H. Hasegawa

S. Ishiguro

National Institute for Fusion Science (NIFS-Repository)

Recently, it is reported that long-lived coherentstructures “blobs” in scrape-off layer (SOL) of magneticconfinement fusion devices1) propagate from the edge ofcore plasma to the first wall. Blobs are believed to trans-port a plasma into the far (second) SOL across magneticfield lines. Many theoretical and numerical works basedon two-dimensional reduced fluid models have been per-formed and dynamics of blobs have been investigated1).In this kind of macroscopic model, kinetic effects, such assheath formation between plasma and divertor plate, aretreated under some assumptions and parameterization.In this study, we have developed a three dimensionalelectrostatic plasma particle code with particle absorb-ing boundaries2) for the purpose of investigating blobdynamics including kinetic effects. Configuration of thesimulation is as follows. An external magnetic field ispointing into the z direction. The strength of magneticfield increases in the positive x direction. Particle ab-sorbing boundaries corresponding to divertor plates areplaced in the both ends of z axis. A particle absorbingboundary corresponding to the first wall is also placed atx = 0. In the y direction, periodic boundary conditionis applied.Fig. 1: Propagation of a blob.Fig. 2: Relation between the size of the blob and thepropagation velocity of the blob.Figures 1 and 2 were obtained from results of pre-liminary simulations. Figure 1 shows the electron densitydistribution in the x-y plane at z = Lz/2 at ωpet = 100,800, and 1, 500, where the system size Lx × Ly × Lz is64λDe×64λDe×256λDe and λDe is the Debye length. Theblob is initially located as a column along the externalmagnetic field at around (x, y) = (48λDe, 32λDe). Then,the blob moves to the first wall across the magnetic fieldlines. The mechanism of the blob propagation is as fol-lows. Ions and electrons drift in the positive and negativey direction due to grad-B drift, respectively. Thus, anelectric field in the negative y direction in the blob isformed. As a result, the blob moves in the negative xdirection due to E ×B drift1).Figure 2 shows the relation between the effectivewidth of the blob in the y direction (δb) and the propa-gation velocity of the blob (vb), where cs is the ion acous-tic speed. In Fig. 2, the closed circle refers to results ofsimulations and the solid line representsvb(δb) = vsimb (8√2 λDe)8√2 λDeδb2, (1)where vsimb (8√2 λDe) is the propagation velocity ob-served in the simulation in which the initial blob sizeis given as δb = 8√2 λDe. From the theory based on thetwo-dimensional reduced fluid model, it was found thatthe blob propagation velocity is proportional to δ−2b 1, 3).Figure 2 indicates that the particle simulation results areconsistent with the fluid theory.1) Krasheninnikov, S. I. et al.: J. Plasma Phys. 74(2008) 679 and references therein.2) Ishiguro, S. and Hasegawa, H.: J. Plasma Phys. 72(2006) 1233.3) Krasheninnikov, S. I.: Phys. Lett. A 283 (2001) 368.368§29. Particle Simulation of Plasma Blob DynamicsHasegawa, H., Ishiguro, S.

Particle Simulation of Plasma Blob Dynamics

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