H. "Naitou

S. Tokuda

T. Sonoda

V.K." Decyk

National Institute for Fusion Science (NIFS-Repository)

§3. Gyrokinetic Particle Simulation of Kinetic Internal Kink Modes Naitou, H., Sonoda, T. (Ya1naguchi Univ. Eng.) Tokuda, S. (J.A.E.R.I.) Decyk, V.K. (U.C.L.A.) Parallel co1nputing is prornising the scalable high speed execution of sirnulation codes with the nun1ber of processors. By the use of parallel cornputer, uni1naginable progress in the plas1na sirnula.tion can be expected. The three dirnen-sional rnagneto-inductive gyrokinetic particle code GYR3D [1,2,3] is ported to a rnassively parallel cornputer, Intel Paragon XP /S15-256, which has 256 scalar processors (nodes) con-nected with a two-dimensional mesh topology. The Fortran77 corn piler with rnessage-passing subroutines (NX cornrnunication library) is used. The standard techniques of paralleliza-tion are used such as dornain decornposi tion and data transpose. No special technique was used for the load balance because we used the same size sub-domains and an ahnost equal nurnber of particles per sub-dornain since the density profile is uniform in space. Even in the non-uniform case, it rnay be possible that we do not need the load balance technique be-cause, in the 8 f code, we can select the dis-tribution of rnarkers uniformly in space. We only decomposed the do1nain in the z direction. The best results were obtained when the num-ber of rneshes in z, Nz, is equal to the num-ber of decomposition ,Ndivz· In our case, Nz is usually srnaller than the nurnber of nodes ' ' N1wde · Therefore we used N1wde/ Ndivz repli-cas. It is found that the speed of the par-allelized code scales well with the nurnber of nodes. The speed of the parallelized GYR3D with 256 nodes is rnore than three times faster than that of the vectorized GYR3D on NEC SX-3/24R (vector /parallel cornputer) with one node. This speed ratio is consistent with the ratio of peak speeds of these computers. The parallelized GYR3D code was applied to the sirnulation of the m = 1 (poloidal) and n = 1 (toroidal) kinetic internal kink rnode in tokamaks. A convergence study was done in the limit of extremely large nurnber of particles. The results show the very good energy conser-vation when about eight rnillion particles are used. It is irnportant to note that the nurnber of particles did not influence the linear phase of the instability. Also the nurnber of pa.rti-cles had only a slight influence on the nonlinear phase in the very long runs. This is explained in the following. Usually the error in energy conservation reflects the error in the total ki-netic energy of electrons which cornes frorn the contribution of the high energy electrons. The error in the kinetic energy did not significantly influence the temporal evolution of collective modes. This is because field quantities are de-termind by the lower order rnoments such as charge and current densities. The other finding in this paper is the existence of the quadrupole potential profile around the reconnection point in the secondary phase after the full reconnec-tion. This clear rnode structure is obtained by including higher n rnodes by reducing the tirne step size. It is found that three-dirnensional gyrokinetic particle codes with a 8 f rnethod are very power-ful tools in simulating the kinetic modification of MHD modes. However, more powerful par-allel computers with greater than 1 tera flops are required for silnulation with pararneters of existing and next generation tokamaks. The particle simulations for such tokamaks need in-tensive research on the architecture of parallel computers and on the parallel cornputing algo-rithms. References 1) Naitou, H., Journal of Plasrna and Fusion Research 70 ( 1994) 135. 2) Naitou, H., Tsuda, K., Lee, W.W., Sydora, R.D., Phys. Plasmas 2 (1995) 4257. 3) Naitou, H., Sonoda, T., Tokuda, S., Decyk, V.K., Jouunal of Plasrna and Fusion Research 72 (1996) 259. 249 

Gyrokinetic Particle Simulation of Kinetic Internal Kink Modes

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