Three-dimensional Transport Simulation Research Articles

Experimental and simulation study of impurity transport response to RMPs in RF-heated H-mode plasmas at EAST

Spatial profiles of impurity emission measurements in the extreme ultraviolet (EUV) spectroscopic range in radiofrequency (RF)-heated discharges are combined with one-dimensional and three-dimensional transport simulations to study the effects of resonant magnetic perturbations (RMPs) on core impurity accumulation at EAST. The amount of impurity line emission mitigation by RMPs appears to be correlated with the ion Z for lithium, carbon, iron and tungsten monitored, i.e. stronger suppression of accumulation for heavier ions. The targeted effect on the most detrimental high-Z impurities suggests a possible advantage using RMPs for impurity control. Profiles of transport coefficients are calculated with the STRAHL one-dimensional impurity transport code, keeping $\nu /D$ fixed and using the measured spatial profiles of $\textrm{F}{\textrm{e}^{20 + }}$ , $\textrm{F}{\textrm{e}^{21 + }}$ and $\textrm{F}{\textrm{e}^{22 + }}$ to disentangle the transport coefficients. The iron diffusion coefficient ${D_{\textrm{Fe}}}$ increases from $1.0- 2.0\;{\textrm{m}^2}\;{\textrm{s}^{ - 1}}$ to $1.5- 3.0\;{\textrm{m}^2}\;{\textrm{s}^{ - 1}}$ from the core region to the edge region $(\rho \gt 0.5)$ after the onset of RMPs. Meanwhile, an inward pinch of iron convective velocity ${\nu _{\textrm{Fe}}}$ decreases in magnitude in the inner core region and increases significantly in the outer confined region, simultaneously contributing to preserving centrally peaked $\textrm{Fe}$ profiles and exhausting the impurities. The ${D_{\textrm{Fe}}}$ and ${\nu _{\textrm{Fe}}}$ variations lead to reduced impurity contents in the plasma. The three-dimensional edge impurity transport code EMC3-EIRENE was also applied for a case of RMP-mitigated high-Z accumulation at EAST and compared to that of low-Z carbon. The exhaust of ${\textrm{C}^{6 + }}$ toward the scrape-off layer accompanying an overall suppression of heavier ${\textrm{W}^{30 + }}$ is observed when using RMPs.

Three-Dimensional Transport Simulations and Modeling of Densely Packed CNTFETs

Carbon nanotube (CNT) field-effect transistor (FET) based applications require multiple CNTs with small tube pitches for competing with existing silicon technologies. The associated dense packing of CNTs leads to electrostatic interactions between the tubes also known as screening effects. In this paper, the impact of the latter on the CNT current and charge density has been investigated by means of three-dimensional (3D) device simulation (TCAD), in which the CNT is represented by its actual 3D structure rather than by a one-dimensional (1D) line, allowing to include the effects of an inhomogeneous charge and current density distribution along the CNT circumference. Different analytical models describing the impact of the oxide thickness, CNT diameter and CNT pitch on the oxide capacitance are compared. Using the most accurate model, the dependence of drain current, transconductance, subthreshold slope and transit frequency on the above mentioned parameters has been determined using TCAD simulations.

Method for comparison of tokamak divertor strike point data with magnetic perturbation models

Resonant magnetic perturbations (RMPs) used to control type-I edge-localized modes produce a splitting of the peaks of heat flux profile on the divertor. The additional peaks can appear outside the areas connected to the plasma core by the perturbed field lines due to perpendicular transport. This effect has been studied so far using demanding three-dimensional transport simulations. We present a simple method to quantitatively estimate the heat flux profile that the RMP will produce on the divertor. The method needs only field line tracing to determine the so-called tangle distance function, and knowledge of the heat flux profile on the unperturbed heat flux profile. We find good agreement between this method and a transport simulation.

Comparison of global variance reduction techniques for Monte Carlo radiation transport simulations of ITER

It is known that a priori information on the distribution of neutron fluxes around a neutron transport model can improve the computational efficiency of Monte Carlo transport calculations. The uncertainty lies in finding a method which obtains this information as fast and effortlessly as possible. A number of global variance reduction techniques were tested on the ITER reference neutronics model for their applicability for performing large three-dimensional transport simulations for ITER. Among these, we propose a new method which is computationally fast and efficient in determining the a priori flux information. The new method, known as MAGIC, was found to perform well achieving an efficiency 74 times that of an analogue run; the next most efficient was FW-CADIS, performing 48 times better than analogue.

The influence of dimensionality on simulations of mass recovery from nonuniform dense non-aqueous phase liquid (DNAPL) source zones

The influence of model dimensionality on predictions of mass recovery from dense non-aqueous phase liquid (DNAPL) source zones in nonuniform permeability fields was investigated using a modified version of the modular three-dimensional transport simulator (MT3DMS). Thirty-two initial two- (2D) and three-dimensional (3D) tetrachloroethene–DNAPL source zone architectures, taken from a recent modeling study, were used as initial conditions for this analysis. Commonly employed source zone metrics were analyzed to determine differences between 2D and 3D predictions: (i) down-gradient flux-averaged contaminant concentration, (ii) reductions in contaminant mass flux through a down-gradient boundary, (iii) source zone ganglia-to-pool (GTP) ratio, and (iv) time required to achieve a remediation objective. 3D flux-averaged contaminant concentrations were approximately 3.5 times lower than concentrations simulated in 2D. This difference was attributed to dilution of the contaminant concentrations down gradient of the source zone. Contaminant flux reduction predictions for a given mass recovery were generally 5% higher in 3D simulations than in 2D simulations. The GTP ratio declined over time as mass was recovered in both 2D and 3D simulations. Although the source longevity (i.e., time required to achieve 99.99% mass recovery) differed between individual 2D and 3D realizations, the mean source longevity for the 2D and 3D simulation ensembles was within 2%. 2D simulations tended to over-predict the time required to achieve lower mass recovery levels (e.g. 50% mass recovery) due to a smaller contaminated area exposed to uncontaminated water. These findings suggest that ensemble averages of 2D numerical simulations of DNAPL migration, entrapment, dissolution, and mass recovery in statistically homogenous, nonuniform media may provide reasonable approximations to average behavior obtained using simulations conducted in fully three-dimensional domains.

Least-squares finite-element SN method for solving three-dimensional transport equation

A discrete ordinates finite-element method for solving three-dimensional first-order neutron transport equation is proposed using a least-squares variation. It avoids the singularity in void regions of the method derived from the second-order equation. Different from using the standard Galerkin variation applying to the first-order equation, the least-squares variation results in a symmetric matrix, which can be solved easily and effectively. The approach allows a continuous finite-element. To eliminate the discontinuity of the angular flux on the fixed flux boundary in the spherical harmonics method, the equation is discretized using the discrete ordinates method for angular dependency. A three-dimensional transport simulation code is developed and applied to some benchmark problems with unstructured geometry. The numerical results demonstrate the accuracy and feasibility of the method.

Three-dimensional transport simulations of the dispersal of volcanic aerosol from Mount Pinatubo

Three-dimensional transport simulations of the dispersal of volcanic aerosol from Mount Pinatubo