Dusty plasma effects in hypervelocity impacts
Keywords: Aeronautics and Astronautics.

Meteoroids routinely impact bodies in space, such as spacecraft and small planetary bodies. Orbital debris also threaten spacecraft near Earth. These hypervelocity impacts occur at velocities of a few to tens of km/s, and the rapid conversion of kinetic energy to internal and thermal energy upon impact leads to fracturing, melting, vaporization, and ionization, producing a dense plasma. As the plasma expands from the impact crater, oscillations and instabilities can generate electromagnetic emissions in the radio frequency (RF) range and a broadband electromagnetic pulse (EMP). Moreover, condensed phase (solid and liquid) ejecta, referred to as dust, may acquire a surface charge due to interactions with the plasma, resulting in a dusty plasma. Hypervelocity impact plasmas generated by ground-based light gas gun impacts, which overlap the mass and velocity range of orbital debris, have been described as dusty for decades. Experimental plasma data and hydrodynamic simulations suggest dust charging as a possible explanation for anomalous plasma measurements. The degree of charge attachment and effect on plasma observations, however, remains unquantified. This thesis makes the first quantitative estimate of charge attachment in light gas gun impacts. We use thin-film witness plates to produce the first measurements of microscopic ejecta 1-50 microns in diameter generated by light gas gun impacts on aluminum and powdered regolith simulant targets, which serve as analogues for spacecraft and small solar system bodies, respectively. We then extend dust charging models based on orbital motion limited (OML) theory to the impact environment and build a novel semi-analytic model for dust charging and dynamics across the many orders of magnitude of space, time, and density spanned by the expanding plasma. Combining ejecta measurements, empirical and analytic models for the expanding plasma, and the dust evolution model enables predictions of the charge state of impact debris throughout the expansion. We quantify and propagate experimental and model uncertainties and use stochastic methods to produce bounded estimates of quantities of interest. This work led to a number of key findings. First, extrapolations from measurements of macroscopic ejecta poorly predict the flux of microscopic debris, and the computed particle size distribution predicts a flux of dust to the co-located plasma sensors that agrees in order of magnitude with the number of impulses, indicative of dust, in the plasma signals. Second, the dust charging model suggests that enough electrons attach to ejecta that dusty plasma phenomena are likely. These may affect electromagnetic emissions generated by hypervelocity impacts, especially on granular bodies, such as asteroids and comets. Third, the evolution model, with reasonable inputs, predicts similar arrival times and a surface charge that agrees in sign with experimental plasma measurements. Thermionic emission by hot debris can explain the observation of positively charged dust in impacts on granular regolith analogues. Finally, the ambient environment plays an important role in the charging and evolution of ejecta, and the neutral background present in many light gas gun facilities augments the tendency of debris to charge positively