Svetlana G. Shasharina

Comparison of C++ and Fortran 90 for object-oriented scientific programming

C++ and Fortran 90 are compared as object-oriented languages for use in scientific computing. C++ is a full-featured, object-oriented language that provides support for inheritance and polymorphism. Fortran 90 can mimic some object-oriented features through combinations of its TYPE and MODULE syntax elements, but it lacks inheritance and thus does not permit code reuse to the same extent as C++. Each language has other useful features unrelated to object-oriented programming, but the additional features of Fortran 90 can be included in C++ through the development of class libraries. In contrast, including the additional features of C++ in Fortran 90 would require further development of the Fortran 90 syntax. A critical feature missing in Fortran 90 is the template, which allows C++ programmers to build portable, reusable code and to dramatically improve the efficiency of the evaluation of complex expressions involving user-defined data types.

Physics of confinement improvement of plasmas with impurity injection in DIII-D

External impurity injection into L mode edge discharges in DIII-D has produced clear confinement improvement (a factor of 2 in energy confinement and neutron emission), reduction in all transport channels (particularly ion thermal diffusivity to the neoclassical level), and simultaneous reduction of long wavelength turbulence. Suppression of the long wavelength turbulence and transport reduction are attributed to synergistic effects of impurity induced enhancement of E × B shearing rate and reduction of toroidal drift wave turbulence growth rate. A prompt reduction of density fluctuations and local transport at the beginning of impurity injection appears to result from an increased gradient of toroidal rotation enhancing the E × B shearing. Transport simulations carried out using the National Transport Code Collaboration demonstration code with a gyro-Landau fluid model, GLF23, indicate that E × B shearing suppression is the dominant transport suppression mechanism.

Omnigenity and quasihelicity in helical plasma confinement systems

Omnigenous stellarators, those with bounce averaged drifts lying within the magnetic surfaces, form a much larger class than quasihelical stellarators (i.e., with magnetic-field strength depending on only a single linear combination of the toroidal flux angles) while nevertheless having very good transport properties. It is shown by construction that exactly omnigenous magnetic strengths can be very far from quasihelical, but that all such magnetic strengths are nonanalytic. However, by truncation of the Fourier representation one can obtain analytic magnetic strengths that are very close to being omnigenous while still very far from quasihelical. This indicates that the condition of quasihelicity can be significantly relaxed and yet retain good neoclassical transport. Finally, it is conjectured that reasonably good transport can be obtained by requiring omnigenity for only the deeply trapped and marginally trapped particles. Such fields will have no transition particles and, hence, no separatrix crossing...

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).

Ripple induced stochastic loss of alpha particles in a tokamak

The Fokker-Planck equation is solved for alpha particles in a tokamak, including both pitch angle scattering and slowing down. Losses of the toroidally trapped particles due to stochastic diffusion related to magnetic ripple are accounted for by assuming that all the stochastic orbits leave the device immediately. This assumption permits the problem to be solved by the introduction of a loss cone corresponding to the stochastic region of the velocity space. The eigenvalue problem is solved numerically, to show that the additional stochastic loss of energy due to scattering into the loss cone substitutes about 30% of the birth energy for typical fusion parameters

Grid service for visualization and analysis of remote fusion data

Simulations and experiments in the fusion and plasma physics community generate large datasets at remote sites. Visualization and analysis of these datasets are difficult because of the incompatibility among the various data formats adopted by simulation, experiments, and analysis tools, and the large sizes of analyzed data. Grids and Web services technologies are capable of providing solutions for such heterogeneous settings, but need to be customized to the field-specific needs and merged with distributed technologies currently used by the community. This paper describes how we are addressing these issues in the fusion grid service under development. We also present performance results of relevant data transfer mechanisms.

Transport in non-axisymmetric tori with up-down asymmetry

Transport phenomena arising from the up-down asymmetry of ripple and helical perturbations of the magnetic field in tokamaks and stellarators are investigated. Accounting for this asymmetry is important for devices with vertically displaced magnetic axes or with a single null divertor. The asymmetry manifests itself in different magnitudes of the ripple or helical perturbations in the up and down parts of the magnetic surface. Owing to this difference, the toroidally trapped particles acquire a net radial drift. The collisionless dynamics of the particles in such configurations are studied, as well as the solutions of the kinetic equation. It is shown that, in spite of the net radial drift, the up-down asymmetry does not lead to enhanced diffusive transport, compared with the conventional ripple transport, although the convective loss at low collisionality might be large

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.

First results from core-edge parallel composition in the FACETS project

FACETS (Framework Application for Core-Edge Transport Simulations), now in its second year, has achieved its first coupled core-edge transport simulations. In the process, a number of accompanying accomplishments were achieved. These include a new parallel core component, a new wall component, improvements in edge and source components, and the framework for coupling all of this together. These accomplishments were a result of an interdisciplinary collaboration among computational physics, computer scientists, and applied mathematicians on the team.

Introducing FACETS, the Framework Application for Core-Edge Transport Simulations

The FACETS (Framework Application for Core-Edge Transport Simulations) project began in January 2007 with the goal of providing core to wall transport modeling of a tokamak fusion reactor. This involves coupling previously separate computations for the core, edge, and wall regions. Such a coupling is primarily through connection regions of lower dimensionality. The project has started developing a component-based coupling framework to bring together models for each of these regions. In the first year, the core model will be a 1 ½ dimensional model (1D transport across flux surfaces coupled to a 2D equilibrium) with fixed equilibrium. The initial edge model will be the fluid model, UEDGE, but inclusion of kinetic models is planned for the out years. The project also has an embedded Scientific Application Partnership that is examining embedding a full-scale turbulence model for obtaining the crosssurface fluxes into a core transport code.