National Institutes of Natural Science, National Institute for Fusion Science
Department of Helical Plasma Research, Fundamental Physics Simulation Research Division

Research Activity

Magnetic reconnection in the multi-hierarchy simulation model where particle (micro) model (micro) and fluid (macro) model are self-consistently connected.
Fundamental Physics Simulation Research Division performs cutting-edge simulation studies for open non-equilibrium physics of plasma and fusion, simulation methodologies and visualizations including real 3D visualization by the virtual reality (VR) to establish the basis of a whole fusion plasma simulation.
We are performing simulation studies on microscopic through macroscopic various physical processes, developments of novel simulation methods which connect micro and macro models, scientific visualization development such as virtual reality visualization. Figure illustrates magnetic reconnection in the multi-hierarchy simulation.

Recent Published Paper

"Impurity transport caused by
blob and hole propagations"
H. Hasegawa and S. Ishiguro,
Nuclear Fusion Vol. 57, in press (2017)

In this study, the dynamics between impurity ions and the blob and hole structures have been investigated with the three-dimensional electrostatic particle-in-cell simulation. The simulations have shown the following facts: (1) The dipolar profile of impurity ion density in the blob / the hole is formed. (2) Such a density profile propagates with the blob/ the hole. (3) The effective radial diffusion coefficient for impurity ions by a single blob / hole is comparable to the Bohm diffusion coefficient (the figure shows impurity ion tranport by a blob).
"Study of self-consistent particle flows
in a plasma blob with particle-in-cell simulations"
H. Hasegawa and S. Ishiguro,
Physics of Plasmas Vol.22, 102113 (2015)

In this study, the presence of the spiral current system composed of the diamagnetic and parallel currents in a blob is confirmed by the particle simulation without any assumed sheath boundary models (see the left figure). Furthermore, we have observed that the temperature structure in a blob is formed by the potential structure (see the electron velocity distributions in the right figure).
"Molecular dynamics and Monte Carlo hybrid simulation for fuzzy tungsten nanostructure formation"
A.M. Ito, A. Takayama, Y. Oda, T. Tamura, R. Kobayashi, T. Hattori, S. Ogata, N. Ohno, S. Kajita, M. Yajima, Y. Noiri, Y. Yoshimoto, S. Saito, S. Takamura, T. Murashima, M. Miyamoto and H. Nakamura,
Nuclear Fusion Vol.55, 073013 (2015)
Fuzzy nanostructure on the surface of tungsten materials is generated by exposure to helium plasma. The formation process of the fuzzy nanostructure had been successfully reproduced by Molecular Dynamics and Monte-Carlo hybrid (MD-MC hybrid) simulation.
"Effects of trapped electrons on ion reflection in an oblique shock wave"
M. Toida and J. Inagaki,
Physics of Plasmas Vol.22, 062305 (2015)

A magnetosonic shock wave propagating obliquely to the magnetic field can trap electrons and accelerate them to ultrarelativistic energies. These electrons excite multi- dimensional fluctuations, which significantly influence electron motions.
Effects of trapped electrons on ion motions in an oblique shock wave have been investigated with particle simulations. It is found that the number of accelerated ions via reflection from the shock front is increased because of the multi-dimensional fluctuations excited by the trapped electrons. The figure shows the phase space plots of ions [(b) 2Ds] and test ions [(c) 1Dt] which do not feel multi-dimensional fluctuations. The ions with vx > vsh are reflected.
"Formation of large-scale structures with sharp density gradient through Rayleigh-Taylor growth in a two-dimensional slab under the two-fluid and finite Larmor radius effects"
R. Goto, H. Miura, A. Ito, M. Sato, and T. Hatori,
Physics of Plasmas Vol.22, 032115 (2015)
Density contours in a 2D extended MHD simulation of the Rayleigh-Taylor instability. It is observed that the secondary Kelvin-Helmholtz-type instability forms fine wavy structures along the density contours. 
 "Multi-Hierarchy Simulation of Collisionless Driven Reconnection by Real-Space Decomposition"
S. Usami, R. Horiuchi, H. Ohtani, and M. Den,
Journal of Physics: Conference Series Vol.561, 012021 (2014)

