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We have implemented the Sugama collision operator in the gyrokinetic Vlasov simulation code, GKV, with an implicit time-integration scheme. The new method is versatile and independent of the details of the linearized collision operator, by means of an operator splitting, an implicit time integrator , and an iterative Krylov subspace solver. Numerical tests demonstrate stable computation over the time step size restricted by the collision term. An efficient implementation for parallel computation on distributed memory systems is realized by using the data transpose communication, which makes the iterative solver free from inter-node communications during iteration. Consequently, the present approach achieves enhancement of computational efficiency and reduction of computational time to solution simultaneously, and significantly accelerates the total performance of the application.

A linearized collision operator, which preserves the conservation properties of particles, momentum, and energy, and the self-adjointness relation (H-theorem) for arbitrary particle species, is applied to Eulerian kinetic simulations of multi-species ions and electrons. Velocity-space grid number scans clarify that numerical errors in like-particle and unlike-ion collisions are sufficiently small under turbulence-simulation relevant resolutions. For electron–ion/ion–electron collisions, it is identified that highly steep and localized nature in the velocity-space leads to relatively larger conservation errors in the standard implementation without error corrections. In order to resolve such numerical difficulties, a modified field-particle operator based on the original one is newly constructed, so that the conservation properties are improved to achieve the round-off error levels. It is confirmed that the modified collision operator successfully reproduces the collisional relaxation of multi-species ions and electrons towards their equilibrium distributions satisfying the H-theorem.

Optimization techniques of a plasma turbulence simulation code GKV for improved strong scaling are presented. This work is motivated by multi-scale plasma turbulence extending over multiple spatio-temporal scales of electrons and ions, whose simulations based on the gyrokinetic theory require huge calculations of five-dimensional (5D) computational fluid dynamics by means of spectral and finite difference methods. First, we present the multi-layer domain decomposition of the multi-dimensional and multi-species problem, and segmented MPI-process mapping on 3D torus interconnects, which fully utilizes the bi-section bandwidth for data transpose and reduces the conflicts of simultaneous point-to-point communications. These techniques reduce the inter-node communication cost drastically. Second, pipelined computation-communication overlaps are implemented by using the OpenMP/MPI hybrid parallelization, which effectively mask the communication cost. Careful regulations of the pipeline length and of the thread granularity respectively suppress latencies of MPI, load imbalance and scheduling overheads of OpenMP. Thanks to the above optimizations, GKV achieves excellent strong scaling up to ∼600k cores with high computational performance 782.4 TFlops (8.29% of the theoretical peak) and high effective parallelization rate ∼99.99994% on K, which demonstrates its applicability and efficiency toward a million of cores. The optimized code realizes multi-scale plasma turbulence simulations covering electron and ion scales, and reveals cross-scale interactions of electron- and ion-scale turbulence.

The polymer electrolyte fuel cell (PEFC) cathode is a performance-limiting component due to the slower oxygen reduction kinetics and mass transport limitations imposed by water generated in an electrochemical reaction. This water assists the performance of the PEFC by preventing drying of the polymer electrolyte. Conversely, the water hinders the transport of the reactant species by blocking the pores in the gas diffusion layer. Moreover, the effective electrode area is decreased, causing the cathode channel to become clogged with supersaturated water from the gas diffusion layer. This problem is overcome by separating the gas channel and the waste channel, and installing a water absorption layer (WAL). The new “WAL type” gas channel has an installed WAL in which the designed waste channel is compared with the gas flow characteristics of a conventional cathode gas channel by using the visualization technique. Gas flowing into the WAL type separator is barely blocked before the WAL absorbs water condensed in the cathode gas channel. Therefore, the WAL type separator effectively improves the PEFC performance.