Timothy J. Bartel

Two-dimensional direct simulation Monte Carlo (DSMC) of reactive neutral and ion flow in a high density plasma reactor

We present a two dimensional direct simulation Monte Carlo (DSMC) study of the rarefied reactive flow of neutrals and ions in a low pressure inductively coupled plasma reactor. The spatially-dependent rate coefficients of electron impact reactions and the electrostatic field were obtained from a fluid plasma simulation. Neutral and ion etching of polysilicon with chlorine gas was studied with emphasis on the reaction uniformity along the wafer. Substantial gradients in total gas density were observed across the reactor invalidating the commonly made assumption of constant gas density. The flow was nonequilibrium with differences in the species translational temperatures, and 100 K temperature jumps near the walls. When etching was limited by ions the etch rate was highest at the wafer center. When etching was limited by neutrals, the etch rate was highest at the wafer edge. In such case, the etch uniformity changed significantly depending on the reactivity of the ring surrounding the wafer. The ion angular distribution was several degrees off normal and it was different at the wafer edge compared to the rest of the wafer. >

Modeling and Simulation of Nuclear Fuel Materials

We review the state of modeling and simulation of nuclear fuels with emphasis on the most widely used nuclear fuel, UO2. The hierarchical scheme presented represents a science-based approach to modeling nuclear fuels by progressively passing information in several stages from electronic structure calculations to continuum level simulations. Such an approach is essential to overcome the challenges posed by radioactive materials handling, experimental limitations in modeling extreme conditions and accident scenarios, and the small time and distance scales of fundamental processes. When used in conjunction with experimental validation, this multiscale modeling scheme can provide valuable guidance to development of fuel for advanced reactors to meet rising global energy demand.

DSMC and Navier-Stokes Predictions for Hypersonic Laminar Interacting Flows

Direct Simulation Monte Carlo (DSMC) and NavierStokes calculations are performed for a Mach 11 25 deg.-55 deg. spherically blunted biconic. The conditions are such that flow is laminar, with separation occurring at the cone-cone juncture. The simulations account for thermochemical nonequilibrium based on standard Arrhenius chemical rates for nitrogen dissociation and Millikan and White vibrational relaxation. The simulation error for the Navier-Stokes (NS) code is estimated to be 2% for the surface pressure and 10% for the surface heat flux. The grid spacing for the DSMC simulations was adjusted to be less than the local mean-freepath (mfp) and the time step less than the cell transient time of a computational particle. There was overall good agreement between the two simulations; however, the recirculation zone was computed to be larger for the NS simulation. A sensitivity study is performed to examine the effects of experimental uncertainty in the freestream properties on the surface pressure and heat flux distributions. The surface quantities are found to be extremely sensitive to the vibrational excitation state of the gas at the test section, with differences of 25% found in the surface pressure and 25%-35% for the surface heat flux. These calculations are part of a blind validation comparison and thus the experimental data has not yet been re

Direct Simulation Monte Carlo of Inductively Coupled Plasma and Comparison with Experiments

ABSTRACT Direct simulation Monte Carlo was used to study ion and neutral transport and reaction in a low-gas-pressure highplasma-density inductively coupled reactor with chlorine (electronegative) chemistry. Electron density and temperature were computed by a self-consistent continuum plasma code and were used as input to the direct simulation Monte Carlo code. Simulation results were compared with experimental data taken in a Gaseous Electronics Conference reference cell

Symmetric Body Vortex Wake Characteristics in Supersonic Flow

An extensive experimental investigation of the symmetric body vortex wake was conducted. Cone probe measurements were made on the leeside of an ogive nose circular cylinder for three different supersonic freestream conditions. Measurements of total pressure, Mach number, and three orthogonal velocity components were made at four angles of attack of the body at various axial stations. In the present paper these data are processed to infer the position of the primary body vortex and vortex feeding sheet in the cross-flow plane, local circulation distribution in the cross-flow plane, vortex core size, wake height, and total circulation in the cross-flow plane. A detailed discussion of the results and data processing is presented.

Navier-Stokes and direct simulation Monte Carlo predictions for laminar hypersonic separation

Axisymmetric direct simulation Monte Carlo (DSMC) and Navier‐Stokes simulations are performed as part of a code validation effort for hypersonice ows. The e owe eld examined herein is the Mach 11 laminar e ow over a 25 ‐ 55-deg blunted biconic. Experimental data are available for surface pressure and heat e ux at a Knudsen number Kn=0.019 based on the nose radius. Simulations at a reduced freestream density (Kn=0.057) are performed to explore the region of viability of the numerical methods for hypersonic separated e ows. A detailed and careful effort is made to address the numerical accuracy of these simulations, including iterative and grid convergence studiesforNavier ‐Stokesandtemporal,grid,andparticleconvergencestudiesforDSMC.Goodagreementisfound between the DSMC and Navier ‐Stokes simulation approaches for surface properties as well as velocity proe les within the recirculation zone for the reduced density case. The results obtained indicate that the failure of earlier DSMC simulations at Kn=0.019 is due to insufe cient grid ree nement within the recirculation zone. Furthermore, it is shown that accurate simulations of the biconic at the experimental conditions with the DSMC method are not yet possible due to the extreme computational cost. Nomenclature d = molecular diameter, m f = general solution variable Kn = Knudsen number based on nose radius, ¸=RN L = characteristic length scale, m n = number density, particles/m 3 p = pressure, N/m 2 , order of accuracy q = heat e ux, W/m 2

