Computational Analysis of Hypervelocity Impact Iron Plasma Generation

Hypervelocity impacts occur when objects, such as orbital debris and micrometeoroids in the space environment, strike a target with a velocity greater than the speed of sound of the material. These impactors can be as small as a micron and have velocities between 11-72 km/s. At this speed, a shock will propagate in the material. With sufficient kinetic energy, the impact can ablate the material, create warm condensed matter, and expand into plasma. Comprehending the physics behind this phenomenon is crucial for preventing electrical damage on spacecraft. Unfortunately, the chemistry and thermodynamics governing plasma production from these impacts are not well understood. This work aims to investigate the timescales, pre-expansion densities, and pre-expansion temperatures using a Monte Carlo Collision algorithm. These results can serve as initial conditions for state-of-the-art plasma expansion models for hypervelocity impacts. This work will focus solely on iron gas and free electrons during plasma production.

Initial temperatures and densities for the algorithm are calculated using a thermodynamics-based methodology. This employs the Tillotson equation of state and assumes a Fermi-Diract distribution. An increase in plasma density is observed alongside a decrease in the ground-state neutral density and electron temperature. Near-complete ionization occurs on the order of femtoseconds, which is much faster than other physical processes of interest, such as plasma expansion and electromagnetic radiation. This validates the common assumption that full ionization occurs instantly, allowing the initial ionization steps to be ignored when performing plasma expansion simulations and analyses. The results of this work are consistent with previous computational studies, such as steady-state electron temperatures on the order of magnitude of 0.1 eV and full ionization dependent on impactor speed.