[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: A new additive flux minimization technique is proposed for carrying out
the verification and validation (V&V) of anomalous transport models.
In this approach, the plasma profiles are computed in time dependent
predictive simulations in which an additional effective diffusivity is
varied. The goal is to obtain an optimal match between the computed and
experimental profile. This new technique has several advantages over
traditional V&V methods for transport models in tokamaks and takes
advantage of uncertainty quantification methods developed by the applied
math community. As a demonstration of its efficiency, the technique is
applied to the hypothesis that the paleoclassical density transport
dominates in the plasma edge region in DIII-D tokamak discharges. A
simplified version of the paleoclassical model that utilizes the Spitzer
resistivity for the parallel neoclassical resistivity and neglects the
trapped particle effects is tested in this paper. It is shown that a
contribution to density transport, in addition to the paleoclassical
density transport, is needed in order to describe the experimental
profiles. It is found that more additional diffusivity is needed at the
top of the H-mode pedestal, and almost no additional diffusivity is
needed at the pedestal bottom. The implementation of this V&V
technique uses the FACETS::Core transport solver and the DAKOTA toolkit
for design optimization and uncertainty quantification. The FACETS::Core
solver is used for advancing the plasma density profiles. The DAKOTA
toolkit is used for the optimization of plasma profiles and the
computation of the additional diffusivity that is required for the
predicted density profile to match the experimental profile.
Physics of Plasmas 10/2013; 20(10):2501-. DOI:10.1063/1.4823701 · 2.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We consider multiphysics applications from algorithmic and architectural perspectives, where ‘‘algorithmic’’ includes both mathematical analysis and computational complexity, and ‘‘architectural’’ includes both software and hardware environments. Many diverse multiphysics applications can be reduced, en route to their computational simulation, to a common algebraic coupling paradigm. Mathematical analysis of multiphysics coupling in this form is not always practical for realistic applications, but model problems representative of applications discussed herein can provide insight. A variety of software frameworks for multiphysics applications have been constructed and refined within disciplinary communities and executed on leading-edge computer systems. We examine several of these, expose some commonalities among them, and attempt to extrapolate best practices to future systems. From our study, we summarize challenges and forecast opportunities.
International Journal of High Performance Computing Applications 02/2013; 27(1). DOI:10.1177/1094342012468181 · 1.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 with experimental measurements from the DIII-D tokamak.
[Show abstract][Hide abstract] ABSTRACT: The confinement of H-mode plasmas strongly depends on the 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 heating of the plasma core, neutral fueling, recycling and
particle 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, avoid disruptions and large scale
instabilities such as NTM and 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 with
experimental measurements from major US tokamaks such DIII-D. These
validation efforts allows to discriminate between different models for
transport in the different regions of the H-mode pedestal. ^* This
research is supported by US Department of Energy.
[Show abstract][Hide abstract] ABSTRACT: As various efforts to integrate fusion codes proceed worldwide, standards for sharing data have emerged. In the U.S., the SWIM project has pioneered the development of the Plasma State, which has a flat-hierarchy and is dominated by its use within 1.5D transport codes. The European Integrated Tokamak Modeling effort has developed a more ambitious data interoperability effort organized around the concept of Consistent Physical Objects (CPOs). CPOs have deep hierarchies as needed by an effort that seeks to encompass all of fusion computing. Here, we discuss ideas for implementing data interoperability that is complementary to both the Plasma State and CPOs. By making use of attributes within the netcdf and HDF5 binary file formats, the goals of data interoperability can be achieved with a more informal approach. In addition, a file can be simultaneously interoperable to several standards at once. As an illustration of this approach, we discuss its application to the development of synthetic diagnostics that can be used for multiple codes.
[Show abstract][Hide abstract] ABSTRACT: 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).
