Srinath Vadlamani

FACETS A Framework for Parallel Coupling of Fusion Components

Coupling separately developed codes offers an attractive method for increasing the accuracy and fidelity of the computational models. Examples include the earth sciences and fusion integrated modeling. This paper describes the Framework Application for Core-Edge Transport Simulations (FACETS).

The particle-continuum method: an algorithmic unification of particle-in-cell and continuum methods ✩

A new numerical algorithm that encompasses both the δf particle-in-cell (PIC) method and a continuum method has been developed, which is an extension to Denavit’s [J. Comput. Phys. 9 (1972) 75] original “hybrid” method. In this article we describe this new Particle-Continuum algorithm in general, and we note our methods of interpolation. The issue of phase space convergence of this algorithm is discussed. We analyze the induced numerical diffusion of such an algorithm and compare theory with results. We also created a simple problem that demonstrates this algorithm solves the “growing weight” problem.

Concurrent, parallel, multiphysics coupling in the FACETS project

FACETS (Framework Application for Core-Edge Transport Simulations), is now in its third year. The FACETS team has developed a framework for concurrent coupling of parallel computational physics for use on Leadership Class Facilities (LCFs). In the course of the last year, FACETS has tackled many of the difficult problems of moving to parallel, integrated modeling by developing algorithms for coupled systems, extracting legacy applications as components, modifying them to run on LCFs, and improving the performance of all components. The development of FACETS abides by rigorous engineering standards, including cross platform build and test systems, with the latter covering regression, performance, and visualization. In addition, FACETS has demonstrated the ability to incorporate full turbulence computations for the highest fidelity transport computations. Early indications are that the framework, using such computations, scales to multiple tens of thousands of processors. These accomplishments were a result of an interdisciplinary collaboration among computational physics, computer scientists and applied mathematicians on the team.

Exposing Fortran Derived Types to C and Other Languages

When building large scientific codes, you might have to mix different programming languages. The authors show how to bridge the interoperability gap between Fortran 90/95 and C, and from C to other languages, with working code examples.

Coupled core-edge simulations of H-mode buildup using the Fusion Application for Core-Edge Transport Simulations (FACETS) code

Coupled simulations of core and edge transport in the DIII-D shot number 118897, after the L-H transition but before the first edge localized mode (ELM), are presented. For the plasma core transport, a set of one dimensional transport equations are solved using the FACETS:Core solver. The fluxes in this region are calculated using the GLF23 anomalous transport model and Chang-Hinton neoclassical model. For the plasma edge transport, two-dimensional transport equations are solved using the UEDGE code. Fluxes in the edge region use static diffusivity profiles based on an interpretive analysis of the experimental profiles. Simulations are used to study the range of validity of the selected models and sensitivity to neutral fueling. It has been demonstrated that the increase of neutral influx to the level that exceeds the level of neutral influx obtained from analysis simulations with the UEDGE code by a factor of two results in increased plasma density pedestal heights and plasma density levels in the scrape...

Stress Tests of Transport Models Using FACETS Code

The confinement of H‐mode plasmas strongly depends on the H‐mode pedestal structure. The pedestal provides the boundary conditions for the hot core tokamak region and determines the stability properties of the plasma edge. The structure of H‐mode pedestal depends on many factors such as heating of the plasma core, neutral fueling, recycling and density and thermal transport. It is important to elucidate the primary mechanisms that are responsible for the pedestal structure in order to optimize the tokamak performance, and avoid disruptions and large scale instabilities such as neoclassical tearing mode (NTM) and edge localized modes (ELMs). In this study, the FACETS code is used to test several models for anomalous, paleoclassical and neoclassical transport in the plasma edge of tokamaks. The FACETS code is a new whole‐device integrated modeling code that advances plasma profiles in time using a selection of transport models and models for heating and particle sources. The simulation results are compared ...

Simulation of anomalous transport in tokamaks using the FACETS code

The development of a new parallel framework for integrated modeling of tokamak plasmas is a primary objective of the SciDAC Framework Architecture for Core-Edge Transport Simulations (FACETS) project. The FACETS code will be used to predict the performance of tokamak discharges and to optimize tokamak discharge scenarios. Novel parallel numerical algorithms and solvers have been developed in the FACETS project in order to simulate the multi-scale dynamics of tokamak plasmas. The status of development of modules for anomalous transport in the FACETS code is described in this paper. Mechanisms that are used for coupling 1D anomalous transport in the plasma core together with 2D transport in the plasma edge (in near separatrix and scrape-off-layer regions) are considered. Results of the first verification studies based on predictive modeling of several analytical and experimental equilibria are presented.

The Interaction of Iteration Error and Stability for Linear Partial Differential Equations Coupled through an Interface

We investigate properties of algorithms that are used to solve coupled evolutionary partial differential equations posed on neighboring, nonoverlapping domains, where the solutions are coupled by continuity of state and normal flux through a shared boundary. The algorithms considered are based on the widely used approach of iteratively exchanging boundary condition data on the shared boundary at each time step. There exists a significant and sophisticated numerical analysis of such methods. However, computations for practical applications are often carried out under conditions under which it is unclear if rigorous results apply while relatively few iterations are used per time step. To examine this situation, we derive exact matrix expressions for the propagation of the error due to incomplete iteration that can be readily evaluated for specific discretization parameters. Using the formulas, we show that the universal validity of several tenants of the practitioner’s conventional wisdom are not universally valid.