25G09
Study of Blob Dynamics with Particle Simulation
粒子シミュレーションによるブロブ挙動の研究
Hiroki Hasegawa1 and Seiji Ishiguro1, 2
長谷川裕記1, 石黒静児1, 2
1

Department of Helical Plasma Research, National Institute for Fusion Science322-6 Oroshi-cho, Toki, Gifu 509-5292 Japan1
核融合科学研究所 ヘリカル研究部 〒509-5292 岐阜県土岐市下石町322-6
2
Department of Fusion Science, The Graduate University for Advanced Studies (Sokendai)322-6 Oroshi-cho, Toki, Gifu 509-5292 Japan2
総合研究大学院大学 核融合科学専攻 〒509-5292 岐阜県土岐市下石町322-6
A three dimensional electrostatic plasma particle code has been developed to study blob dynamics including kinetic effects. Some results of preliminary simulations coincide with previous results fromtwo-dimensional reduced fluid model. The relation between observed blob propagation speed and initial
blob size is consistent with the expectation by the fluid theory. The collapse of blob structure by certain instability is also observed.

1. Introduction
Recently, it was reported that the evidence of
non-diffusive plasma transport from the edge ofcore plasma to the first wall in magnetic confinement fusion devices was observed. Such atransport is thought to be brought by long-lived
coherent structures “blobs” in scrape-off layer(SOL) [1]. Blobs propagate across magnetic fieldlines and are believed to transport a plasma into
the far SOL. Many theoretical and numericalstudies of blobs have been performed on the basisof two-dimensional reduced fluid models anddynamics of blobs have been investigated [1].
However, kinetic effects, such as sheath formation between a SOL plasma and a divertor
plate, are treated under some assumptions andparameterization in that kind of macroscopic
model.
In this study, we have developed a three dimensional electrostatic plasma particle code withparticle absorbing boundaries [2] in order tostudy blob dynamics including kinetic effects.
Results of preliminary simulations indicate thatblobs move to the first wall across the magnetic
field lines. Obtained relation between the observed propagation speed of the blob and the initial effective width of the blob in the poloidal
direction is consistent with the expectation by thefluid theory. In no sheath case, it is observed thata blob evolves to a mushroom-shaped object. This
fact is in agreement with previous results of thefluid model simulations [3].
2. Simulation Method
We use a three-dimensional electrostatic parti-cle code with full particle dynamics to investigate
blob propagation. Configuration of the simulation
is as follows. An external magnetic field ispointing into the z direction (equivalent to thetoroidal direction). The strength of magnetic field
increases in the positive x direction (equivalent tothe counter radial direction). Particle absorbing
boundaries corresponding to divertor plates areplaced in the both ends of z axis. A particle absorbing boundary corresponding to the first wallis also placed at x = 0. In the y direction (equivalent to the poloidal direction), periodic boundary
condition is applied. The schematic figure of thesimulation system is shown in Fig. 1. The shaded
planes in Fig. 1 refer to absorbing boundaries.

Fig. 1. Configuration of the simulation

3. Results of preliminary simulations
We now show results of preliminary simulations.
Figure 2 displays electron density distributions in
the x-y plane at z = Lz/2 at pe t = 100, 800, and1,500, where the system size Lx x Ly x Lz is 64 Dex 64 De x 256 De and De is the Debye length. Theblob is initially located as a column along the ambient magnetic field at around (x, y) = (48 De, 32
De). This figure indicates that the blob moves tothe first wall across the magnetic field lines as thesize is given as b = 11.3 De. From the theory basedon the two-dimensional reduced fluid model, it wasfound that the blob propagation velocity is proportional to b-2 [1, 4]. Thus, Fig. 3 indicates that theparticle simulation results are consistent with thefluid theory.
On the other hand, Fig. 4 gives a result in thecase that periodic boundary condition is applied in
the z direction, i.e., there is no sheath. As shown in
Fig. 4, the blob evolves mushroom-shaped structure
by certain instability. This fact is in agreement withprevious results of the two-dimensional reduced
fluid model simulations [3].

Fig. 2. Electron density distribution at pe t = 100, 800,and 1,500

Fig. 4. Electron density distribution obtained by simulation in the case of no sheath

4. Summary
We have developed a three dimensional electrostatic plasma particle code to study blob dynamics including kinetic effects. We then showed
results of preliminary simulations. Some results
coincide with previous results from the fluidmodel.
The close investigation into the effect of kinetic dynamics on blob propagation will be one of
topics in future work.

Fig. 3. Relation between blob size and blob propagation
speed

fluid model expected.
We show in Fig. 3 the relation between the effective width of the blob in the y direction (b) and thepropagation velocity of the blob (vb), where cs is theion acoustic speed. In Fig. 3, the closed circle refers
to results of simulations and the solid line represents() ,(1)

where vb (11.3 De) is the propagation speed observed in the simulation in which the initial blobsim

Acknowledgments
This work is performed with the support and under the auspices of the NIFS Collaboration Research programs (NIFS11KNSS021 and NIFS11
KNXN224) and supported by a Grant-in-Aid for
Scientific Research from Japan Society for thePromotion of Science (KAKENHI 23740411).
References[1] S. I. Krasheninnikov, D. A. D’Ippolito, and J. R.
Myra: J. Plasma Phys. 74 (2008) 679 and references
therein.
[2] S. Ishiguro and H. Hasegawa: J. Plasma Phys. 72(2006) 1233.
[3] N. Bian, S. Benkadda, J.-V. Paulsen, and O. E. Garcia: Phys. Plasmas 10 (2003) 671.
[4] S. I. Krasheninnikov: Phys. Lett. A 283 (2001) 368.