Albina Tropina

The high temperatures present in hypersonic flows produce a non-negligible amount of electronically excited states in NO, N2, CN, and C2, where the carbon-containing species are produced from ablation of a carbon vehicle's surface. The inclusion of electronically excited states in a chemical kinetics scheme will change the properties of the gas, for which the scheme is being modeled. In this work, we developed Ablative Solver for High Eenthalpy Nonequilibrium, a finite-rate chemical kinetics library for gas–material reactions and electronic excitation within two-temperature computational fluid dynamics solver Hy2Foam, to calculate the changes to the population distributions and surface properties of the hypersonic flow. The model considers chemical and thermal non-equilibrium, ablative material response to surface heating and reactions, and electronic excitation of gas-phase species of a reentry flow for the first time. We show in a number of different hypersonic cases that when these excited states exist in the gas phase, population distributions of both carbon-containing, air, and charged species, as well as mass blowing rates, change significantly. For a case at Mach 24 using an augmented air–carbon-ablation model of gas–material interaction, the mass blowing rate changes by up to 5%, and for Mach 8, the mass blowing rate changes by up to 10%.

In this work, the decay of femtosecond laser-induced plasmas has been explored through both experimental and numerical studies. Understanding the plasma decay in different gas compositions at varying background pressures is crucial in order to establish the capability of localized control of laser energy addition that ultimately can enable various applications. This study investigates the decay dynamics of femtosecond laser-induced plasmas in a varying N2–O2 mixture at varying pressures, with a focus on the role of oxygen in the plasma life and optimization of dual-pulse energy deposition of a second, nanosecond laser pulse. Experimental and simulation analyses show how the presence of oxygen affects the dual-pulse laser plasma decay. Using the self-consistent three-temperature plasma kinetic solver, the key reactions that dominate the temporal behavior of plasma are identified, such as recombination with N4+, electron attachment with O2, associative ionization producing N2+ or NO+, and photo-detachment. The findings are expected to contribute to a deeper understanding of plasma kinetics in both N2-only and N2–O2 environments at various pressures and be of value for advancing the understanding of laser-induced plasmas, optimizing localized energy addition, and advancing low-temperature plasma applications.

This study presents a detailed plasma kinetics model for laser-induced non-equilibrium plasmas in atmospheric pressure air, incorporating a self-consistent energy balance and refined rate expressions within a three-temperature framework. The model is validated against experimental data of femtosecond-laser-induced filaments, showing good agreement in electron dynamics and gas temperature. The analysis focuses on femtosecond filament decay kinetics and characteristic properties across varying initial electron densities and electron temperatures, including cases with oxygen addition and its influence on decay behavior. The study further examines energy coupling between the femtosecond filament and nanosecond laser pulse, identifying dominant kinetic pathways and optimal time delays through a comparative analysis of single-pulse and dual-pulse plasmas. Additionally, results indicate that temporal shaping of the nanosecond laser pulse intensity enhances dual-pulse performance relative to Gaussian pulses.

The paper presents a detailed analysis of the change in the refractive index field during the decay of laser plasma in nitrogen at atmospheric pressure. The mathematical model combines a three-temperature nonequilibrium plasma model with detailed kinetics and a semi-classical refractive index model based on species polarizabilities. A detailed characterization of the femtosecond plasma for a wide range of initial electron density and electron temperature forms the basis of the refractive index analysis. The study provides a quantitative evaluation of the effects of vibrational nonequilibrium, dissociation, ionization, and hydrodynamics on the refractive index field. The fast temporal evolution of the refractive index at early times during the laser plasma decay is explained by the exponential decay of the electron density, with dissociation and vibrational excitation of molecular nitrogen being equally important factors. The study of relative contributions to the evolution of the refractive index field has significance for diagnostics, laser energy deposition, and wave-guiding.

In this paper we study defocusing and non-paraxial effects associated with nanosecond pulsed laser beam propagation through femtosecond laser generated plasmas. Simulations were performed using a non-reduced wave propagation equation to highlight the role of spatial beam shaping of the nanosecond pulse in improving the laser propagation characteristics. We show that optimizing the shape of the laser intensity profile at the focus using a beam with the ring-liked focus helps to reduce both defocusing and non-paraxial effects.

In this work, the potential for hypersonic leading edge cooling by electron emission is demonstrated. To overcome space charge limitations the experiments are carried out in an argon discharge at 1 Torr. Cooling is observed with time-resolved measurements of the electron emission current and surface temperature, taking advantage of well controlled laser heating of the emitting surface and time accurate surface pyrometry. For the ignited mode of the plasma discharge, surface cooling by the electron emission is directly observed, leading to an estimated cooling rate of 1.6 ± 0.2 MW m⁻². Higher cooling rates with self sustained plasmas for space charge mitigation are expected using cesium transpiration. A two-dimensional model of heat transfer has been developed, which reproduces well the experimentally observed cooling dynamics. Parametric tests of emitter materials with various work functions show that for effective surface cooling by electron emission, the optimal work function must be less than 3.0 eV. This result indicates that electron cooling can be a promising thermal protection method for leading edges of hypersonic vehicles in flight.

This paper presents a self-consistent multi-dimensional mathematical model and a numerical approach for simulating the low-temperature plasma induced by the femtosecond laser filament. Addressing limitations in current models, we analyze key aspects of the laser plasma behavior, including plasma generation, detailed chemical kinetics, energy exchange channels, total energy balance, and hydrodynamics. The developed model and LOTASFOAM code are applied to study the temporal and spatial decay of the plasma produced by a femtosecond laser pulse in pure nitrogen at atmospheric pressure. The paper also includes a discussion on the spatial and temporal dynamics of electronically excited states of nitrogen in the decaying laser plasma.

We studied the process of laser heating and evaporation of a single droplet taking into account absorption of the laser radiation, evaporation process and vapor flow dynamics. The developed mathematical model and solver were validated against available experimental and numerical data for the ethanol and water droplet. The potential for the control of heating and evaporation processes by the laser pulse was shown. For pulsed laser heating the maximum temperature of droplets does not depend on the droplet size and depends only on the peak intensity of the laser radiation. For the peak irradiance close to the transition to the boiling regime, temporal dynamics of the droplet temperature follow the same dependence regardless of the droplet size. A selective transition to the boiling regime for larger size droplets is observed for the repetitive pulsed laser heating. Analysis of the absorption of laser radiation by the ensemble of droplets was given to describe the laser bleaching effect. The results show that the desired path can be cleared of droplets using the laser pulse, which helps to improve long range communication and remote sensing applications.