Parallel, Distributed and Network-Based Processing (PDP), 2010 18th Euromicro International Conference on; 03/2010
[Show abstract][Hide abstract] ABSTRACT: The FACETS code, a whole-device integrated modeling code that self-consistently computes plasma profiles for the plasma core and edge in tokamaks, has been recently developed as a part of the SciDAC project for core-edge simulations. A choice of transport models is available in FACETS through the FMCFM interface . Transport models included in FMCFM have specific ranges of applicability, which can limit their use to parts of the plasma. In particular, the GLF23 transport model does not include the resistive ballooning effects that can be important in the tokamak pedestal region and GLF23 typically under-predicts the anomalous fluxes near the magnetic axis . The TGLF and GYRO transport models have similar limitations . A combination of transport models that covers the entire discharge domain is studied using FACETS in a realistic tokamak geometry. Effective diffusivities computed with the FMCFM transport models are extended to the region near the separatrix to be used in the UEDGE code within FACETS. 1. S. Vadlamani et al. (2009) %First time-dependent transport simulations using GYRO and NCLASS within FACETS (this meeting).2. T. Rafiq et al. (2009) %Simulation of electron thermal transport in H-mode discharges Submitted to Phys. Plasmas.3. C. Holland et al. (2008) %Validation of gyrokinetic transport simulations using %DIII-D core turbulence measurements Proc. of IAEA FEC (Switzerland, 2008)
[Show abstract][Hide abstract] ABSTRACT: A general purpose time-domain plasma simulation algorithm has been
constructed and implemented in the VORPAL software framework. It is
able to represent the complex physical boundaries of the ICRF antenna
structure, and complex magnetic topology of the edge region. This
time-domain algorithm is now being supplemented with a sub-grid boundary
sheath model based upon the work of D'Ippolito et al. We verify the
model against known behavior from frequency-domain sheath calculations
in 1-dimension. We also test the model in 3-D simulation including RF
launcher geometry, and compare results with related models being
implemented in 1, 2, and 3-D full wave solvers. This new model will
provide realistic estimates of power loss due to short range sheaths. We
will also present possible strategies for treating mid and long range
sheaths within the model.  D. N. Smithe, Physics of Plasmas,
Vol. 14 056104 (2007).  D. A. D'Ippolito and J. R. Myra, Phys.
Plasmas 13, 102508 (2006).
[Show abstract][Hide abstract] ABSTRACT: Atmospheric pressure plasmas have application in plasma processing and plasma lighting. One common technique for creating these plasmas is to use RF to drive a discharge in a gas column. A main unexplained feature of these discharges is a radial contraction of the plasma, believed to depend in part on the depth to which the RF wave penetrates the gas column. We present collisional particle-in-cell (PIC) simulations of such a system, specifically looking at how the collisions modify the plasma skin depth for these parameters and geometries. We find the skin depth changes roughly 30% from 0.53 mm to 0.71 mm as the temperature increases from 0 to 500 eV for parameters relevant to atmospheric pressure plasmas in use today.
[Show abstract][Hide abstract] ABSTRACT: Laser driven wakefield accelerators produce accelerating fields thousands of times those achievable in conventional radio-frequency accelerators, offering compactness and ultrafast bunches to potentially extend the frontiers of high energy physics and enable laboratory scale ultrafast radiation sources. Realization of this potential requires understanding of accelerator physics to advance beam performance and stability, and particle simulations model the highly nonlinear, kinetic physics required. One-to-one simulations of experiments provide new insight for optimization and development of 100 MeV to GeV and beyond laser accelerator stages, and on production of reproducible and controllable low energy spread beams with improved emittance (focusability) and energy through control of injection.
