Scientific Discovery Through Advanced Computing Research Articles

A reduced model for the ITER divertor based on SOLPS solutions for ITER Q = 10 baseline conditions: B. A reduced model based on reversed-direction two point modeling * ∗This manuscript has been partially authored by UT-Battelle, LLC, under Contract DE-AC05-00OR22725 with the US Department of Energy. This material is based upon work supported the Scientific Discovery through Advanced Computing

Edge codes such as SOLPS coupled to neutral codes such as EIRENE have become so comprehensive and sophisticated that they now constitute, in effect, ‘code-experiments’ that, as for actual experiments, can benefit from interpretation using simple models and conceptual frameworks, i.e. reduced models. The first task is the identification of options for the reduced model control parameters that are best suited for control of the action of the divertor, i.e. for control of target power loading and sputter–erosion, primarily. A strong correlation between the electron temperature at the divertor target, T e,t, and the neutral deuterium D2 density at the target, n D2,t, flux-tube resolved, has recently been reported for a number of code studies including SOLPS-4.3 modeling of a set of ∼50 ITER baseline cases: Q DT = 10, q 95 = 3, P SOL = 100 MW, metallic walls, and Ne seeding (Pitts et al 2019). Part A of the present study reports new results for largely the same ITER cases, confirming the strong correlation reported earlier between local values of T e,t, and (i) n D2,t, and (ii) normalized volumetric losses of power and pressure in the divertor. Strong correlations have now also been found, and are reported here for the first time, between T e,t and all of the divertor target quantities of practical interest. A physical explanation for this surprising result has not as yet been fully identified; nevertheless it has encouraging implications for reduced modeling of the ITER divertor. For such ITER conditions, (i) the global Ne injection rate, InjNe (Ne s−1), and (ii) the electron temperature at the location on the target where the peak power deposition occurs, T e,t@q⊥,pk (eV), are found to be promising reduced model control parameters. In this part B, a reduced model for the ITER divertor is developed and described in detail, based on reversed-direction two point modeling, Rev2PM. The input to the reduced model is a value of the variable pair for a chosen case and the output are values of the various target as well as divertor-entrance quantities of practical interest, e.g. q ⊥,pk, the electron density at the X-point, n e,Xpt, etc. The reduced model was quantitatively characterized using one half of the code cases; it was then used to successfully predict (replicate) the code values of e.g. n e,Xpt for the other half.

A reduced model for the ITER divertor based on SOLPS solutions for ITER Q = 10 baseline conditions: A. identifying options for the control parameters* *This manuscript has been partially authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy. This material is based upon work supported the Scientific Discovery through Advanced Computing (SciDAC) project on Plasma-Surface

Edge codes such as SOLPS coupled to neutral codes such as EIRENE have become so comprehensive and sophisticated that they now constitute, in effect, ‘code-experiments’ that, as for actual experiments, can benefit from interpretation using simple models and conceptual frameworks, i.e. reduced models. The first task is the identification of options for the reduced model control parameters that are best suited for control of the action of the divertor, i.e. for control of target power loading and sputter-erosion, primarily. A strong correlation between the electron temperature at the divertor target, T e,t, and the neutral deuterium D2 density at the target, n D2,t, flux-tube resolved, has recently been reported for a number of code studies including SOLPS-4.3 modeling of a set of ∼50 ITER baseline cases: Q DT = 10, q 95 = 3, P SOL = 100 MW, metallic walls, and Ne seeding (Pitts et al 2019 Nucl. Mater. Energy 20 100696). This part A of the present study reports new results for largely the same ITER cases, confirming the strong correlations reported earlier between local values of T e,t, and (i) n D2,t, and (ii) normalized volumetric losses of power and pressure in the divertor. Strong correlations have now also been found, and are reported here for the first time, between T e,t and all of the divertor target quantities of practical interest. A physical explanation for this surprising result has not been fully identified; nevertheless it has encouraging implications for reduced modeling of the ITER divertor. For such ITER conditions, (i) the global Ne injection rate, InjNe (Ne s−1), and (ii) the electron temperature at the location on the target where the peak power deposition occurs, T e,t @q ⊥,pk (eV), are found to be promising reduced model control parameters. In the companion report, part B, a reduced model for the ITER divertor is developed and described in detail, based on reversed-direction 2 point modelling, Rev2PM. The input to the reduced model is a value of the variable pair and the output are values of the various target as well as divertor-entrance quantities of practical interest, e.g. q ⊥,pk, n e,Xpt (the electron density at the poloidal location of the X-point), etc. In part B the reduced model is quantitatively characterized using one half of the code cases; it is then used to successfully predict (replicate) the code values of e.g. n e,Xpt for the other half of the cases.

