S. Ethier

Princeton University, Princeton, New Jersey, United States

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Publications (95)70.5 Total impact

  • William Tang, Bei Wang, Stephane Ethier
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    ABSTRACT: The primary goal of the extreme scale plasma turbulence studies described in this paper is to gain new insights on confinement scaling in magnetic fusion systems by using powerful, world-class supercomputers to carry out simulations with unprecedented resolution and temporal duration. New insights have been gained on the key question of how the turbulent transport of plasma particles and associated confinement scale from present generation devices to much larger ITER-size plasmas. In particular, new results from large-scale simulation studies have demonstrated that improvement in confinement as devices grow larger takes place far more gradually, and with significantly lower loss rates, than less-powerful computer simulations have indicated in research carried out over the past decade.
    Computing in Science and Engineering 05/2014; · 1.25 Impact Factor
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    ABSTRACT: Collaborative science demands global sharing of scientific data. But it cannot leverage universally accessible cloud-based infrastructures like Drop Box, as those offer limited interfaces and inadequate levels of access bandwidth. We present the Scibox cloud facility for online sharing scientific data. It uses standard cloud storage solutions, but offers a usage model in which high end codes can write/read data to/from the cloud via the APIs they already use for their I/O actions. With Scibox, data upload/download volumes are controlled via Data Reduction-functions stated by end users and applied at the data source, before data is moved, with further gains in efficiency obtained by combining DR-functions to move exactly what is needed by current data consumers. We evaluate Scibox with science applications and their representative data analytics - the GTS fusion and the combustion image processing - demonstrating the potential for ubiquitous data access with substantial reductions in network traffic.
    2014 IEEE International Parallel & Distributed Processing Symposium (IPDPS); 05/2014
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    ABSTRACT: Reliable predictive simulation capability addressing confinement properties in magnetically confined fusion plasmas is critically-important for ITER, a 20 billion dollar international burning plasma device under construction in France. The complex study of kinetic turbulence, which can severely limit the energy confinement and impact the economic viability of fusion systems, requires simulations at extreme scale for such an unprecedented device size. Our newly optimized, global, ab initio particle-in-cell code solving the nonlinear equations underlying gyrokinetic theory achieves excellent performance with respect to \textquotedblleft time to solution at the full capacity of the IBM Blue Gene/Q on 786,432 cores of Mira at ALCF and recently of the 1,572,864 cores of Sequoia at LLNL. Recent multithreading and domain decomposition optimizations in the new GTC-P code represent critically important software advances for modern, low memory per core systems by enabling routine simulations at unprecedented size (130 million grid points ITER-scale) and resolution (65 billion particles).
    Proc. ACM/IEEE Conf. on Supercomputing (SC), Denver, CO; 11/2013
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    ABSTRACT: The Gyrokinetic Toroidal Code (GTC) uses the particle-in-cell method to efficiently simulate plasma microturbulence. This work presents novel analysis and optimization techniques to enhance the performance of GTC on large-scale machines. We introduce cell access analysis to better manage locality vs. synchronization tradeoffs on CPU and GPU-based architectures. Our optimized hybrid parallel implementation of GTC uses MPI, OpenMP, and NVIDIA CUDA, achieves up to a 2× speedup over the reference Fortran version on multiple parallel systems, and scales efficiently to tens of thousands of cores.
    International Journal of High Performance Computing Applications 10/2013; 27(4):454-473. · 1.63 Impact Factor
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    ABSTRACT: Verification and historical perspective are presented on the gyrokinetic particle simulations that discovered the device size scaling of turbulent transport and indentified the geometry model as the source of the long-standing disagreement between gyrokinetic particle and continuum simulations.
    Plasma Science and Technology 12/2012; 14(12):1125. · 0.60 Impact Factor
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    ABSTRACT: I/O bottlenecks in HPC applications are becoming a more pressing problem as compute capabilities continue to outpace I/O capabilities. While double-precision simulation data often must be stored losslessly, the loss of some of the fractional component may introduce acceptably small errors to many types of scientific analyses. Given this observation, we develop a precision level of detail (APLOD) library, which partitions double-precision datasets along user-defined byte boundaries. APLOD parameterizes the analysis accuracy-I/O performance tradeoff, bounds maximum relative error, maintains I/O access patterns compared to full precision, and operates with low overhead. Using ADIOS as an I/O use-case, we show proportional reduction in disk access time to the degree of precision. Finally, we show the effects of partial precision analysis on accuracy for operations such as k-means and Fourier analysis, finding a strong applicability for the use of varying degrees of precision to reduce the cost of analyzing extreme-scale data.
    Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis (SC12); 11/2012
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    ABSTRACT: New results from global nonlinear gyrokinetic simulations with the GTS code show that strong flow shear can drive a negative compressibility mode [1-3] unstable in tokamak geometry in some experimentally relevant parameter regimes. The modes reside in a low-k range, similar to that of ITG mode, with smaller but almost constant growth rate over a wider kθ range, while the mode frequency increases strongly with kθ. More interestingly, the flow shear modes show significantly finite k//, unlike ITG and TEM. The nonlinear energy transfer to longer wavelength via toroidal mode coupling and corresponding strong zonal flow and geodestic acoustic mode (GAM) generation are shown to play a critical role in the nonlinear saturation of the instability. The associated turbulence fluctuations can produce significant momentum and energy transport, including an intrinsic torque in the co-current direction. Remarkably, strong ``resonance'' in the fluctuations and associated transport peaks at the lowest order rational surfaces with integer q-number (rather than fractional), consistent with theoretical calculation. As a consequence, local ``corrugations'' are generated in all plasma profiles (temperatures, density and toroidal rotation), potentially impacting transport barrier formation near the rational surface. Discussions on flow optimization for minimizing plasma transport will be reported.[4pt] [1] N. Mattor and P. H. Diamond, Phys. Fluids 31, 1180 (1988).[0pt] [2] P. J. Catto et al., Phys. Fluids 16, 1719 (1973).[0pt] [3] M. Artun and W. M. Tang, Phys. Fluids B4, 1102 (1992).
    10/2012;
  • W. W. Lee, S. Ethier, J. Ganesh
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    ABSTRACT: The use of a generalized weight-based particle simulation scheme suitable for simulating tokamak turbulence is reported. The scheme, which is a generalization of the perturbed distribution schemes developed earlier for PIC simulations, is now capable of handling the full distribution of the particles in the simulation. Specifically, we can simulate both the delta-f and the full-F particles within the same code. Its development [1] is based on the concept of multiscale expansion, which separates the scale lengths of the background inhomogeneity from those associated with the perturbed distributions, and on the fact that the intrinsic particle noise level is troublesome only in the beginning of the simulation, where the signal to noise ratio is low. But, when the signal to noise ratio becomes higher afterwards, we can gradually turn on the the full-F particles without interfering with the ensuing fluctuations. We will report on the simulation studies using GTC [2] for the ion temperature gradient (ITG) driven instabilities in the presence of zonal flows. The physics of steady state transport in tokamaks will be discussed.[4pt] [1] W. W. Lee, T. G. Jenkins and S. Ethier, Comp. Phys. Comm. 182, 564 (2011).[0pt] [2] Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang, R. White Science 281, 1835 (1998).
    10/2012;
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    ABSTRACT: Energetic ion transport has been studied using a global gyrokinetic nonlinear simulation in the presence of ion temperature gradient (ITG) driven turbulence. The measured transport and its nature show dependence on the system size of the tokamak expressed as the ratio of plasma minor radius (a) to the thermal ion Larmor radius (ρi). It increases with system size initially and then tends to saturate at larger system size. The nature of transport, on the other hand, exhibits nondiffusive character for smaller system size which eventually becomes diffusive one as the system size becomes larger.
    Physics of Plasmas 04/2012; 19(4). · 2.25 Impact Factor
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    ABSTRACT: Efficient handling of large volumes of data is a necessity for exascale scientific applications and database systems. To address the growing imbalance between the amount of available storage and the amount of data being produced by high speed (FLOPS) processors on the system, data must be compressed to reduce the total amount of data placed on the file systems. General-purpose loss less compression frameworks, such as zlib and bzlib2, are commonly used on datasets requiring loss less compression. Quite often, however, many scientific data sets compress poorly, referred to as hard-to-compress datasets, due to the negative impact of highly entropic content represented within the data. An important problem in better loss less data compression is to identify the hard-to-compress information and subsequently optimize the compression techniques at the byte-level. To address this challenge, we introduce the In-Situ Orthogonal Byte Aggregate Reduction Compression (ISOBAR-compress) methodology as a preconditioner of loss less compression to identify and optimize the compression efficiency and throughput of hard-to-compress datasets.
