Plasma Formation in Ambient Fluid from Hypervelocity Impacts

Ionization and plasma plumes have been detected in some hypervelocity impact experiments. However, knowledge of the plasma’s origin, composition, and energy is limited. We hypothesize that in atmospheric hypervelocity impacts, the ambient gas ionizes and contributes significantly to the plasma, despite the small amount of energy it receives. To test this hypothesis, we develop a fluid-solid coupled computational model that combines the multi-material compressible Navier-Stokes equations, a complete thermodynamic equation of state for each material, and a non-ideal, multi-species Saha equation for ionization prediction. Material interfaces are tracked using an extended two-equation level set method, and the interfacial mass, momentum, and energy fluxes are computed by the FIVER method. The impact of tantalum on soda-lime glass (SLG) within argon gas is simulated, with impact velocity varied between 3 km/s and 7 km/s. We show that for impact velocities above 4 km/s, ionization is clearly detected in both argon and SLG. The temperature and plasma density are both higher in the argon gas than in SLG and tantalum. In general, the results show that the plasma's density and energy depend on both impact velocity and the material combination, including the ambient gas. The plasma's composition further reflects the properties (e.g., ionization energies) of the chemical elements in each material. Acknowledgement: Office of Naval Research (ONR).