Molecular Structure and Ion Transport Near Graphite-Electrolyte Interfaces in Lithium-Ion Batteries

The performance of lithium-ion secondary batteries is strongly tied to ionic transport through electrode-electrolyte interfaces. Ion solvation structures near the interface as well as the chemistry of the electrode surface, in particular the edge termination of graphite electrodes, can impact Li ion transport into and out of the electrode. Understanding these effects is critical to optimizing battery performance. We have performed molecular dynamics simulations of organic liquid electrolytes in contact with graphite electrodes, with and without electric field, to understand the relationship between ion solvation structure and transport near the interface. Prototypical electrolytes consisting of ethylene carbonate (EC) or ethyl methyl carbonate (EMC) organic solvent containing dissolved LiPF6 salt were studied, in contact with armchair-edge oriented graphite electrodes. Various surface terminations of the graphite, including H and OH, were compared. We find that Li+ maintains a well-structured solvation shell even when an electric field is present. The bulkier PF6 – anion does not present a strong solvation structure and exhibits much higher mobility in an electric field. We further observe an energy barrier for Li+ insertion into graphite, particularly when the surface is hydroxyl terminated. Lithium extraction, however, is facile even against an applied electric field. The energetics of Li+ insertion and extraction were analyzed using density functional theory to sample the free energy surfaces. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Support for this work was provided through Scientific Discovery through Advanced Computing (SciDAC) program funded by U.S. Department of Energy, Office of Science, Advanced Scientific Computing Research and Basic Energy Sciences.

HPCToolkit: performance tools for scientific computing

As part of the U.S. Department of Energy's Scientific Discovery through Advanced Computing (SciDAC) program, science teams are tackling problems that require simulation and modeling on petascale computers. As part of activities associated with the SciDAC Center for Scalable Application Development Software (CScADS) and the Performance Engineering Research Institute (PERI), Rice University is building software tools for performance analysis of scientific applications on the leadership-class platforms. In this poster abstract, we briefly describe the HPCToolkit performance tools and how they can be used to pinpoint bottlenecks in SPMD and multi-threaded parallel codes. We demonstrate HPCToolkit's utility by applying it to two SciDAC applications: the S3D code for simulation of turbulent combustion and the MFDn code for ab initio calculations of microscopic structure of nuclei.

Managing locality in grand challenge applications: a case study of the gyrokinetic toroidal code

Achieving high performance with grand challenge applications on today's large-scale parallel systems requires tailoring applications for the characteristics of the modern microprocessor architectures. As part of the US Department of Energy's Scientific Discovery through Advanced Computing (SciDAC) program, we studied and tuned the Gyrokinetic Toroidal Code (GTC), a particle-in-cell code for simulating turbulent transport of particles and energy in burning plasma, developed at Princeton Plasma Physics Laboratory. In this paper, we present a performance study of the application that revealed several opportunities for improving performance by enhancing its data locality. We tuned GTC by performing three kinds of transformations: static data structure reorganization to improve spatial locality, loop nest restructuring for better temporal locality, and dynamic data reordering at run-time to enhance both spatial and temporal reuse. Experimental results show that these changes improve execution time by more than 20% on large parallel systems, including a Cray XT4.

Data management and analysis for the Earth System Grid

The international climate community is expected to generate hundreds of petabytes of simulation data within the next five to seven years. This data must be accessed and analyzed by thousands of analysts worldwide in order to provide accurate and timely estimates of the likely impact of climate change on physical, biological, and human systems. Climate change is thus not only a scientific challenge of the first order but also a major technological challenge. In order to address this technological challenge, the Earth System Grid Center for Enabling Technologies (ESG-CET) has been established within the U.S. Department of Energy's Scientific Discovery through Advanced Computing (SciDAC)-2 program, with support from the offices of Advanced Scientific Computing Research and Biological and Environmental Research. ESG-CET's mission is to provide climate researchers worldwide with access to the data, information, models, analysis tools, and computational capabilities required to make sense of enormous climate simulation datasets. Its specific goals are to (1) make data more useful to climate researchers by developing Grid technology that enhances data usability; (2) meet specific distributed database, data access, and data movement needs of national and international climate projects; (3) provide a universal and secure web-based data access portal for broad multi-model data collections; and (4) provide a wide-range of Grid-enabled climate data analysis tools and diagnostic methods to international climate centers and U.S. government agencies. Building on the successes of the previous Earth System Grid (ESG) project, which has enabled thousands of researchers to access tens of terabytes of data from a small number of ESG sites, ESG-CET is working to integrate a far larger number of distributed data providers, high-bandwidth wide-area networks, and remote computers in a highly collaborative problem-solving environment.

