Root Growth of Carbon and Boron Nitride Nanotubes in Plasma of High Pressure Arc
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Physics - Plasma physics↗
Engineering topics
Ethier, S
Publications and source records attributed to Ethier, S.
Radial transport of energetic ions in the presence of trapped electron mode turbulence
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.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Characteristics of turbulence-driven plasma flow and origin of experimental empirical scalings of intrinsic rotation
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. In particular, 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 basis 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 $R/L_{T_{e}}$ and $R/L_{n_{e}}$ 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. Moreover discussed are the experimental tests proposed to validate findings from these gyrokinetic simulations.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Trapped Electron Mode Turbulence Driven Intrinsic Rotation in Tokamak Plasmas
Progress from global gyrokinetic simulations in understanding the origin of intrinsic rotation in toroidal plasmas is reported. The turbulence-driven intrinsic torque associated with nonlinear residual stress generation due to zonal flow shear induced asymmetry in the parallel wave number spectrum is shown to scale close to linearly with plasma gradients and the inverse of the plasma current, qualitatively reproducing experimental empirical scalings of intrinsic rotation. The origin of current scaling is found to be enhanced k{sub ||} symmetry breaking induced by the increased radial variation of the safety factor as the current decreases. The intrinsic torque is proportional to the pressure gradient because both turbulence intensity and zonal flow shear, which are two key ingredients for driving residual stress, increase with turbulence drive, which is R/L{sub T{sub e}} and R/L{sub n{sub e}} for the trapped electron mode.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Skel: Generative Software for Producing Skeletal I/O Applications
Massively parallel computations consist of a mixture of computation, communication, and I/O. As part of the co-design for the inevitable progress towards exascale computing, we must apply lessons learned from past work to succeed in this new age of computing. Of the three components listed above, implementing an effective parallel I/O solution has often been overlooked by application scientists and was usually added to large scale simulations only when existing serial techniques had failed. As scientists teams scaled their codes to run on hundreds of processors, it was common to call on an I/O expert to implement a set of more scalable I/O routines. These routines were easily separated from the calculations and communication, and in many cases, an I/O kernel was derived from the application which could be used for testing I/O performance independent of the application. These I/O kernels developed a life of their own used as a broad measure for comparing different I/O techniques. Unfortunately, as years passed and computation and communication changes required changes to the I/O, the separate I/O kernel used for benchmarking remained static no longer providing an accurate indicator of the I/O performance of the simulation making I/O research less relevant for the application scientists. In this paper we describe a new approach to this problem where I/O kernels are replaced with skeletal I/O applications automatically generated from an abstract set of simulation I/O parameters. We realize this abstraction by leveraging the ADIOS middleware's XML I/O specification with additional runtime parameters. Skeletal applications offer all of the benefits of I/O kernels including allowing I/O optimizations to focus on useful I/O patterns. Moreover, since they are automatically generated, it is easy to produce an updated I/O skeleton whenever the simulation's I/O changes. In this paper we analyze the performance of automatically generated I/O skeletal applications for the S3D and GTS codes. We show that these skeletal applications achieve performance comparable to that of the production applications. We wrap up the paper with a discussion of future changes to make the skeletal application better approximate the actual I/O performed in the simulation.
99 GENERAL AND MISCELLANEOUS↗
A synthetic diagnostic for validation of electron gyroradius scale turbulence simulations against coherent scattering measurements
Comparison between spectra of short-scale density fluctuations measured with coherent electromagnetic scattering experiments and those extracted from space-resolved numerical simulations is affected by a number of systematic errors. These include the locality of scattering measurements, the different domain covered (space-resolved simulations versus wavenumber resolved measurements), and the stationarity of simulated nonlinear spectra. To bridge the gap between theory-simulations and experiments, a synthetic diagnostic for high- k scattering measurements has been developed. This synthetic scattering predicts the propagation of the beam in an anisotropic, inhomogeneous plasma and accounts for the spatial variation of the instrumental transfer function. Here the latter, in particular, is proven to provide an important calibration factor not only for the simulated spectra, but also for the measured ones, allowing the use of the synthetic diagnostic in predictive mode. Results from a case study for National Spherical Torus Experiment plasmas using high- k</em. tangential scattering system and the gyrokinetic tokamak simulation code are presented.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Nonlinear flow generation by electrostatic turbulence in tokamaks
Global gyrokinetic simulations have revealed an important nonlinear flow generation process due to the residual stress produced by electrostatic turbulence of ion temperature gradient (ITG) modes and trapped electron modes (TEMs). In collisionless TEM (CTEM) turbulence, nonlinear residual stress generation by both the fluctuation intensity and the intensity gradient in the presence of broken symmetry in the parallel wavenumber spectrum is identified for the first time. Concerning the origin of the symmetry breaking, turbulence self-generated low frequency zonal flow shear has been identified to be a key, universal mechanism in various turbulence regimes. Simulations reported here also indicate the existence of other mechanisms beyond E × B shear. The ITG turbulence driven “intrinsic” torque associated with residual stress is shown to increase close to linearly with the ion temperature gradient, in qualitative agreement with experimental observations in various devices. In CTEM dominated regimes, a net toroidal rotation is driven in the cocurrent direction by intrinsic torque, consistent with the experimental trend of observed intrinsic rotation. The finding of a “flow pinch” in CTEM turbulence may offer an interesting new insight into the underlying dynamics governing the radial penetration of modulated flows in perturbation experiments. Lastly, simulations also reveal highly distinct phase space structures between CTEM and ITG turbulence driven momentum, energy, and particle fluxes, elucidating the roles of resonant and non-resonant particles.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Application Acceleration on Current and Future Cray Platforms
Abstract not provided
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