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Measuring the shock Hugoniot data of liquid nitrogen using a cryogenic system for shock compression

The study of homonuclear diatomic molecules under high pressure and high temperature is a fundamental problem of condensed matter physics. In this research work, a cryogenic target (CT) was built to liquefy the gas and execute shock compression. The characteristics of the CT and the diagnostic system are explained in detail. We performed a shock compression of liquid nitrogen by using a two-stage light-gas gun at pressures up to 93 GPa (0.93 Mbar). Impactor velocities were measured with the magnetic velocimetry system, with a precision of 0.2%. The optical waveforms were recorded with the Doppler pin system, then further fast Fourier transform obtained velocity profiles in the sample. The measured velocity profiles were used to identify optical reflectance and obtain first-shock velocities, independent of the sample thickness above dissociative pressure (>30 GPa). The measured shock velocities had an uncertainty of less than 1%. First particle velocities were calculated by impedance matching, and the second velocities were directly calculated from the velocity profiles in an LiF anvil. The experimental shock Hugoniot results were observed to be consistent with those of the previous work. However, the principal Hugoniot softened above 27 GPa, and the uncertainties in the first and second-shock volumes were less than 0.7% and 3%, respectively.