Creating a collaborative space to share data, visualization, and knowledge

Collaboration is an effective approach to problem solving. Most large-scale scientific investigations are highly interdisciplinary and collaborative, with project investigators often geographically distributed. For example, ITER [1] is a joint international research and development project that aims to demonstrate the scientific and technical feasibility of fusion power. The partners in the project are distributed in more than ten countries, and it is imperative that they share their creations and findings since greater advances are only made with collective efforts. Similarly, I am participating in the U.S. Department of Energy's SciDAC (Scientific Discovery through Advanced Computing) program [2], which sponsors a dozen high-profile science projects. Each of the project teams involves researchers from several different universities and national laboratories. Clearly, there is an explicit need to provide support for collaborative work. How can these researchers effectively share their data, problem solving strategies, and research findings without time and place constraint? The answer is web-based collaboratories, which have been created for many major science projects. However, most of these collaboratories are simply data repositories with web interfaces. For instance, users do not get to see the associations between data and users, which are as valuable as the data itself. These associations can be large and very complex, and thus hard to comprehend. Visualization can play a key role in both knowledge discovery and managing potentially complex and high-dimensional collaborative workspace. I have been thinking about how to address the need to support collaborative data analysis and visualization, and how to direct the visualization research community towards the development of such needed technologies. I am going to describe some of the ongoing efforts to facilitate sharing visualization resources that will provide the eventual support for the kind of collaborative workspace I have in mind.

Results of the PERI survey of SciDAC applications

The Performance Engineering Research Institute (PERI) project focuses on achieving superior performance for Scientific Discovery through Advanced Computing (SciDAC) applications on leadership class machines through cutting-edge research in performance modeling and automated performance tuning. This focus requires coordinated activities to engage SciDAC application teams. The initial application engagement activity was a survey of these teams to determine their performance goals, the criticality of those goals, current performance of their applications, application characteristics relevant to performance and their plans for future optimization. Using a web-based questionnaire, PERI researchers have worked with application developers to provide this information for over twenty-five applications. This paper describes the initial analysis of the application characteristics and performance goals, as well as current and future application engagement activities driven by these results. While the survey was conducted primarily to meet PERI needs, the results represent a snapshot of the state of SciDAC code development and may be of use to the DOE community at large. Overall, the results show that SciDAC application teams are engaged in significant new code development, which will require flexible performance optimization techniques that can improve performance as the applications evolve.

Building a global federation system for climate change research: the earth system grid center for enabling technologies (ESG-CET)

The recent release of the Intergovernmental Panel on Climate Change (IPCC) 4th Assessment Report (AR4) has generated significant media attention. Much has been said about the US role in this report, which included significant support from the Department of Energy through the Scientific Discovery through Advanced Computing (SciDAC) and other Department of Energy (DOE) programs for climate model development and the production execution of simulations. The SciDAC-supported Earth System Grid Center for Enabling Technologies (ESG-CET) also played a major role in the IPCC AR4: all of the simulation data that went into the report was made available to climate scientists worldwide exclusively via the ESG-CET At the same time as the IPCC AR4 database was being developed, the National Center for Atmospheric Research (NCAR), a leading US climate science laboratory and a ESG participant, began publishing model runs from the Community Climate System Model (CCSM), and its predecessor the Parallel Coupled Model (PCM) through ESG In aggregate, ESG-CET provides seamless access to over 180 terabytes of distributed climate simulation data to over 6,000 registered users worldwide, who have taken delivery of more than 250 terabytes from the archive. Not only does this represent a substantial advance in scientific knowledge, it is also a major step forward in how we conduct the research process on a global scale. Moving forward, the next IPCC assessment report, AR5, will demand multi-site metadata federation for data discovery and cross-domain identity management for single sign-on of users in a more diverse federation enterprise environment. Towards this aim, ESG is leading the effort in the climate community towards standardization of material for the global federation of metadata, security, and data services required to standardize, analyze, and access data worldwide.

Computational issues and algorithm assessment for shock/turbulence interaction problems

The paper provides an overview of the challenges involved in the computation of flows with interactions between turbulence, strong shockwaves, and sharp density interfaces. The prediction and physics of such flows is the focus of an ongoing project in the Scientific Discovery through Advanced Computing (SciDAC) program. While the project is fundamental in nature, there are many important potential applications of scientific and engineering interest ranging from inertial confinement fusion to exploding supernovae. The essential challenges will be discussed, and some representative numerical results that highlight these challenges will be shown. In addition, the overall approach taken in this project will be outlined.