Spatial profiles of the magnetic field lines. By means of our multi-hierarchy simulations, the influence of macroscopic dynamics on microscopic physics of magnetic reconnection is investigated. Dynamical behaviours of reconnection that is, steay reconnection (Fig.(a)) or intermittent reconnection (Fig. (b)) depend sensitively on plasma inflows from the MHD domain.
"Structure transitions induced by the Hall term in homogeneous and isotropic
magnetohydrodynamic turbulence"
H. Miura and K. Araki,
Physics of Plasmas Vol.21, 072313 (2014)
Isosurfaces of the enstrophy density (colored) and the current density (grey) in freely decaying homogeneous and isotropic Hall MHD turbulence in a simulation of 10243 grid points.
 "Suppression effects of Weibel instability for fast electron divergence"
H. Sakagami, T. Johzaki, T. Taguchi and K. Mima,
EPJ Web of Conference Vol.59, 17016 (2013)

As the growth rate of the Weibel instability is absolutely large, little suppression effects can be expected even if the background electron temperature increase to more than 100 keV. This magnetic field does not remain at rest and shows turbulent behaviors, enhancing the divergence of fast electrons. To control the location of Bz, punched out targets are introduced, where many vacuum holes are punched out as shown in left Figure. Background electrons that carry the return current can only flow in the gaps between holes, because the sheath field prevents them from crossing over the hole. Thus stable magnetic fields are induced and grow up to almost ±300 MG, which are shown in right figure. Fast electrons go forward with a meandering motion, and reduction of divergence can be expected. Unfortunately, this structure simultaneously disturbs fast electron propagation, and the fast electron beam intensity also decreases because the number of electrons in such energy range is reduced.
 "Development of Multi-Hierarchy Simulation Model with Non-Uniform Space Grids for Collisionless Driven Reconnection"
S. Usami, R. Horiuchi, H. Ohtani, and M. Den,
Physics of Plasmas Vol.20, 061208 (2013)

Multi-hierarchy simulation result of magnetic reconnection. The left and right panels display magnetic field lines and fluid velocity vectors. The multi-hierarchy model is progressed from the previous model as follows. First, the boundary condition in the downstream is improved from periodic to open, thus outflow can go out across the downstream boundary. Furthermore, non-uniform space grids are adopted in the macro hierarchy so as to calculate larger region.
 "Analytic Equilibria of High-Beta Tokamaks with Toroidal and Poloidal Flows and Pressure Anisotropy Associated with Parallel Heat Flux"
A. Ito and N. Nakajima,
Journal of the Physical Society of Japan Vol.82, 064502 (2013)

Profiles of pressures and the magnetic flux in the midplane (left) and the radial component of the diamagnetic current in a poloidal cross section (right) obtained from the analytic solution for tokamak equilibria with flow and pressure anisotropy. 
"Formation and Classification of Amorphous Carbon by Molecular Dynamics Simulation"
A. M. Ito, A. Takayama, S. Saito, and H. Nakamura,
Japanese Journal of Applied Physics Vol.52, 01AL04 (2013)

(Color online) Animation snapshots in the MD simulation for transition from graphite to diamond at a pressure of 15 GPa. The green and red spheres indicate sp2 and sp3 carbon atoms, respectively. The white lines show the side of the simulation box under the periodic boundary condition. 
"Kinetic effects on robustness of electron magnetohydrodynamic structures"
M. Hata, H. Sakagami, and A. Das,
Physics of Plasmas Vol.20, 042303 (2013)

(a) Electron orbit in the case of R=8 at the temperature of 4 keV, where black, blue, and pink lines indicate orbits and color contour represents magnetic field in the z direction.
(b) Schematic for sustained mechanism of electron's orbit, where red shading and yellow arrows express magnetic field in the z direction and radial electric field, and purple and white circles and blue-drifted-circle represent Larmor motion, circular motion via ExB drift, and electron orbit that results from Larmor motion and ExB drift, respectively.
"Macro- and microphysics of magnetic reconnection in a multi-hierarchy open system"
R. Horiuchi, M. Den, T. Tanaka, H. Ohtani, and S. Usami,
Plasma Physics and Controlled Fusion Vol.55, 014008 (2013)

