S2-4 Fusion Plasma Simulation and Computation Science in the Future
核融合プラズマシミュレーションと次世代計算科学
Hayashi Takaya
林 隆也
Theory and Data Analysis Division, National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu, 509-5292,
Japan
核融合科学研究所理論・データ解析研究系，〒509-5292 岐阜県土岐市下石町322-6

Plasma physics has directed its attention to the potential power of computer simulation as a
powerful methodology of science since the twilight of the evolution of computer. With the great
help of noticeable progress in supercomputer, investigation of fusion plasmas through computer
simulation has been deepened significantly. For future direction of computer simulation, it’simportant to develop an algorism that solves accurately processes in each scale layer while
exchanging necessary information between layers.
1. Introduction
Plasma physics has directed its attention to thepotential power of computer simulation as a
powerful methodology of science since the twilightof the evolution of computer. In particular, it has
been trying to capture the nature of plasmas fully ina self-consistent manner as much as possible byusing large-scale computer simulation method. Thisis because complex natures that are often expressed
as nonlinearity, nonequilibrium, and openness are
considered to be essential in describing behavior ofplasmas. Remarkable properties, such as
self-organization or structure formation, are very
often observed in such systems, and plasma has
been found to be an exemplar of those properties.
Those natures require cultivation of new
methodology to understand the mechanism ofprocesses that is beyond conventional methods inphysics. Fusion plasma physics is not an exception,
which is a branch of plasma physics. Due toexistence of strong gradient in various parametersand influx/outflux of energy with external region,
fusion plasmas have been found to reveal itself
variety of remarkable phenomena. With the great
help of noticeable progress in supercomputer,investigation of fusion plasmas through computer
simulation has been deepened significantly.
Nowadays computer simulation aims to give
good prediction of real fusion plasma experiments,
which contributes to effisient execution ofexperiments as well as reduction of construction
costs for new devices. The properties of plasma areso complicated that we need to develop further
sophisticated modeling to realize precise prediction
for overall behavior of plasmas, however.

2. Prospect of Fusion Plasma Simulation
The problem is whether or not we could give
good prediction of fusion plasmas if performance ofcomputer is enhanced. It is true that we can havefiner grid points as well as longer time evolution
when computer performance is increased. In thisway, phenomena in a specific scale of space andtime can be investigated more accurately. It’s
important to continue this effort.
It has been recognized recently, however, thatvarious scales of space and time appear in actual
fusion plasmas, which consist of macroscale andmicroscale (ion and electron), and they are notindependent. Conventional approach of plasma
physics has been based on an assumption thatphysics processes can be split into several layers ofscale, and overall nature can be captured if physics
of each layer is understood. In actual plasma
phenomena, the interaction between different scalelayers may not be able to be neglected, even if it isweak. The weak interaction could change theprocesses drastically when it is accumulated. The
challenge is to develop a new method that can treat
multiple layer structure simultaneously andself-consistently. The difference in time scale
between macro and micro scale can be extended to
10 to the 8th in fusion plasmas, and it is understood
immediately that a model that try to cover all scales
in one equation and grid system is not realistic. For
future direction of computer simulation, it’s
important to develop an algorism that solves
accurately processes in each layer while exchanging
necessary information between layers. The multiple
layer structure is not specific in fusion plasmas, but
is quite universal in all fields of natural science aswell as social science. The algorism that isdeveloped in plasma physics can be exported toother fields in science.
Shown in Fig.1 is a possible example ofinteraction between macroscale and microscale
phenomena. Figure 1 (a) shows global evolution ofInternal Reconnection Event that is observed inspherical tokamak experiments [1]. As the nameindicates, magnetic reconnection plays importantrole in the processes. MHD simulation is executedin Fig.1 (a). For a case of higher temperature
confinement, collisionless reconnection is supposed
to be important in the mechanism of reconnection,
which is not fully described by the MHD model.
Shown in Fig.1 (b) is a microscale simulation of thecollisionless reconnection [2], which reveals
detailed physics around the X points ofreconnection. Self-consistent combination of those
two simulations can clarify the overall behavior ofthe plasma.
(a)

(b)

Fig.1 (a) Macroscale MHD simulation of Internal
Reconnection Event in spherical tokamak [1]. (b)Microscale simulation of collisionless magnetic
reconnection [2].
3. DiscussionNumerical analysis technique of linear stability
for fusion plasmas has been developed to a highlysophisticated level. Nonlinear simulation for eachlayer of scale has revealed many exciting processes.
Nevertheless, we believe it’s a time to open
doorway to investigate physics of multi scale layers.
A lot of surprises may be waiting.
In many fields of science, computer simulation is
now considered to be the third method of science
following experiment and theory. We haveproposed to call it “simulation science”, in which
we aim to search for truth that can be reached only
through computer simulation.
For the purpose of pursuing simulation science,
it’s also important to organize a simulation
researcher group that consists of intensive
interaction among researchers on simulation,
computerusage,algorism,andtheory.
Collaboration with surrounding researchers is
another important factor.
The Earth Simulator is a bright star of hope forsimulation science. It has kept the world record ofperformance for more than two and half years. A
good combination of computer vendor andsimulation researcher would create a new era ofsimulation science.

Acknowledgments
I acknowledge discussions with simulation
researchers at NIFS.
References[1] N.Mizuguchi, T.Hayashi and T.Sato, Phys.
Plasmas 7 (2000) 940.
[2] R.Horiuchi et al, Comp. Phys. Comm (2004) inpress.