Optical characterization methods for hypervelocity impact generated plasma
Keywords: Aeronautics and Astronautics.

The rapid advancement of technology and ease of accessibility to space have introduced a fundamental change in space, allowing globalization of space programs and commercialization of space activities. Consequently, a more congested and competitive space environment introduces new challenges and threats to spacecraft designers, operators, and users in protecting their space assets. Among these threats includes hypervelocity impacts from microparticles in the form of meteoroids and space debris. These microparticles travel at speeds between 11.2 and 72.8 km/s with respect to the Earth and can impact spacecraft forming a small and dense plasma. This plasma can generate a strong optical emission, known as an impact flash, and an electromagnetic pulse (EMP). The impact flash and EMP can lead to spacecraft electrical anomalies when the impacted spacecraft surface (target) carries high electrical potential due to various space weather effects. Two parameters of the impact plasma that strongly determine its behavior and electrical anomaly risk are its temperature and density. To understand the microparticle hypervelocity impact plasma and its associated threat to spacecraft electronics, we need to determine the impact plasma temperature and density under a range of space operating conditions, such as different spacecraft charging conditions. The understanding of hypervelocity impact flash phenomenon and its connection to the impact plasma under these conditions is critical for our knowledge of hypervelocity impact effects and the development of a reliable optical characterization method in predicting impact plasma properties, impact conditions and risks. We establish three objectives to achieve this overarching goal. First, we create an optical method that enables the characterization of the impact plasma using impact flash measurements. Second, we investigate the generation mechanism of the impact flash from the impact plasma. Third, we examine the correlation between the impact flash and impact conditions, such as the impactor mass, impact velocity, target electrical condition, and spacecraft damage risk. In this dissertation, we develop a non-intrusive optical method to study the impact plasma via its continuum optical emission spectrum. This optical method includes a theoretical model and calibrated sensors to characterize the impact plasma. Our theoretical model (Impact Plasma Generated Flash model) is the first of its kind to qualitatively define the temporal stages of the behavior of the impact flash and plasma. This model evaluates the approximate temperature and density of the plasma where the continuum radiation dominates and theorizes the process by which the plasma generates the impact flash. The photometry sensor suite developed in this dissertation consists of three spectral photomultiplier tubes (450, 550, and 600 nm), which enable intensity, spectrum, and geometry measurements of the impact flash. This sensor suite was deployed at a ground-based 3 MV electrostatic dust accelerator and a light gas gun facility. Using this optical characterization method, we demonstrate new results on hypervelocity impacts with various target electrical conditions as a control variable to study the relationship between the impact plasma and impact flash. Our result suggests that the impact flash continuum spectrum was produced by a combination of the following mechanisms: Bremsstrahlung radiation (a more general form of blackbody radiation) due to the acceleration of charged particles via spacecraft surface electrical conditions or thermal motion; Bremsstrahlung radiation due to the oscillating internal electric field; and recombination radiation from the plasma particles. The impact flash was found to emit blackbody radiation in the early time after impact (~800 ns) and evolve from an optically thick continuum emission to an optically thin continuum emission during this time window. Using blackbody and Bremsstrahlung spectra to estimate the plasma density and temperature, we found that an impact velocity range of 15 to 40 km/s yielded average plasma temperatures at signal intensity peaks between 3500 and 8000 K depending on the surface electrical conditions. The initial plasma density and temperature (0.5 - 2 eV) from optical measurements are in good agreement with previous experimental and simulation results of the impact plasma. Thus, these new results are consistent with the existing literature. The strong resemblance between the impact plasma and impact flash expansion geometry, the consistent plasma diffusion process measured by the optical method, and the strong connections between the target electrical condition and the impact flash temperature all suggest that the impact flash originates from the impact plasma. Therefore, this dissertation enables the future use of the impact flash measurements as an assessment tool for impact conditions, impact plasma properties and the risk of spacecraft electrical anomalies.