Global MHD simulation of geomagnetosphere reconnection with kinetic resistivity. Spatial profiles of magnetic field lines and pressure are plotted when reconnection sets in (top panels) and when flux rope is formed (bottom panels). The left and right panels stand for the results for an effective resistivity model based on steady collisionless driven reconnection, and those for a constant resistivity model. Color on the lines presents the Bz intensity.
"Visualization of Particle Trajectories in Time-Varying Electromagnetic Fields by CAVE-Type Virtual Reality System"
N. Ohno, H. Ohtani, D. Matsuoka, R. Horiuchi,
Plasma and Fusion Research Vol.7, 1401001 (2012)

CAVE visualization of the time-varying data at (1) T=0, (2) 25 T, and (3) 50ΔT, where ΔT is time interval of storing the simulation data. The color contours on xy and yz planes are ion temperature and a reconnection component of magnetic field, respectively. The blue and white lines are magnetic field and ion trajectories, respectively. The magnetic field lines are drawn by the data at the each time, while the ion trajectories are shown as time-tracing lines from T=0.
 "Effects of long rarefied plasma on fast electron generation for FIREX-I targets"
H. Sakagami, A. Sunahara, T. Johzaki, and H. Nagatomo,
Laser and Particle Beams Vol.30, 103-109 (2012)

These figures show time evolutions of averaged core electron temperature over the dense region (ρ>10 g/cm3) without rarefied plasmas(purple) and with rarefied plasmas of different densities, 0.01(green), 0.05(red), 0.1(blue), 0.5(yellow) and 1(gray) critical density for (a) Lrare=150 and (b) 300 μm.
"Simulation of Plasma Flow Injection with Multi-Hierarchy Model Aiming Magnetic Reconnection Studies"
S. Usami, H. Ohtani, R. Horiuchi, and M. Den,
Communications in Computational Physics Vol.11, 1006-1021 (2012)

Bird's eye view of the mass density in the multi-hierarhcy simulation. At the initial state, mass density is uniform. Plasmas smoothly and continuously flow from the MHD to PIC domains through the interface domain.
"Visualization of Three-Dimensional MHD Simulation Data to Study Fine Unstable Motions in a Helical Torus Device"
H. Miura,

Isosurfaces of the pressure in a 3D MHD simulation of a hot magnetized plasma dominated by the ballooning instability are visualized. The flute-like structures of the isosurfaces indicates growth of the ballooning mode. The yellow circle indicates the tangential port of the LHD.
"Equilibria of toroidal plasmas with toroidal and poloidal Flow in high-beta
reduced magnetohydrodynamic models"
A. Ito and N. Nakajima,
Nuclear Fusion Vol.51, 123006 (2011)

Grad-Shafranov type equations for high-beta toroidal equilibria with toroidal and poloidal flow, two-fluid and finite Larmor radius effects and pressure anisotropy.
"Scientific Visualization of Plasma Simulation Results and Device Data in Virtual-Reality Space"
H. Ohtani, Y. Tamura, A. Kageyama, and S. Ishiguro,
IEEE Transactions on Plasma Science Special Issue - Images in Plasma Science 2011, Vol.39 (11) 2472-2473 (2011) 

Scientific visualization of the equilibrium plasma in the Large Helical Device (LHD) with the realistic description by the virtual-reality system `CompleXcope.' Blue surface shows the isosurface of plasma pressure. Green and magenta lines are the single magnetic-field line and single particle orbit, respectively.
"Hybrid Simulation between Molecular Dynamics and Binary Collision Approximation Codes for Hydrogen injection into Carbon Materials"
S. Saito, A. M. Ito, A. Takayama, T. Kenmotsu, and H. Nakamura,
Journal of Nuclear Materials, doi:10.1016/j.jnucmat.2010.12.233, (2011)

The left panel shows thirty trajectories of incident hydrogen that moves in a graphite material calculated by BCA-MD hybrid simulation. The gray and black lines in the left panel are obtained by the binary collision approximation (BCA) and the molecular dynamics (MD) simulation, respectively. The right panel shows a detailed image of the MD simulation part.
"Multi-scale simulation for plasma science"
S. Ishiguro, S. Usami, R. Horiuchi, H. Ohtani, A. Malukov, and M. M. Skoric,
Journal of physics: Conference Series Vol. 257, 012026 (2010) 

Structure of primal Equation Free Projective Integration PlasmaSimulation.
"Simulation Data Analysis by Virtual Reality System"
H. Ohtani, N. Ohno, N. Mizuguchi, M. Shoji, and S. Ishiguro,
Plasma and Fusion Research Vol.5, S2109 (2010)