    Proceedings of the 28th IEEE International Conference on Data Engineering (ICDE); 04/2012
  • W. W. Lee, S. Ethier
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    ABSTRACT: A generalized weight-based particle simulation schemes suitable for simulating microturbulence in magnetic fusion plasmas, where the zeroth-order inhomogeneity is important, has recently been developed [1]. The schemes is a generalization of the perturbative simulation schemes developed earlier for PIC simulations [2]. The new two-weight scheme, which can simulate both the perturbed distribution and the full distribution within the same code, has now been implemented to simulate tokamak plasmas using the GTC code [3]. Its development is based on the concept of multiscale expansion, which separates the scale lengths of the background inhomogeneity from those associated with the perturbed distributions. The code starts out as a delta-f code and gradually evolves into a full-F code, as such the delta-f part can help us with the noise issue in the linear stage and the full-F part can be useful in the fully nonlinear stage when the particle weights become too large or it becomes necessary to simulate realistic situations where sinks and sources become important.[4pt] [1] W. W. Lee, T. G. Jenkins and S. Ethier, Comp. Phys. Comm. 182, 564 (2011).[0pt] [2] S. E. Parker and W. W. Lee, Phys. Fluids B 5, 77 (1993).[0pt] [3] Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang and R. White, Science 281, 1835 (1998).
    03/2012;
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    ABSTRACT: Exploding dataset sizes from extreme-scale scientific simulations necessitates efficient data management and reduction schemes to mitigate I/O costs. With the discrepancy between I/O bandwidth and computational power, scientists are forced to capture data infrequently, thereby making data collection an inherently lossy process. Although data compression can be an effective solution, the random nature of real-valued scientific datasets renders lossless compression routines ineffective. These techniques also impose significant overhead during decompression, making them unsuitable for data analysis and visualization, which require repeated data access.To address this problem, we propose an effective method for In situ Sort-And-B-spline Error-bounded Lossy Abatement (ISABELA) of scientific data that is widely regarded as effectively incompressible. With ISABELA, we apply a pre-conditioner to seemingly random and noisy data along spatial resolution to achieve an accurate fitting model that guarantees a ⩾0.99 correlation with the original data. We further take advantage of temporal patterns in scientific data to compress data by ≈ 85%, while introducing only a negligible overhead on simulations in terms of runtime. ISABELA significantly outperforms existing lossy compression methods, such as wavelet compression, in terms of data reduction and accuracy.We extend upon our previous paper by additionally building a communication-free, scalable parallel storage framework on top of ISABELA-compressed data that is ideally suited for extreme-scale analytical processing. The basis for our storage framework is an inherently local decompression method (it need not decode the entire data), which allows for random access decompression and low-overhead task division that can be exploited over heterogeneous architectures. Furthermore, analytical operations such as correlation and query processing run quickly and accurately over data in the compressed space. Copyright © 2012 John Wiley & Sons, Ltd.
    Concurrency and Computation Practice and Experience 01/2012; · 0.85 Impact Factor
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    ABSTRACT: The size and scope of cutting-edge scientific simulations are growing much faster than the I/O and storage capabilities of their runtime environments. The growing gap gets exacerbated by exploratory dataâ"intensive analytics, such as querying simulation data for regions of interest with multivariate, spatio-temporal constraints. Query-driven data exploration induces heterogeneous access patterns that further stress the performance of the underlying storage system. To partially alleviate the problem, data reduction via compression and multi-resolution data extraction are becoming an integral part of I/O systems. While addressing the data size issue, these techniques introduce yet another mix of access patterns to a heterogeneous set of possibilities. Moreover, how extreme-scale datasets are partitioned into multiple files and organized on a parallel file systems augments to an already combinatorial space of possible access patterns. To address this challenge, we present MLOC, a parallel Multilevel Layout Optimization framework for Compressed scientific spatio-temporal data at extreme scale. MLOC proposes multiple fine-grained data layout optimization kernels that form a generic core from which a broader constellation of such kernels can be organically consolidated to enable an effective data exploration with various combinations of access patterns. Specifically, the kernels are optimized for access patterns induced by (a) queryâ"driven multivariate, spatio-temporal constraints, (b) precisionâ"driven data analytics, (c) compressionâ"driven data reduction, (d) multi-resolution data sampling, and (e) multiâ"file data partitioning and organization on a parallel file system. MLOC organizes these optimization kernels within a multiâ"level architecture, on which all the levels can be flexibly re-ordered by userâ"defined priorities. When tested on queryâ"driven exploration of compressed data, MLOC demon- trates a superior performance compared to any state-of-the-art scientific database management technologies.
