these experiments,baseplate(stainless steel 304),low-Z gas,and transparent windows were used to compose a "sandwich”target.The shock wave propagated and reflected between the baseplate and window due to their higher shock impedance,making the gas material being compressed.The states of gas material in the shock process were obtained by impedance-matching method and the accuracy of the results intensively relied on release paths of stainless steel 304.However,there is rarely release measurements of stainless steel 304 being published up to now,especially in the multi-Mbar regime.The release paths of stainless steel 304 used for impedance-matching in the multiple shock reverberating compression were therefore been simulated with a particular release model.So,whether the release model could accurately describe the release process of stainless steel 304 in the experiments would matter significantly for the final results.The release models urgently need to be checked by experimental measurements.For that,we measured the release paths of stainless steel 304 in a pressure range of 178 GPa to 318 GPa by use of an isentropic expanding method.The release models of stainless steel 304 were checked by the measurements.The major results conclude as follows:1)An experimental plan based on the isentropic expanding method was put forward for the measurement of isentropic release paths of stainless steel 304.Detailed method was that several standard materials were affixed to the rear surface of stainless steel 304,which could be released from an only shocked state to multiple pressure states(according to the different impedance of the standard materials).2)The published Hugoniot data of several materials using in the experiments(TC4,LY12,MB2,and PMMA)were estimated for their applicability of being used as a standard.And the Hugoniot data of PMMA in the pressure range of 6 GPa to 51 GPa was measured by new experiments,for the big uncertainty of the old data in literatures.Then new relationships of the D-u data of all the standards were displayed,providing evidence for the accurate ascertainment of the release points of stainless steel 304.3)Three release paths of stainless steel 304,beginning from an initial shocked pressure as 178 GPa,254 GPa,and 318 GPa respectively,were measured by isentropic expanding method in the experiments.The three initial shock state crossed the solid phase,solid-liquid mixed phase,and liquid phase and could provide evidence for the checkup of release models.4)The release paths of stainless steel 304 in the experiments were calculated by mirror reflection model and a particular isentropic release model.The two release models were checked by the experimental results,and the effect of the shock-melting was also been analyzed.The understandings get as follows:First,the release process can be well described by the mirror reflection model when the difference between the release pressure and the initial pressure of the release process is sufficiently small.But,the mirror line will more and more depart away from the experimental measurements along with the increase of that difference.So,when stainless steel 304 is used as a standard for impedance-matching in the shock experiments,the mirror reflection model can only be used when the shock impedance of the material of interest is close to that of stainless steel 304.Second,the particular isentropic release model can well describe the release process of stainless steel 304 whether its initial shocked state is solid,solid-liquid mixture,or liquid.Therefore,this kind of isentropic release model is available and reliable when stainless steel 304 is used as the impedance-matching standard in the multiple shock compression experiments for low-Z gas.Moreover,the effect of shock-melting can be ignored in our experimental pressure range.