Christopher Hudson Moore

Parallel 3D Hybrid Continuum/DSMC Method for Unsteady Expansions Into a Vacuum

We present the application of a unidirectional unsteady coupling between a continuum solver and a three dimensional parallel Direct Simulation Monte Carlo (DSMC) code. Two different problems have been considered: the spherically symmetric expansion of a water vapor cloud into a vacuum and the late stages of a comet impact on the Moon. In both cases, unsteady data pre-computed from a continuum solution are used as input to the DSMC code at a fixed interface. The DSMC results were then compared to the continuum results downstream of the interface in a region of mutual validity in order to validate our approach. The DSMC results for the expansion flow showed good agreement with the analytic solution for the density, velocity and temperature downstream of the interface. Similarly, for the comet impact simulations, the DSMC density agrees well with the solution from the continuum solver, the SOVA hydrocode. A slightly hotter solution is however obtained downstream of the interface using the DSMC code compared to the hydrocode solution.

Simulation Of Plasma Interaction With Io's Atmosphere

One dimensional Direct Simulation Monte Carlo (DSMC) simulations are used to examine the interaction of the jovian plasma torus with Io’s sublimation atmosphere. The hot plasma sweeps past Io at ∼57 km/s due to the external Jovian magnetic and corotational electric fields and the resultant energetic collisions both heat and dissociate the neutral gas creating an inflated, mixed atmosphere of SO2 and its daughter products. The vertical structure and composition of the atmosphere is important for understanding Io’s mass loading of the plasma torus, electron excited aurora, and Io’s global gas dynamics. Our 1D simulations above a fixed location on the surface of Io allows the O+ and S+ ions to drift down into the domain where they then undergo elastic and charge exchange collisions with the neutral gas. Each electron’s position is determined by the motion of a corresponding ion; however, the electrons retain their own velocity components which are then used during elastic, ionization, and excitation collisio...

Incorporating radiation transport into particle-based plasma simulations

Summary form only given. In an effort to expand modern Particle-in-Cell (PIC) plasma simulations, a method for including radiation transport is examined. Tracking the dynamics of radiation transport in plasma simulations is inherently difficult and most models use either a semi-empirical or a propagator function approach. Here, discrete photons are emitted from excited state species with a state-dependent wavelength. This self-produced emission from the plasma is broadened according to natural and Doppler line widths resulting in a Voigt profile and emitted isotropically. By directly tracking the velocities of the excited species that emit radiation, the Doppler shift is easily found and the expensive convolution calculation for the Voigt profile is avoided. Absorption cross sections of radiation by the background neutral gas cross sections for ground-state are determined as a function of the emission profile. As such, discrete photon particles also have the advantage of being easily coupled to an existing collision routine such as Direct Simulation Monte Carlo (DSMC) or Monte Carlo Collision (MCC). Simulations of helium and air discharges demonstrate the effectiveness of this method for determining non-equilibrium emission spectra and incorporating energy-dependent photo-processes (e.g. photoemission, photo-ionization).

DSMC Simulations of the Plasma Bombardment on Io's Sublimated and Sputtered Atmosphere.

The DSMC method is used to model the interaction of the jovian plasma torus with Io’s SO2 sublimation and sputtered atmosphere just prior to eclipse. The SO2 frost sublimes on the warm dayside and photo and neutral chemistry, the dominant source of the daughter species (SO, O2, O, and S) are included. To model the plasma interaction with the sublimation atmosphere, a two-timestep method is utilized in which the neutrals are assumed to be stationary while electrons and ions are moved and collided over a much smaller timestep. The dominant ion-neutral interactions (non-reactive and resonant charge exchange) are included. Sputtering of SO2 molecules from the frost-covered surface is dependent on the incident ion energy and the surface frost temperature. Io’s surface is assumed to be uniformly covered by SO2 surface frosts with the temperature computed based on radiative equilibrium with insolation. We investigate the effect that the plasma interaction with Io’s atmosphere has on atmospheric composition and structure, circumplanetary winds, and the escape rate of material from Io to the plasma torus. The dense sublimation atmosphere reduces sputtering from SO2 surface frosts over much of the dayside; however, sputtering was found to be a significant contributor to the nightside atmosphere. The plasma pressure on the sublimation atmosphere has a substantial effect on the day-to-night winds. Not only does the plasma pressure induce an overall retrograde wind in Io’s atmosphere just prior to entry into eclipse, but the atmospheric scale height is reduced by the plasma pressure on the trailing hemisphere. Molecular oxygen is a minor species on the dayside but is found to be the dominant nightside species because it is non-condensable and the loss rates due to atmospheric escape or dissociation are slow.

Loki - A lava lake in rarefied circumplanetary cross flow

The interaction between Io’s largest hot spot, Loki, and Io’s circumplanetary winds is simulated using the direct simulation Monte Carlo (DSMC) method. Our three‐dimensional simulation models the rarefied pressure‐driven boundary layer flow over a “hot” disk in the presence of a weak gravitational field. The pressure gradient which forces winds away from the subsolar point toward the nightside is caused by the variation in insolation over the surface. The rarefaction varies strongly with time of day due to the exponential dependence of the vapor pressure on the surrounding surface frost temperature (KnHS≈1×10−4 to 0.5 where KnHS = λ/R, λ is the mean free path, and R is Loki’s effective radius). The spread of heat from the hot spot, the equilibration of pressure over the hot spot, and separation of the boundary layer are examined. The spread of heat away from the hot spot is approximately controlled by δ = tRADU/R (tRAD is the radiation time scale and U is the mean wind speed). For cross flow speed conside...

