Monte Carlo Module Research Articles

EDGE2D modelling of edge profiles obtained in JET diagnostic optimized configuration

Nine type-I ELMy H-mode discharges in diagnostic optimized configuration in JET are analysed with the EDGE2D/NIMBUS package. EDGE2D solves the fluid equations for the conservation of particles, momentum and energy for hydrogenic and impurity ions, while neutrals are followed with the two-dimensional Monte Carlo module NIMBUS. Using external boundary conditions from the experiment, the perpendicular heat conductivities χi,e and the particle transport coefficients D, v are varied until good agreement between code result and measured data is obtained. A step-like ansatz is used for the edge transport parameters for the outer core region, the edge transport barrier and the outer scrape-off layer. The time-dependent effect of edge localized modes on the edge profiles is simulated with an ad hoc ELM model based on the repetitive increase of the transport coefficients χi,e and D. The values of the transport coefficients are matched to experimental data mapped to the outer midplane, in the course of which radial shifts of experimental profiles of the order of 1 cm caused by the accuracy limit of the equilibrium reconstruction are taken into account. Simulated divertor profiles obtained from the upstream transport ansatz and the experimental boundary conditions agree with measurements, except a small region localized at the separatrix strike points which is supposed to be affected by direct ion losses. The integrated analysis using EDGE2D modelling, although still limited by the marginal spatial resolution of individual diagnostics, allows the characterization of profiles in the edge/pedestal region and supplies additional information on the separatrix position. The steep density gradient zone inside the separatrix shrinks compared to the electron temperature with increasing density, indicating the effect of the neutral penetration depth becoming shorter than the region of reduced transport.

Evaluation of the first commercial Monte Carlo dose calculation engine for electron beam treatment planning.

The purpose of this study is to perform a clinical evaluation of the first commercial (MDS Nordion, now Nucletron) treatment planning system for electron beams incorporating Monte Carlo dose calculation module. This software implements Kawrakow's VMC++ voxel-based Monte Carlo calculation algorithm. The accuracy of the dose distribution calculations is evaluated by direct comparisons with extensive sets of measured data in homogeneous and heterogeneous phantoms at different source-to-surface distances (SSDs) and gantry angles. We also verify the accuracy of the Monte Carlo module for monitor unit calculations in comparison with independent hand calculations for homogeneous water phantom at two different SSDs. All electron beams in the range 6-20 MeV are from a Siemens KD-2 linear accelerator. We used 10,000 or 50,000 histories/cm2 in our Monte Carlo calculations, which led to about 2.5% and 1% relative standard error of the mean of the calculated dose. The dose calculation time depends on the number of histories, the number of voxels used to map the patient anatomy, the field size, and the beam energy. The typical run time of the Monte Carlo calculations (10,000 histories/cm2) is 1.02 min on a 2.2 GHz Pentium 4 Xeon computer for a 9 MeV beam, 10 x 10 cm2 field size, incident on the phantom 15 x 15 x 10 cm3 consisting of 31 CT slices and voxels size of 3 x 3 x 3 mm3 (total of 486,720 voxels). We find good agreement (discrepancies smaller than 5%) for most of the tested dose distributions. We also find excellent agreement (discrepancies of 2.5% or less) for the monitor unit calculations relative to the independent manual calculations. The accuracy of monitor unit calculations does not depend on the SSD used, which allows the use of one virtual machine for each beam energy for all arbitrary SSDs. In some cases the test results are found to be sensitive to the voxel size applied such that bigger systematic errors (>5%) occur when large voxel sizes interfere with the extensions of heterogeneities or dose gradients because of differences between the experimental and calculated geometries. Therefore, user control over voxelization is important for high accuracy electron dose calculations.

The development of a stochastic physiologically-based pharmacokinetic model for lead

This presentation describes the development of a prototype Monte Carlo module for the physiologically-based pharmacokinetic (PBPK) model for lead, created by Dr Ellen O'Flaherty. The module uses distributions for the following: exposure parameters (soil and dust concentrations, daily soil and ingestion rate, water lead concentration, water ingestion rate, air lead concentration, inhalation rate and dietary lead intake); absoption parameters; and key pharmacokinetic parameters (red blood binding capacity and half saturation concentration). Distributions can be specified as time-invariant or can change with age. Monte Carlo model predicted blood levels were calibrated to empirically measured blood lead levels for children lining in Midvale, Utah (a milling/smelting community). The calibrated model was then evaluated using blood lead data from Palmerton, Pennsylvania (a town with a former smelt) and Sandy, Utah, (a town with a former smelt and slag piles). Our initial evaluation using distributions for exposure parameters showed that the model accurately predicted geometric (GM) blood lead level of Palmerton and Sandy and slightly over predicted the GSD. Consideration of uncertainty in red blood cell parameters substantially inflated the GM. Future model development needs to address the correlation among parameters and the use of parameters for long-term exposure derived from short-term studies.

EGS-Ray, ein Programm zur Visualisierung von Monte-Carlo-Rechnungen in der Strahlenphysik

A Windows program is introduced which allows a relatively easy and interactive access to Monte Carlo techniques in clinical radiation physics. Furthermore, this serves as a visualization tool of the methodology and the results of Monte Carlo simulations. The program requires only little effort to formulate and calculate a Monte Carlo problem. The Monte Carlo module of the program is based on the well-known EGS4/PRESTA code. The didactic features of the program are presented using several examples common to the routine of the clinical radiation physicist.