Scientific visualization of both simulation result and experimental device data. This is the view from outside of LHD vessel device.Simulation result about LHD is obtained by MHD simulation, and pressure profile is shown as green isosurface.
"Simulation of feedback instability in the coupled magnetosphere-ionosphere system"
Hiroki Hasegawa, Nobuaki Ohno, and Tetsuya Sato,
Journal of Geophysical Research Vol.115, A08304 (2010) 

The left panel shows the distribution of the field-aligned current density perturbation at the ionospheric height, which is obtained by a nonlinear simulation. The right panel represents the distribution of the linear growth rate calculated from the quasi-steady state data through the integrated feedback instability theory. This figure indicates that the areas where longitudinally striated structures appear are consistent with the prediction by the theory.
"Influences of ballooning modes with moderate wave number on MHD equilibrium in LHD"
H. Miura and N. Nakajima,
Nuclear Fusion Vol.50, 054006 (2010)

A 3D view of a full-3D MHD simulation of LHD. Contours of the pressure are drawn on the poloidal cross-section.The red region represents β = p/(B02 /2) = 3.7% , while the green colour is p/(B02 /2) = 1%.
"Magnetic Reconnection Controlled by Multi-Hierarchy Physics in an Open System"
R. Horiuchi, S. Usami, H. Ohtani, and T. Moritaka,
Plasma and Fusion Research Vol.5, S2006 (2010) 

Spatial profiles of E + ve x B (left), Bx/Te (middle), and off-diagonal component of electron pressure tensor Peyz (right) in the (y, z) plane in the presence of drift-kink instability.
"Improved Open Boundary Model for Plasma Particle Simulations"
Hiroki HASEGAWA and Tetsuya SATO
Plasma and Fusion Research Vol.5, 020 (2010)

This figure shows the result of a test simulation. The top, middle, and bottom panels present the spatial profile of electric potential and distributions of electrons and ions in space-velocity phase space, respectively. This result indicates that double layer creation by ion-acoustic instability also occurs in a case where the electron drift velocity is comparable to the electron thermal velocity.
"First Demonstration of Collisionless Driven Reconnection in a Multi-Hierarchy Simulation"
S. Usami, H. Ohtani, R. Horiuchi, and M. Den,
Plasma and Fusion Res. Vol.4, 049 (2009)

Magnetic lines of force and fluid velocity vectors in a multi-hierarchy simulation.
"Equation-Free Coarse Grained Projective Integration Method for Multi-Scale Plasma Simulation"
ISHIGURO Seiji and SKORIC Milos, M.
J. Plasma Fusion Res. Vol.85, 593-596 (2009)

 Concept of Equation-Free Projective Integration Method.
"Relativistic Laser-Plasma Interactions"
Milos M. Skoric
AIP Conf. Proc. Vol.1188, 15-34 (2009)

Snapshots of spatial laser pulse energy from 1D Fluid (left) and 2D PIC (right) simulations show strong spatial modulation, depletion and pulse break-up due to backward SRS.
"Generation control of fast electron beam by low density foam for FIREX-I"
H. Sakagami, T. Johzaki, H. Nagatomo, and K. Mima
Nucl. Fusion, Vol.49, 075026 (2009)

Maximum averaged core electron temperatures as a function of foam density. Red circle, blue triangle, purple square and green diamond indicate the foam thickness of 40, 60, 70 and 80 micron, respectively.
"Open Boundary Condition for Particle Simulation in Magnetic Reconnection Research"
H.Ohtani and R.Horiuchi
Plasma and Fusion Research Vol.4 (2009) 024.

Comparison of the results of short (top) and long (bottom) -simulation boxes to check whether the open boundary condition fulfills its function. The results of the short -simulation box effectively mimic those of the long -simulation box.
"Analytic high-beta tokamak equilibria with poloidal-sonic flow"
A. Ito and N. Nakajima,
Plasma Phys. Control. Fusion Vol.51, 035007 (2009)

Pressure isosurfaces (black) and magnetic flux surfaces (gray) for (a) sub-and (b) super-poloidal-sonic flows.
"Electron Force Balance in Steady Collisionless-Driven Reconnection"
B. Li and R. Horiuchi,
Physical Review Letters Vol.101, 215001 (2008)

Spatial profiles of each term in the z direction(top) and inflow direction (bottom) of the electron momentum equation along the vertical line passing the X point in the steady state.