Iterative addition of parallel temperature effects to finite-difference simulation of radio-frequency wave propagation in plasmas
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Green, David L↗
Engineering topics
Berry, Lee A
Publications and source records attributed to Berry, Lee A.
Time parallelization of advanced operation scenario simulations of ITER plasma
This work demonstrates that simulations of advanced burning plasma operation scenarios can be successfully parallelized in time using the parareal algorithm. CORSICA - an advanced operation scenario code for tokamak plasmas is used as a test case. This is a unique application since the parareal algorithm has so far been applied to relatively much simpler systems except for the case of turbulence. In the present application, a computational gain of an order of magnitude has been achieved which is extremely promising. A successful implementation of the Parareal algorithm to codes like CORSICA ushers in the possibility of time efficient simulations of ITER plasmas.
Samaddar, D.↗
Event-Based Parareal: A data-flow based implementation of parareal
Parareal is an iterative algorithm that, in effect, achieves temporal decomposition for a time-dependent system of differential or partial differential equations. A solution is obtained in a shorter wall-clock time, but at the expense of increased compute cycles. The algorithm combines a fine solver that solves the system to acceptable accuracy with an approximate coarse solver. The critical task for the successful implementation of parareal on any system is the development of a coarse solver that leads to convergence in a small number of iterations compared to the number of time slices in the full time interval, and is, at the same time, much faster than the fine solver. Fast coarse solvers may not lead to sufficiently rapid convergence, and slow coarse solvers may not lead to significant gains even if the number of iterations to convergence is satisfactory. We find that the difficulty of meeting these conflicting demands can be substantially eased by using a data-driven, event-based implementation of parareal instead of the conventional algorithm where solver tasks are executed sequentially. For given convergence properties, the event-based approach relaxes the speed requirements on the coarse solver by a factor of , where is the number of iterations required for a converged solution. This may, for many problems, lead to an efficient parareal implementation that would otherwise not be possible or would require substantial coarse solver development.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Mechanisms for the convergence of time-parallelized, parareal turbulent plasma simulations
Parareal is a recent algorithm able to parallelize the time dimension in spite of its sequential nature. It has been applied to several linear and nonlinear problems and, very recently, to a simulation of fully-developed, two-dimensional drift wave turbulence. The mere fact that parareal works in such a turbulent regime is in itself somewhat unexpected, due to the characteristic sensitivity of turbulence to any change in initial conditions. This fundamental property of any turbulent system should render the iterative correction procedure characteristic of the parareal method inoperative, but this seems not to be the case. In addition, the choices that must be made to implement parareal (division of the temporal domain, election of the coarse solver and so on) are currently made using trial-and-error approaches. Here, we identify the mechanisms responsible for the convergence of parareal of these simulations of drift wave turbulence. We also investigate which conditions these mechanisms impose on any successful parareal implementation. The results reported here should be useful to guide future implementations of parareal within the much wider context of fully-developed fluid and plasma turbulent simulations.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Predicting High Harmonic Ion Cyclotron Heating Efficiency in Tokamak Plasmas
Not Available
Green, D. L.↗
Simulation of High-Harmonic Fast-Wave Heating on the National Spherical Tokamak Experiment
Images associated with radio-frequency heating of low-confinement mode plasmas in the National Spherical Tokamak Experiment, as calculated by computer simulation, are presented. The AORSA code has been extended to simulate the whole antenna-to-plasma heating system by including both the kinetic physics of the well-confined core plasma and a poorly confined scrape-off plasma and vacuum vessel structure. The images presented show the 3-D electric wave field amplitude for various antenna phasings. Visualization of the simulation results in 3-D makes clear that -30 degrees phasing excites kilo-volt per meter coaxial standing modes in the scrape-off plasma and shows magnetic-field-aligned whispering-gallery type modes localized to the plasma edge.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
A Dependency-Driven Formulation of Parareal: Parallel-in-Time Solution of PDEs as a Many-Task Application
Parareal is a novel algorithm that allows the solution of time-dependent systems of differential or partial differential equations (PDE) to be parallelized in the temporal domain. Parareal-based implementations of PDE problems can take advantage of this parallelism to significantly reduce the time to solution for a simulation (though at an increased total cost) while making effective use of the much larger processor counts available on current high-end systems. In this paper, we present a dynamic, dependency-driven version of the parareal algorithm which breaks the final sequential bottleneck remaining in the original formulation, making it amenable to a "many-task" treatment. We further improve the cost and execution time of the algorithm by introducing a moving window for time slices, which avoids the execution of tasks which contribute little to the final global solution. We describe how this approach has been realized in the Integrated Plasma Simulator (IPS), a framework for coupled multiphysics simulations, and examine the trade-offs among time-to-solution, total cost, and resource utilization efficiency as a function of the compute resources applied to the problem.
99 GENERAL AND MISCELLANEOUS↗
Time parallelization of plasma simulations using the parareal algorithm
Simulation of fusion plasmas involve a broad range of timescales. In magnetically confined plasmas, such as in ITER, the timescale associated with the microturbulence responsible for transport and confinement timescales vary by an order of 10^6 10^9. Simulating this entire range of timescales is currently impossible, even on the most powerful supercomputers available. Space parallelization has so far been the most common approach to solve partial differential equations. Space parallelization alone has led to computational saturation for fluid codes, which means that the walltime for computaion does not linearly decrease with the increasing number of processors used. The application of the parareal algorithm to simulations of fusion plasmas ushers in a new avenue of parallelization, namely temporal parallelization. The algorithm has been successfully applied to plasma turbulence simulations, prior to which it has been applied to other relatively simpler problems. This work explores the extension of the applicability of the parareal algorithm to ITER relevant problems, starting with a diffusion-convection model.
Samaddar, D.↗