Navier-Stokes and DSMC simulations for hypersonic laminar shock-shock interaction flows

DSMC and Navier-Stokes simulations are performed as part of a code validation effort for hypersonic flows. The flowfield examined herein is the Mach 11, laminar flow over a 25 deg 55 deg blunted biconic for which experimental data are available for surface pressure and heat flux. Considerable effort is made to address the numerical accuracy of all simulations including iterative and grid convergence studies for Navier-Stokes and temporal, grid, and particle convergence studies for DSMC. Simulations of the biconic at a reduced freestream density are performed to explore the region of viability of the numerical methods for hypersonic separated flows. Excellent agreement is found between the DSMC and Navier-Stokes simulation approaches for surface properties as well as velocity profiles within the recirculation zone. The results of the rarefied biconic study indicate that the failure of prior DSMC simulations at the experimental densities is due to insufficient grid refinement within the recirculation zone. Additional DSMC and Navier-Stokes simulations are performed for the blunted 25 deg forecone using fine computational meshes to address discrepancies between the simulations and the experiment for the forecone heating. The results of this highly refined forecone study provide strong evidence for the presence of a bias error in the freestream conditions.  1 † Senior Member of Technical Staff, MS 0825, E-mail: cjroy@sandia.gov, Member AIAA ‡ Member of Technical Staff, MS 0827, E-mail: magalli@sandia.gov, Member AIAA § Principal Member of Technical Staff, MS 0820, E-mail: tjbarte@sandia.gov, Member AIAA # Principal Member of Technical Staff, MS 0825, E-mail: jlpayne@sandia.gov, Member AIAA * Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000. This paper is declared a work of the U. S. Government and is not subject to copyright protection in the United States. Nomenclature Cp specific heat at constant pressure, J/kgK f general solution variable h specific enthalpy, J/kg p pressure, N/m2, order of accuracy q heat flux, W/m2 R specific gas constant, J/kgK (= 296.8 for N2) RN nose radius, m (= 0.00635) r grid refinement factor s specific entropy, J/kgK T translational temperature, K t time, ms u axial velocity component, m/s V velocity magnitude, m/s x axial coordinate, m y radial coordinate, m γ ratio of specific heats μ absolute viscosity, Ns/m2

Parallelized hybrid Monte Carlo simulation of stress-induced texture evolution

A parallelized hybrid Monte Carlo (HMC) methodology is devised to quantify the microstructural evolution of polycrystalline material under elastic loading. The approach combines a time explicit material point method (MPM) for the mechanical stresses with a calibrated Monte Carlo (cMC) model for grain boundary kinetics. The computed elastic stress generates an additional driving force for grain boundary migration. The paradigm is developed, tested, and subsequently used to quantify the effect of elastic stress on the evolution of texture in nickel polycrystals. As expected, elastic loading favors grains which appear softer with respect to the loading direction. The rate of texture evolution is also quantified, and an internal variable rate equation is constructed which predicts the time evolution of the distribution of orientations.

Direct simulation Monte Carlo (DSMC) of rarefied gas flow during etching of large diameter (300-mm) wafers

Strong density gradients of a gas species can be sustained even at very low pressures when the reaction probability of that species is high. Under these conditions, inlet gas arrangements are very important to reaction uniformity.

Hybrid Monte Carlo Simulation of Stress-Induced Texture Evolution with Inelastic Effects

A hybrid Monte Carlo (HMC) approach is employed to quantify the influence of inelastic deformation on the microstructural evolution of polycrystalline materials. This approach couples a time explicit material point method (MPM) for deformation with a calibrated Monte Carlo model for grain boundary motion. A rate-independent crystal plasticity model is implemented to account for localized plastic deformations in polycrystals. The dislocation energy difference between grains provides an additional driving force for texture evolution. This plastic driving force is then brought into a MC paradigm via parametric links between MC and sharp-interface (SI) kinetic models. The MC algorithm is implemented in a parallelized setting using a checkerboard updating scheme. As expected, plastic loading favors texture evolution for grains that have a bigger Schmid factor with respect to the loading direction, and these are the grains most easily removed by grain boundary motion. A macroscopic equation is developed to predict such texture evolution.