One-dimensional magneto-hydrodynamic simulation of the magnetic drive isentropic compression experiments on primary test stand

Abstract

The 10 MA primary test stand (PTS), the most powerful pulse power generator in China, is used to obtain isentropic compression of Al samples under a pressure of about 100 GPa. The high performance of laser-triggered gas switches enables the precise synchronization of the 24 modules according to the required timing sequence. This advantage makes the PTS a very good platform for dynamic material compression with fundamental capability of pulse shaping. Tens of isentropic compression experiments have been conducted on the PTS, among which two distinct loading profiles were designed and used to obtain distinct compression processes. The first current, which is used to obtain a shockless compression, has a relatively smooth rise, and the rise-rate keeps almost constant during the 400 ns-long compression. The second current shape has a mild rise but a sharp ends, which is designed to make an artificial turn-point in the velocity history, which is helpful for the numerical code verification. The current profile, as well as the sample thickness, is optimized by a one-dimensional magneto-hydrodynamic (1D MHD) code MADE1D coupled with a full circuit model for the PTS. The equation of state and conductivity model used here have a wide coverage in the density, temperature and pressure range. The strength of material and its constitution model are also taken into consideration to simulate the elastic and plastic flow of metal at relatively low pressure and temperature. Compared with the experimental results, the simulated velocity at the sample/window interface is found to agree well with the measurement for most of the cases. This suggests that the MHD simulations with the circuit model are able to reflect the main process of the loading history, and help to analyze and elucidate the phenomena contributing to the compression. It shows that the current waveform is one of the most important factors that affect the loading process. For the PTS and strip-line electrodes it uses, a current rise ratio less than 15 kA/ns helps to obtain a smooth off-Hugoniot pressure rise. The temperature rise due to the pdV work is very small, and most of the sample material, except those in the skin layer where current passes through, keeps solid during the compression. However, for a current rises at 40 kA/ns or more, the ramp loading wave could be sharpened into a shock within the sample thicker than 1.2 mm. Based on the PTS flexibility of pulse shaping, a wide range of desired load processes can be gained by designing and controlling the load current and sample thickness precisely.