Collisional Particle-In-Cell Investigations of RF Emissions in Hypervelocity Impact Plasmas

Abstract

Hypervelocity impact plasmas are formed when projectiles greater than the speed of sound in a material (typically 5-10 km/s) strike an object. The high energy associated with this impact causes vaporization and ionization of both the projectile and the target material, forming a mixture of dust, gas, and plasma, or dusty plasma. During the initial expansion of plasma, the high plasma density leads to highly coupled physics and collisions. For hypervelocity impacts onto spacecraft, the latter stages of expansion into the surrounding vacuum are largely collision-less. It has been observed that under certain conditions, radio frequency (RF) emission is produced as a result of the hypervelocity impact. Currently, no consensus regarding the physical mechanism behind the RF emission has been found. Computational models are beneficial tools to investigate plasma physics. In particular, particle-based kinetic methods such as the particle-in-cell (PIC) model are useful due to their ability to capture kinetic and non-Maxwellian effects of plasmas. Previous computational works have modeled the development of the plasma from initial impact [1] and RF emission associated with the collisionless expansion of plasma into the surrounding vacuum [2]. From the former study, RF emission was produced in plasmas for particles with velocities greater than 14 km/s. This RF emission is associated with the transition of the plasma from partial to full ionization. In the latter study, the RF emission simulated was a higher frequency than detected by experiments. This is hypothesized to be due to the initial conditions of the study, which assume a simple bulk separation of the plasma caused by a difference in ion and electron bulk velocities. The exact dynamics behind the bulk separation is required to be simulated more thoroughly. It is expected that the physics is dominated by the transition of the plasma from a highly collisional regime to a collisionless regime. Thus, this work focuses on the addition of collisional PIC schemes and the influence of external electric fields , simulating more accurate bulk separation physics to better understand the mechanisms behind RF emissions in hypervelocity impact plasmas.