    Parallel Processing (ICPP), 2012 41st International Conference on; 01/2012
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    ABSTRACT: The size and scope of cutting-edge scientific simulations are growing much faster than the I/O subsystems of their runtime environments, not only making I/O the primary bottleneck, but also consuming space that pushes the storage capacities of many computing facilities. These problems are exacerbated by the need to perform data-intensive analytics applications, such as querying the dataset by variable and spatio-temporal constraints, for what current database technologies commonly build query indices of size greater than that of the raw data. To help solve these problems, we present a parallel query-processing engine that can handle both range queries and queries with spatio-temporal constraints, on B-spline compressed data with user-controlled accuracy. Our method adapts to widening gaps between computation and I/O performance by querying on compressed metadata separated into bins by variable values, utilizing Hilbert space-filling curves to optimize for spatial constraints and aggregating data access to improve locality of per-bin stored data, reducing the false positive rate and latency bound I/O operations (such as seek) substantially. We show our method to be efficient with respect to storage, computation, and I/O compared to existing database technologies optimized for query processing on scientific data.
    Parallel & Distributed Processing Symposium (IPDPS), 2012 IEEE 26th International; 01/2012
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    ABSTRACT: The nature of transport of hot ions is studied in the presence of microturbulence generated by the trapped electron mode in a Tokamak using massively parallel, first principle based global nonlinear gyrokinetic simulation, and with the help of a passive tracer method. Passing and trapped hot ions are observed to exhibit inverse and inverse square scaling with energy, while those with isotropic pitch distribution are found to exhibit inverse dependence on energy. For all types of hot ions, namely, isotropic, passing, and trapped, the radial transport appears to be subdiffusive for the parameters considered.
    Physics of Plasmas 11/2011; 18(11):112510-112510-9. · 2.25 Impact Factor
  • W. X. Wang, S. Ethier
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    ABSTRACT: Global gyrokinetic simulations show that ion temperature gradient (ITG) and trapped electron mode (TEM) turbulence can drive a significant parallel current in meso-scale (zonal current). The underlying dynamics is closely related to nonlinear plasma flow generation [1] by turbulent residual stress [2]. However, unlike toroidal momentum which is mostly carried by ions, the turbulent current is mainly carried by electrons in the laboratory frame, and shows finer radial scale in comparison with poloidal and toroidal zonal flows. In both collisonless TEM and ITG turbulence, substantial electron current is first generated in the positive direction of magnetic field and remains quasi-stationary in post-saturation phase. The current generation by turbulence exhibits the similar characteristic dependence on plasma parameters as that of plasma flow generation [3]. Specifically, it increases with pressure gradient, decreases with equilibrium current Ip and increases with the radial variation of safety factor. Also discussed are interesting phase space structures between TEM and ITG turbulence driven current to elucidate the roles of resonant and non-resonant electrons. In collaboration with T. S. Hahm, P. H. Diamond (UCSD), F. L. Hinton (UCSD), A. H. Boozer (Columbia U.), G. Rewoldt, W. M. Tang and W. W. Lee. [1] W. X. Wang et al., Phys. Plasmas 17, 072511 (2010). [2] P. H. Diamond et al., Phys. Plasmas 15, 012303 (2008). [3] W. X. Wang et al., Phys. Rev. Lett. 106, 085001 (2011).
    11/2011;
  • W. W. Lee, S. Ethier, R. Ganesh
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    ABSTRACT: A generalized weight-based particle simulation schemes suitable for simulating microturbulence in magnetic fusion plasmas, where the zeroth-order inhomogeneity is important, has recently been developed [1]. The schemes is a generalization of the perturbative simulation schemes developed earlier for PIC simulations [2]. The new two-weight scheme, which can simulate both the perturbed distribution (δf ) and the full distribution (total-F) within the same code, has now been extended to simulate tokamak plasmas using the GTC code [3]. Its development is based on the concept of multiscale expansion, which separates the scale lengths of the background inhomogeneity from those associated with the perturbed distributions. In this paper, we will demonstrate the correctness and the usefulness of such a code, which starts out as a δf code and gradually evolves into a full-F code. The δf part would help us with the noise issue in the linear stage and the full-F part of the code could be useful when the particle weights become too large or it becomes necessary to simulate the realistic situation where the sinks and sources for the simulation become important. [1] W. W. Lee, T. G. Jenkins and S. Ethier, Comp. Phys. Comm. 182, 564 (2011). [2] S. E. Parker and W. W. Lee, Phys. Fluids B 5, 77 (1993). [3] Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang and R. White, Science 281, 1835 (1998).
