H. Hasegawa

S. Ishiguro

National Institute for Fusion Science (NIFS-Repository)

In recent magnetic confinement fusion device exper-iments, it has been observed that the density profile inthe far scrape-off layer (SOL) is flatter than the expo-nential distribution which is expected from a diffusionmodel1). Motivated by these experiments, some theoret-ical and computational studies about SOL plasmas havebeen made. Then a theory of plasma blob dynamics hasbeen suggested as the mechanism of the non-diffusive(convective) radial transport2). The plasma blob is a fila-mentary coherent structure along the magnetic field line,appears intermittently, and propagates from the edge ofcore plasma to the first wall. Such a structure is believedto transport a plasma into the far SOL across magneticfield lines. Many authors have studied dynamics of blobson the basis of two-dimensional reduced fluid models3).In such kind of macroscopic models, however, kineticeffects, such as sheath formation between plasma anddivertor plate and velocity difference between ions andelectrons, are considered under some assumptions andtreated as some adjustable parameters. Thus, in thisstudy, we investigate blob dynamics with the first prin-ciples method, that is, a three dimensional electrostaticplasma particle 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 and in the both ends of z axis asthe shaded planes shown in Fig. 1. The plane at x = 0and others at z = 0 and Lz correspond to the first walland divertor plates, respectively. In the y direction (cor-responding to the poloidal direction), periodic bound-ary condition is applied. A blob is initially located asa column along the ambient magnetic field (as shown inFig. 1). The initial density configuration of a blob in thecross section is given by the Gaussian distribution withthe width δb.In the simulations, we found the spontaneous elec-tric current system in a blob. And then, we have an-alyzed this current system in this fiscal year. Figure 2shows the streamlines of the electric current in a blob.Here, the viewing direction in Fig. 2 is about the sameas that in Fig. 1, the system size Lx × Ly × Lz is64λDe × 64λDe × 256λDe, and the initial blob size isδb = 4λDe. Figure 2 indicates that the current system ina blob has a spiral configuration due to the diamagneticcurrent and the parallel flows.Fig. 1: Configuration of the simulation.Fig. 2: Streamlines of the electric current in a blob.1) Umansky, M. V. et al.: Phys. Plasmas 5 (1998) 3373.2) Krasheninnikov, S. I.: Phys. Lett. A 283 (2001) 368.3) Krasheninnikov, S. I. et al.: J. Plasma Phys. 74(2008) 679 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) 2401060.411§8. Spontaneous Current System in a Plasma Coherent StructureHasegawa, H., Ishiguro, S.

Spontaneous Current System in a Plasma Coherent Structure

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