03aD11P
負イオン源における 3D3VPIC 法を用いたプラズマメニスカス形成及びビームハロに関する研究
Study of Plasma Meniscus Formation and Beam Halo in Negative Ion Sources Using
3D3V PIC Model
西岡 宗 1、宮本 賢治 2、奥田 晋 3、後藤 一平 1、畑山 明聖 1、深野 あづさ 4
S. Nishioka1, K. Miyamoto2, S. Okuda3, I. Goto1, A. Hatayama1 and A. Fukano41

慶應義塾大学、2 鳴門教育額、3 東芝、4 東京都立産業技術高等専門学校

1

Keio Univ.2 Naruto Univ. of Education, 3 Toshiba, 4 Tokyo Metropolitan Collage of Industrial Technology

1. Background, purpose
The neutral beam injection (NBI) system is one of thepromising candidates for heating and current drive in future
fusion reactors. In the NBI-System, the negative ion sources
are required to produce high energy, high current density andlong pulse negative ion beams.
In surface-produced negative ion sources of NBI system
for fusion device, it has been reported that negative ion beam
consists of two Gaussian parts, namely, a beam core withgood beam optics and a beam halo with poor beam optics.
Since the beam halo cause the heat loads on the acceleration
grid, it may limit the injection power and the beam pulse
length. Thus, to understand the physical mechanism of thebeam halo formation in negative ion sources is inevitable forthe suppression of the beam halo.
Recently, it is reported by the 2D3V-PIC (Particle In Cell)
simulation that the beam halo is possibly caused byrelatively deep penetration of an H- ion emitting surface(plasma meniscus) into the source1. However, the simulation
results don’t agree well quantitatively with the experimental
result. Therefore, for quantitative analysis of the beam halo,
our PIC code is extended to the 3D3V-PIC model. The
purpose of this study is to verify mechanism of the beam
halo formation and plasma meniscus formation with the3D3V-PIC model. In this study, the plasma meniscus and theconsequent beam halo fraction are compared between the 2Dand 3D models.
2. Simulation model
The simulation model in the present study is almost thesame as the previous 2D3VPIC model1, 2, 3. The equation ofmotion, the Poisson’s equation and the simulation domain isextended to the three dimensions.
Figure 1 shows a schematic view of our 3D simulation
model. The volume of the simulation domain is 17mm ×19mm × 19mm in the x, y, and z directions, respectively. Thesimulation domain includes the plasma grid (PG) with asingle aperture. The thickness of the PG and the radius of thePG aperture are 2 mm and 6 mm, respectively
The normalization of the physical quantities, reduced-size
scaling, various boundary conditions and the main physicalparameter are based on ref [1].

BFIG. 1 3D geometry of the simulation volume showing one plasma gridaperture.

(a)Plasmameniscus(b)

n%H-Plasma
meniscusFIG. 2 Negative ion density distribution(a) 2D3V PIC model (b) 3D3VPIC model

3. Simulation Result
Before full 3D calculations, we have done the 2D
calculation to compare the 3D results with those by the 2Dmodel. Figure 2 shows the negative ion density profiles of(a) by the 2D3VPIC model, and (b) by the 3D3VPIC modelin the x-y plane (z=0mm). In Fig. 2(a) and (b), the broken
line show the contour with gradφ = 0. This contour defines aplasma meniscus.
As seen from the comparison between Fig. 2(a) and (b), theplasma meniscus in the 2D model penetrates more deeply
into the plasma source region and the curvature is larger. In
the 3D model, the aperture is modeled as a hole while it ismodeled as a slit in the 2D models. This is a reason why thepenetration of the meniscus is small in the 3D model.
The beam halo fraction to the total beam current isestimated to be 51.5% in the 2D model while around 6.3% in
the 3D model. This value reasonably agrees with the fraction
observed in the experiments quantitatively4, 5.
4. Reference
1. K. Miyamoto, S. Okuda, A. Hatayama, M. Hanada, and A. Kojima, Appl.
Phys. Lett., 102, 023512-1 (2013).
2. S. Okuda, K. Miyamoto, T. Fukuyama, S. Nishioka, A. Hatayama, A.
Fukano, M. Hanada, A. Kojima, AIP conference proceeding, 1515, 107(2013).
3. S. Kuppel, D. Matsushita, A. Hatayama, and M. Bacal, J. Appl. Phys.,
109, 013305, (2011).
4. K. Miyamoto, Y. Fujiwara, T. Inoue, N. Miyamoto, A. Nagase, Y. Ohara,Y. Okumura, and K. Watanabe, AIP conference proceedings, 380, 360(1996).
5. H.P.L. de Esch, L. Svensson, Fusion Engineering and Design 86, 363(2011).