H. Hasegawa

S. Ishiguro

National Institute for Fusion Science (NIFS-Repository)

Recent papers have reported that the density profilein the far scrape-off layer (SOL) in magnetic confinementfusion devices is flatter than the exponential distribu-tion which is expected from a diffusion model1). Mo-tivated by these experiments, some authors have stud-ied SOL plasmas theoretically. Then a theory of plasmablob dynamics has been suggested as the mechanism ofthe non-diffusive (convective) radial transport2). Theplasma blob is a filamentary coherent structure alongthe magnetic field line, appears intermittently, and prop-agates from the edge of core plasma to the first wall.Such a structure is believed to transport a plasma intothe far SOL across magnetic field lines. Many pieces ofevidence that such structures are produced have beenreported3). Further, many authors have investigated dy-namics of blobs on the basis of two-dimensional reducedfluid models3). In such kind of macroscopic models,however, kinetic effects, such as sheath formation be-tween plasma and divertor plate and velocity differencebetween ions and electrons, are considered under someassumptions and treated as some adjustable parameters.Thus, in this study, we investigate kinetic effects on blobdynamics with a three dimensional electrostatic plasmaparticle simulation4, 5).Figure 1 shows the configuration of the particle sim-ulation. An external magnetic field is pointing into the zdirection (corresponding to the toroidal direction). Thestrength of magnetic field increases in the positive x di-rection (corresponding to the counter radial direction) as2LxB0/(3Lx − x) where Lx, Ly, and Lz are the systemsize in the x, y, and z directions and B0 is the magneticfield strength at x = Lx. Particle absorbing boundariesare placed at x = 0 as the shaded planes shown in Fig. 1.The plane at x = 0 corresponds to the first wall. In they (corresponding to the poloidal direction) and z direc-tions, periodic boundary condition is applied. A coherentstructure is initially set as a column along the externalmagnetic field (as shown in Fig. 1). The initial densityconfiguration of the structure in the cross section is givenby the Gaussian distribution with the width δb. The sys-tem size Lx × Ly × Lz is 64λDe × 64λDe × 16λDe. Theinitial radius of the structure is δb = 4.0λDe.Figure 2 shows the electron density distributions inthe x-y plane at z = Lz/2 at ωpet = 800 where the ion-to-electron temperature ratio is Ti/Te = 0.25 and themagnetic strength is set as |Ωe|/ωpe = 10, 5, and 1 inthe upper, middle, and bottom panels, respectively.As shown in Fig. 2, the symmetry breaking in a blobprofile occurs when the magnetic strength is decreased.This fact is thought to indicate that the effect of Larmorradius induces the symmetry breaking.Fig. 1: Configuration of the simulation.Fig. 2: Electron density distributions in the x-y planeat z = Lz/2 at ωpet = 800 where the magnetic strengthis set as |Ωe|/ωpe = 10, 5, and 1 in the upper, middle,and bottom panels, respectively.1) Umansky, M. V. et al.: Phys. Plasmas 5 (1998) 3373.2) Krasheninnikov, S. I.: Phys. Lett. A 283 (2001) 368.3) D’Ippolito, D. A. et al.: Phys. Plasmas 18 (2011)060501 and references therein.4) Ishiguro, S. and Hasegawa, H.: J. Plasma Phys. 72(2006) 1233.5) Hasegawa, H. and Ishiguro, S.: Plasma Fusion Res. 7(2012) 2401060.412§9. Kinetic Effect on Plasma Coherent Structure DynamicsIshiguro, S., Hasegawa, H.

Kinetic Effect on Plasma Coherent Structure Dynamics

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