Facilities and Methods: A New Initiative in Nuclear Theory: UNEDF

The U.S. Department of Energy announced in December 2006 the project on the physics of nuclei in its SciDAC (Scientific Discovery through Advanced Computing) program. SciDAC brings together physical and computer scientists to solve important problems that require the most advanced computer resources for their computation [1]. The new project, called “Building a universal nuclear energy density functional” (UNEDF) is in theoretical nuclear physics. Its goal is to make a qualitative improvement in the accuracy and reliability of theoretical methods for calculating nuclear properties such as energies, spectra, and low-energy reactions.

The collaborative tokamak control room

Magnetic fusion experiments keep growing in size and complexity resulting in a concurrent growth in collaborations between experimental sites and laboratories worldwide. In the US, the National Fusion Collaboratory Project is developing a persistent infrastructure to enable scientific collaboration for all aspects of magnetic fusion energy research by creating a robust, user-friendly collaborative environment and deploying this to the more than 1000 US fusion scientists in 40 institutions who perform magnetic fusion research. This paper reports on one aspect of the project which is the development of the collaborative tokamak control room to enhance both collocated and remote scientific participation in experimental operations. This work includes secured computational services that can be scheduled as required, the ability to rapidly compare experimental data with simulation results, a means to easily share individual results with the group by moving application windows to a shared display, and the ability for remote scientists to be fully engaged in experimental operations through shared audio, video, and applications. The project is funded by the USDOE Office of Science, Scientific Discovery through Advanced Computing (SciDAC) Program and unites fusion and computer science researchers to directly address these challenges.

Supernovae, Rotation, and Bipolar Explosions

In this paper, we discuss the possible role of rotation in supernova blast energetics and morphology, and speculate on the origin of Cas A’s and SN1987A’s ejecta fields. Two “explosive” phenomena may be associated with most core collapses, the neutrinodriven supernova itself and an underenergetic jet-like ejection that follows. The latter may be a magnetic wind that easily penetrates the debris created by the much more energetic supernova. We speculate that many core-collapse supernova remnants should have sub-dominant jet-like features. In Cas A, we associate this sub-dominant collimated wind with the “jet/counter-jet” structure observed. We suggest that the actual Cas A explosion itself is at nearly right angles to this jet, along a rotation axis that coincides with the bulk of the ejecta, the iron lobes, and the putative direction of motion of the point source. It may be that when rotation becomes sufficiently rapid that the strong-neutrinodriven-supernova/weak-jet duality switches to a strong-MHD-jet scenario that might be associated with hypernovae, and in some cases GRBs. Finally, we present a calculation using a new 2D multi-group, flux-limited radiation/hydrodynamics code we have recently developed for the simulation of core-collapse supernovae. We discuss the rotation-induced anisotropy in the neutrino radiation field, neutrino heating, and the neutrino flux vectors and speculate on rotation’s possible role in the supernova mechanism. ∗ We acknowledge discussions with Todd Thompson, Jeremiah Murphy, Casey Meakin, Itamar Lichtenstadt, and Moath Jarrah. Support for this work is provided in part by the Scientific Discovery through Advanced Computing (SciDAC) program of the DOE, grant number DE-FC02-01ER41184. In addition, we thank Jeff Fookson and Neal Lauver of the Steward Computer Support Group for their invaluable help with the local Beowulf cluster and acknowledge the use of the NERSC/LBNL/seaborg and ORNL/CCS/cheetah machines.

New multigrid solver advances in TOPS

In this paper, we highlight new multigrid solver advances in the Terascale Optimal PDE Simulations (TOPS) project in the Scientific Discovery Through Advanced Computing (SciDAC) program. We discuss two new algebraic multigrid (AMG) developments in TOPS: the adaptive smoothed aggregation method (αSA) and a coarse-grid selection algorithm based on compatible relaxation (CR). The αSA method is showing promising results in initial studies for Quantum Chromodynamics (QCD) applications. The CR method has the potential to greatly improve the applicability of AMG.

Building the US National Fusion Grid: results from the National Fusion Collaboratory Project

The US National Fusion Collaboratory Project is developing a persistent infrastructure to enable scientific collaboration for all aspects of magnetic fusion research. The project is creating a robust, user-friendly collaborative software environment and making it available to more than 1000 fusion scientists in 40 institutions who perform magnetic fusion research in the United States. In particular, the project is developing and deploying a national Fusion Energy Sciences Grid (FusionGrid) that is a system for secure sharing of computation, visualization, and data resources over the Internet. The FusionGrid goal is to allow scientists at remote sites to fully participate in experimental and computational activities as if they were working at a common site thereby creating a virtual organization of the US fusion community. The project is funded by the USDOE Office of Science, Scientific Discovery through Advanced Computing (SciDAC) Program and unites fusion and computer science researchers to directly address these challenges.