Journal of Physics Conference Series 09/2007; 78(1). DOI:10.1088/1742-6596/78/1/012021
[Show abstract][Hide abstract] ABSTRACT: Plasma accelerator methods offer the potential to reduce the size of moderate and high energy accelerators by factors of 1000. In the past few years great advances have been made in the production of low emittance, high quality (i.e., monoenergetic) electron beams with energies between .1 and 1 GeV using ultra-fast (< 50 femtoseconds), high power (> 10TW) lasers. The most noticeable of these advances were the experimental results presented in the "Dream Beam" issue of Nature and in a recent issues of Physical Review Letters, Nature, and Nature Physics. The experimental progress have been made due to advances in lasers, diagnostics, plasma sources, and the knowledge of how to control of this highly nonlinear acceleration process. And this experimental progress has occurred simultaneously with and been in part due to advances in modeling capabilities. Using a hierarchy of particle-in-cell (PIC) codes OSIRIS, VORPAL, and QuickPIC, we have performed numerous full scale 3D simulations using parameters quoted from the Nature and Nature Physics articles. Our simulations have predicted results, provided agreement between simulations and experiments (within the shot-to-shot variations of the experiments), and provided insight into the complicated physics of the experiments. Most importantly, as our confidence in the fidelity of our methods increases we can now guide the planning of new experiments, and probe parameters that are not yet available. Thereby providing a "road map" for generating high quality, high-charge 10 to 100 GeV electron beams for use in high-energy physics and light sources.
Journal of Physics Conference Series 09/2007; 78(1). DOI:10.1088/1742-6596/78/1/012077
[Show abstract][Hide abstract] ABSTRACT: Experiments have observed significant toroidal and poloidal flows in their plasmas [1-3]. This has led to the need for a flowing equilibrium solver that is equipped to handle both fixed and free boundaries. In order to achieve this, the mathematical properties of the flow equilibrium equations need to be better understood. Difficulty arises when analyzing these equations since they are not only fully non-linear, but also the differential operator itself is coupled to an algebraic equation. We discuss the method and process used to formulate the equilibrium problem with flow effects in a computationally tractable form and the methods available to solve the resulting equations. Initial results from an implementation of the equilibrium equations are also presented.  S.K. Erents, A.V. Chankin, G.F. Matthews, P.C. Stangeby, Plasma Phys. Controlled Fusion 42, 905 (2000).  T.S. Taylor, H.St. John, A.D. Turnbull, et al. Plasma Phys. Controlled Fusion 36 B229 (1994).  M. Ono, S.M. Kaye, Y.K.M. Peng et al. Nucl. Fusion 40, 557 (2000).
[Show abstract][Hide abstract] ABSTRACT: Plasma-based lepton acceleration concepts are a key element of the long-term R&D portfolio for the U.S. Office of High Energy Physics. There are many such concepts , but we consider only the laser (LWFA) and plasma (PWFA) wakefield accelerators. We present a summary of electromagnetic particle-in-cell (PIC) simulations for recent LWFA and PWFA experiments. These simulations, including both time explicit algorithms and reduced models, have effectively used terascale computing resources to support and guide experiments in this rapidly developing field. We briefly discuss the challenges and opportunities posed by the near-term availability of petascale computing hardware.
Journal of Physics Conference Series 09/2006; 46(1). DOI:10.1088/1742-6596/46/1/030
[Show abstract][Hide abstract] ABSTRACT: The Framework Architecture for Core-Edge Transport Simulations (FACETS) is a SciDAC project for self-consistent simulations of core-edge-wall transport in tokamaks using leadership class computers . For analysis of transient peak power load handling, PFC erosion/deposition and lifetime, plasma impurity contamination, and hydrogen retention issues in FACETS, we developed the 1D continuum code WALLPSI . WALLPSI simulates highly non-linear transport, release and trapping of hydrogen species in wall, and calculates the wall temperature and emerging impurity fluxes. We present progress on the development of an interface to allow WALLPSI to be invoked from within the multiple-component FACETS infrastructure. Each wall segment is modeled by WALLPSI instance which are all run concurrently on separate CPUs. FACETS provides the mechanism for coupling the wall to plasma transport code UEDGE. The results are presented showing non-linear variation of hydrogen species wall inventory in response to incident plasma fluxes and abrupt changes in edge plasma parameters caused by wall switching from net pumping regimes to net outgassing ones using an initial slab edge plasma setup.  J.Cary et al J.Physics CS 125(2008)012040  A.Pigarov et al JNM 390(2009)192