Dielectric-directed surface flashover under atmospheric conditions

Summary form only given. High-voltage arc formation near a dielectric material is a complex process by which surface charging, secondary electron emission, and photoelectron emission modify the local electric field to determine the arc path and breakdown threshold. Strong electric field enhancement at the triple-point junction of dielectric, metal, and atmosphere may act to generate initiating electrons to seed prompt formation of streamers. This study investigates the fundamental role of dielectrics in influencing voltage breakdown threshold, statistical time lag, and reproducibility under high voltage conditions with and without external ultraviolet stimulation. We are investigating whether field emission at triple points can sufficiently minimize variance in atmospheric breakdown. The experimental data presented here are from the Advanced Component Development laboratory at Sandia National Laboratories. The experiments were performed in dry air at 600-Torr using a low-inductance test-stand. Dielectric granules were placed on a planar brass electrode, offset from a rounded brass rod electrode which defined the 1-mm gap. 200 μm dielectric granules minimally affect electric field in the majority of the gap, but set up high field cathode triple points on the ground plane. We will discuss how dielectric material properties impact surface charging, electron emission, and material conversion, thereby directing the flashover path. Concurrent continuum modelling of surface flashover examines the mechanisms of electron absorption and emission at the surface (c.f. H. Hjalmarson et al.) Furthermore, these flashover experiments are to be used in validation efforts of our PIC-DSMC code (c.f. “Development and Validation of PIC-DSMC Air Breakdown Model in the Presence of Dielectric Particles”).

Io’s Atmospheric Freeze‐out Dynamics in the Presence of a Non‐condensable Species

One dimensional direct simulation Monte Carlo (DSMC) simulations are used to examine the effect of a trace non‐condensable species on the freeze‐out dynamics of Io’s sulfur dioxide sublimation atmosphere during eclipse and egress. Due to finite ballistic times, essentially no collapse occurs during the first 10 minutes of eclipse at altitudes above ∼100 km, and hence immediately after ingress auroral emission morphology above 100 km should resemble that of the immediate pre‐eclipse state. In the absence of a non‐condensable species the sublimation SO2 atmosphere will freeze‐out (collapse) during eclipse as the surface temperature drops. However, rapid collapse is prevented by the presence of even a small amount of a perfect non‐condensable species due to the formation of a static diffusion layer several mean free paths thick near the surface. The higher the non‐condensable mole fraction, the longer the collapse time. The effect of a weakly condensable gas species (non‐zero sticking/reaction coefficient) w...

Modeling Io’s Sublimation‐Driven Atmosphere: Gas Dynamics and Radiation Emission

Io’s sublimation‐driven atmosphere is modeled using the direct simulation Monte Carlo method. These rarefied gas dynamics simulations improve upon earlier models by using a three‐dimensional domain encompassing the entire planet computed in parallel. The effects of plasma impact heating, planetary rotation, and inhomogeneous surface frost are investigated. Circumplanetary flow is predicted to develop from the warm subsolar region toward the colder night‐side. The non‐equilibrium thermal structure of the atmosphere, including vibrational and rotational temperatures, is also presented. Io’s rotation leads to an asymmetric surface temperature distribution which is found to strengthen circumplanetary flow near the dusk terminator. Plasma heating is found to significantly inflate the atmosphere on both day‐ and night‐sides. The plasma energy flux also causes high temperatures at high altitudes but permits relatively cooler temperatures at low altitudes near the dense subsolar point due to plasma energy depleti...

Development of PIC-DSMC air breakdown model in the presence of a dielectric: Breakdown time sensitivity to self-absorption and photoemission

Summary form only given. Electrical breakdown between electrodes in the presence of a dielectric cylinder is simulated using an electrostatic particle-in-cell (PIC) code that models particle-particle collisions using the direct simulation Monte Carlo (DSMC) method. In this talk we will present recent work on sensitivity of the breakdown time delay to the dielectric photoemission yield and to inclusion of self-absorption of photons by the background gas. Validation of the simulation model is being performed against prior experimental data on breakdown across a 13mm rod-to-plane gap with a 10mm dielectric cylinder in the middle1. The dielectric cylinder provides both an electron source by photoemission from low energy photons and enhances the reduced field (thus changing the plasma radiation spectrum)2.The model includes electron-neutral elastic, excitation, ionization, and attachment collision chemistry; ion and photon induced electron emission from surfaces; ion-neutral collisions; and self-absorption, photoionization, and photodissociation. The model tracks excited state neutrals which can be quenched through collisions with the background gas and surfaces or spontaneously emit a photon (isotropically) and transition to a lower state. Each simulated photon from an emission event is given a wavelength based on the transition that includes natural and Doppler broadening3. Emitted photons have an energy dependent probability of causing photoemission from the dielectric or electrode surfaces, as pre-computed by a separate electron Monte Carlo transport code4.

Development of PIC-DSMC model for laser-trigged vacuum switch

Laser-triggered vacuum switches (LTVS) are used for low-jitter, low-inductance, synchronized high current pulsed-power switching. Single-pulse laser irradiation (UV, visible, or IR) heats a target cathode material, ablating the cathode surface and creating a plasma within the gap. UV irradiation may additionally seed the gap with photoemitted electrons. Reported LTVS have laser energies 10s μJ-10s mJ; cathode materials include KCl, Ti, W, and graphite1.