31 Search Results

Molecular dynamics investigation of hexagonal boron nitride sputtering and sputtered particle characteristics
The sputtering of hexagonal boron nitride (hBN) by impacts of energetic xenon ions is investigated using a molecular dynamics (MD) model. The model is implemented within an opensource MD framework that utilizes graphics processing units to accelerate its calculations, allowing the sputtering process to be studied in much greater detail than has been feasible in the past. Integrated sputter yields are computed over a range of ion energies from 20 eV to 300 eV, and incidence angles from 0° to 75°. Sputtering of boron is shown to occur at energies as low as 40 eV at normal incidence, and sputtering of nitrogen atmore » 
Thermal relaxation of molecular oxygen in collisions with nitrogen atoms
Investigation of O{sub 2}–N collisions is performed by means of the quasiclassical trajectory method on the two lowest ab initio potential energy surfaces at temperatures relevant to hypersonic flows. A complete set of bound–bound and bound–free transition rates is obtained for each precollisional rovibrational state. Special attention is paid to the vibrational and rotational relaxations of oxygen as a result of chemically nonreactive interaction with nitrogen atoms. The vibrational relaxation of oxygen partially occurs via the formation of an intermediate NO{sub 2} complex. The efficient energy randomization results in rapid vibrational relaxation at low temperatures, compared to other molecular systemsmore » 
High fidelity modeling of thermal relaxation and dissociation of oxygen
A master equation study of vibrational relaxation and dissociation of oxygen is conducted using statespecific O{sub 2}–O transition rates, generated by extensive trajectory simulations. Both O{sub 2}–O and O{sub 2}–O{sub 2} collisions are concurrently simulated in the evolving nonequilibrium gas system under constant heat bath conditions. The forced harmonic oscillator model is incorporated to simulate the statetostate relaxation of oxygen in O{sub 2}–O{sub 2} collisions. The system of master equations is solved to simulate heating and cooling flows. The present study demonstrates the importance of atomdiatom collisions due to the extremely efficient energy randomization in the intermediate O{sub 3} complex.more » 
Resolution of the vibrational energy distribution function using a direct simulation Monte Carlomaster equation approach
The direct simulation Monte Carlo (DSMC) method is the primary numerical technique for analysis of rarefied gas flows. While recent progress in computational chemistry is beginning to provide vibrationally resolved transition and reaction cross sections that can be employed in DSMC calculations, the particle nature of the standard DSMC method makes it difficult to use this information in a statistically significant way. The current study introduces a new technique that makes it possible to resolve all of the vibrational energy levels by using a master equation approach along with temperaturedependent transition rates. The new method is compared to the standardmore » 
Perturbation analysis of ionization oscillations in Hall effect thrusters
A perturbation analysis of ionization oscillations, which cause low frequency oscillations of the discharge plasma, in Hall effect thrusters is presented including the electron energy equation in addition to heavyspecies transport. Excitation and stabilization of such oscillations, often called the breathing mode, are discussed in terms of the growth rate obtained from the linear perturbation equations of the discharge plasma. The instability induced from the ionization occurs only when the perturbation in the electron energy is included while the neutral atom flow contributes to the damping of the oscillation. Effects of the electron energy loss mechanisms such as wall heatmore » 
Quantum and quasiclassical collisional dynamics of O{sub 2}–Ar at high temperatures
A hypersonic vehicle traveling at a high speed disrupts the distribution of internal states in the ambient flow and introduces a nonequilibrium distribution in the postshock conditions. We investigate the vibrational relaxation in diatomatom collisions in the range of temperatures between 1000 and 10 000 K by comparing results of extensive fully quantummechanical and quasiclassical simulations with available experimental data. The present paper simulates the interaction of molecular oxygen with argon as the first step in developing the aerothermodynamics models based on first principles. We devise a routine to standardize such calculations also for other scattering systems. Our results demonstrate verymore » 
Mode transition of a Hall thruster discharge plasma
A Hall thruster is a crossfield plasma device used for spacecraft propulsion. An important unresolved issue in the development of Hall thrusters concerns the effect of discharge oscillations in the range of 10–30 kHz on their performance. The use of a high speed Langmuir probe system and ultrafast imaging of the discharge plasma of a Hall thruster suggests that the discharge oscillation mode, often called the breathing mode, is strongly correlated to an axial global ionization mode. Stabilization of the global oscillation mode is achieved as the magnetic field is increased and azimuthally rotating spokes are observed. A hybriddirect kinetic simulationmore »
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"Boyd, Iain"
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