Quadrilateral Finite Element Monte Carlo Simulation of Complex Shape Compound FETs

The complex recess and gate shape of modem compound FETs greatly affect the device parasitics and therefore impose the need for proper description of the device geometry and surface conditions in any practical device simulations. In this paper we describe a new Monte Carlo (MC) module incorporated in our Heterojunction 2D Finite element FET simulator H2F [1]. The module combines realistic quadrilateral finite‐element description of the device geometry with realistic particle simulation of the non‐equilibrium hot carrier transport in short recess gate compound FETs. A Single Programme Multiple Data (SPMD) parallel approach makes it possible to use our MC simulator for practical design work, generating the necessary I‐V characteristics in parallel. The capabilities of the finite element MC module are illustrated in example simulations of a 200nm pseudomorphic HEMT fabricated in the Nanoelectronics Research Centre of Glasgow University.

Coupled Lagrangian Monte Carlo PDF–CFD Computation of Gas Turbine Combustor Flowfields With Finite-Rate Chemistry

A coupled Lagrangian Monte Carlo Probability Density Function (PDF)-Eulerian Computational Fluid Dynamics (CFD) technique is presented for calculating steady three-dimensional turbulent reacting flow in a gas turbine combustor. PDF transport methods model turbulence-combustion interactions more accurately than conventional turbulence models with an assumed shape PDF. The PDF transport equation was solved using a Lagrangian particle tracking Monte Carlo (MC) method. The PDF modeled was over composition only. This MC module has been coupled with CONCERT, which is a fully elliptic three-dimensional body-fitted CFD code based on pressure correction techniques. In an earlier paper (Tolpadi et al., 1995), this computational approach was described, but only fast chemistry calculations were presented in a typical aircraft engine combustor. In the present paper, reduced chemistry schemes were incorporated into the MC module that enabled the modeling of finite rate effects in gas turbine flames and therefore the prediction of CO and NOx emissions. With the inclusion of these finite rate effects, the gas temperatures obtained were also more realistic. Initially, a two scalar scheme was implemented that allowed validation against Raman data taken in a recirculating bluff body stabilized CO/H2/N2-air flame. Good agreement of the temperature and major species were obtained. Next, finite rate computations were performed in a single annular aircraft engine combustor by incorporating a simple three scalar reduced chemistry scheme for Jet A fuel. This three scalar scheme was an extension of the two scalar scheme for CO/H2/N2 fuel. The solutions obtained using the present approach were compared with those obtained using the fast chemistry PDF transport approach (Tolpadi et al., 1995) as well as the presumed shape PDF method. The calculated exhaust gas temperature using the finite rate model showed the best agreement with measurements made by a thermocouple rake. In addition, the CO and NOx emission indices were also computed and compared with corresponding data.

Monte Carlo Probability Density Function Method for Gas Turbine Combustor Flowfield Predictions

A coupled Lagrangian Monte Carlo (MC) probability density function (PDF), Eulerian computational e uid dynamics (CFD) technique is presented for calculating steady three-dimensional turbulent reacting e ow in a gas turbine combustor. PDF transport methods model turbulence ‐ combustion interactions more accurately than conventional turbulence models with an assumed-shape PDF. The PDF was over composition only. The PDF transport equation was solved using a Lagrangian particle-tracking MC method. This MC module has been coupled with CONCERT, which is a fully elliptic three-dimensional bodye tted CFD code based on pressure correction techniques. CONCERT calculates the mean velocity and mixing frequency e eld that are required by the composition PDF in the MC module, whereas the MC module computes the PDF from which the mean density e eld is extracted and supplied to CONCERT. This modeling approach was initially validated against Raman data taken in a recirculating bluff body stabilized e ame. The computed mixture fraction and its variance (as obtained from the calculated PDF ) compared very well against the corresponding measurements made along several radial lines at different axial downstream positions and along the axis. A typical single annular aircraft engine combustor was also analyzed. In this preliminary study, the e owe eld, fuel, and temperature distribution were obtained based on the assumption of fast chemistry. The solutions obtained using the present approach were compared with those obtained using a presumed-shape PDF method. The comparison of the calculated exhaust gas temperatures using these two approaches with measurements made by a thermocouple rake appeared to indicate better agreement with the PDF transport technique.

Finite element Monte Carlo simulation of recess gate compound FFTs

In this paper we report on a new Monte Carlo module for simulation of recess gate compound FETs incorporated in our Heterojunction 2D Finite element simulator H2F. The shape of the gate and recess determines the device parasitics to a great extent and strongly influences the performance of these devices. A finite element discretization scheme has been used to model precisely the device geometry. The module combines the accurate description of the device geometry with realistic particle simulation of hot carrier transport. The flexibility of our approach is illustrated in example simulations of FETs fabricated in the Nanoelectronics Research Centre at Glasgow University.

The effect of subwafer dielectrics on plasma properties in plasma etching reactors

Nonplanar electrode topographies in plasma etching reactors are known to perturb plasma properties. In this article results from a computational study of plasma etching reactors having nonuniform dielectric structures below the wafer are presented. The system is an inductively coupled plasma reactor having a 13.56 MHz bias applied to the substrate. The model we have used is a hybrid simulation consisting of electromagnetics, electron Monte Carlo and fluid kinetics modules, and an off-line plasma chemistry Monte Carlo simulation. We found that the subwafer dielectric adds a series capacitance to the sheath and wafer resulting in voltage division of the applied potential between the sheath, wafer, and dielectric. This produces a smaller sheath potential and smaller sheath thickness above the dielectric. The ion energy distribution is therefore depressed in the vicinity of the dielectric. The effect is more severe at high plasma densities where the capacitance of the sheath is larger compared to the subwafer dielectric.