    11/2011;
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    ABSTRACT: In the last two experimental campaigns, the low aspect ratio NSTX has explored physics issues critical to both toroidal confinement physics and ITER. Experiments have made extensive use of lithium coatings for wall conditioning, correction of non-axisymmetric field errors and control of n = 1 resistive wall modes (RWMs) to produce high-performance neutral-beam heated discharges extending to 1.7 s in duration with non-inductive current fractions up to 0.7. The RWM control coils have been used to trigger repetitive ELMs with high reliability, and they have also contributed to an improved understanding of both neoclassical tearing mode and RWM stabilization physics, including the interplay between rotation and kinetic effects on stability. High harmonic fast wave (HHFW) heating has produced plasmas with central electron temperatures exceeding 6 keV. The HHFW heating was used to show that there was a 20–40% higher power threshold for the L–H transition for helium than for deuterium plasmas. A new diagnostic showed a depletion of the fast-ion density profile over a broad spatial region as a result of toroidicity-induced Alfvén eigenmodes (TAEs) and energetic-particle modes (EPMs) bursts. In addition, it was observed that other modes (e.g. global Alfvén eigenmodes) can trigger TAE and EPM bursts, suggesting that fast ions are redistributed by high-frequency AEs. The momentum pinch velocity determined by a perturbative technique decreased as the collisionality was reduced, although the pinch to diffusion ratio, Vpinch/χ, remained approximately constant. The mechanisms of deuterium retention by graphite and lithium-coated graphite plasma-facing components have been investigated. To reduce divertor heat flux, a novel divertor configuration, the 'snowflake' divertor, was tested in NSTX and many beneficial aspects were found. A reduction in the required central solenoid flux has been realized in NSTX when discharges initiated by coaxial helicity injection were ramped in current using induction. The resulting plasmas have characteristics needed to meet the objectives of the non-inductive start-up and ramp-up program of NSTX.
    Nuclear Fusion 08/2011; 51(9):094011. · 3.24 Impact Factor
  • Cray User Group (CUG); 05/2011
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    ABSTRACT: Toroidal plasma flow driven by turbulent torque associated with nonlinear residual stress generation is shown to recover the observed key features of intrinsic rotation in experiments. Specifically, the turbulence-driven intrinsic rotation scales close to linearly with plasma gradients and the inverse of the plasma current, qualitatively reproducing empirical scalings obtained from a large experimental data base. The effect of magnetic shear on the symmetry breaking in the parallel wavenumber spectrum is identified. The origin of the current scaling is found to be the enhanced k∥ symmetry breaking induced by increased radial variation of the safety factor as the current decreases. The physics origin for the linear dependence of intrinsic rotation on the pressure gradient comes from the fact that both turbulence intensity and the zonal flow shear, which are two key ingredients for driving the residual stress, are increased with the strength of the turbulence drives, which are R/LTe and R/Lne for the collisionless trapped electron mode (CTEM). Highlighted results also include robust radial pinches in toroidal flow, heat and particle transport driven by CTEM turbulence, which emerge “in phase,” and are shown to play important roles in determining plasma profiles. Also discussed are the experimental tests proposed to validate findings from these gyrokinetic simulations.
    Physics of Plasmas 04/2011; 18(4):042502-042502-12. · 2.25 Impact Factor

Publication Stats

452 Citations
70.50 Total Impact Points

Institutions

  • 1835–2013
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, New Jersey, United States
  • 2009
    • The University of Warwick
      Coventry, England, United Kingdom
  • 2004–2008
    • University of California, Irvine
      • Department of Physics and Astronomy
      Irvine, California, United States
  • 2002
    • University of California, San Diego
      • Department of Physics
      San Diego, California, United States