22P152-P
Analysis of H- Ion Emissive Surface in the Extraction Region in Hydrogen
Negative Ion Sources
水素負イオン源の引き出し領域における表面生成水素負イオンの解析
Toshishige Fukuyama1), Nobuhiko Kameyama1), Susumu Okuda1), Shojiro Koga1),
Katsuyoshi Tsumori2), Haruhisa Nakano2), Akiyoshi Hatayama1)
福山俊茂1), 亀山伸彦1), 奥田 晋1), 古賀章二郎1), 津守克嘉2), 仲野治久2), 畑山明聖1)
Faculty of Science and Technology, Keio University
3-14-1, Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan1)
慶應義塾大学理工学部〒223-8522 神奈川県横浜市港北区日吉3-14-1
National Institute for Fusion Science322-6, Oroshi-cho, Toki, Gifu 509-5292,Japan2)
核融合科学研究所〒509-5292 岐阜県土岐市下石町322-6
To optimize the H- production and extraction condition is one of the key issues for developing negative ionsources. The profile of plasma density and electrostatic potential with and without extraction grid (EG)voltage are analyzed with 2D PIC modeling for a NIFS-R&D source. The results make clear the processes
forming double ion (H+ and H-) and electron poor plasma in the vicinity of the plasma grid (PG) which has
been recently observed in the experiments of the NIFS R&D source. In addition, the present numericalmodeling gives useful insight into the formation mechanism of the plasma meniscus and H- extraction
process in such a double ion plasma.

1. Introduction
In negative ion sources of neutral beam
injectors (NBI) for large fusion devices such as
LHD, one of the key issues is to optimize the Hproduction and extraction condition. For theoptimization, it is indispensable to understand theformation mechanism of the ion emissive surface(so-call plasma meniscus) and its location/shapearound the extraction hole. Recently, in aNIFS-R&D ion source scaled a half size of theLHD ones, the following interesting experimental
observations have been reported under the“surface” H- production case with the Cs-seeding[1] : (1) Plasma layer consisting of H+ and Hions (i.e., electrons are excluded from the layer.)
is formed in the vicinity of the PG, and (2) thethickness of the plasma layer is relatively large(at least the layer has a thickness of ~15mm fromthe PG by Langmuir probe measurements).
The purpose of this study is to understand theformation mechanism of the plasma meniscus andits location/shape around the extraction hole with
2D Particle-in-Cell (PIC) modeling. The profileof plasma density and potential with and without
EG voltage are analyzed with 2D PIC model in amodel geometry near the PG of the NIFS-R&D
source.
2. Simulation Model
The motion of charged particles (H+ ions, H- ions,and electrons) is solved in their self-consistentelectric field using the PIC method [2]. The
trajectories are first calculated with the equation ofmotion for each particle with a leap-frog integrator[3]. The charge density is obtained at each meshpoint from the particle location by using a linear
interpolation. The Poisson equation is then solved ateach mesh point by the Bi-CGSTAB method [4].
Figure 1 shows the model geometry used in thefollowing numerical calculations. The model
geometry is based on the NIFS-R&D ion source.
The system size of the present simulation is scaled
as 1/38. The x-axis is taken parallel to the directionof the H- extraction beam through the aperture,
while the y-axis is parallel to the PG surface.
The space coordinates ( x, y ) and the time t arenormalized as follows: ~x = x / λ De , ~y = y / λ De ,~and t = ωpet , where λ De and ωpe are theelectron Debye length and the electron plasma
frequency, respectively, calculated from the initial
electron density ne and temperature Te ,respectively. The electrostatic potential Φ andmagnetic~flux density ~B are also normalized asfollows: Φ = eΦ / kTe , B = B /( meωpe / e) ,where eand k are the unit charge and the Boltzmann const,respectively. Also, the Electron Deflection Magnets(EDM) with an infinite length in the z direction is
installed in the EG. The magnetic field is calculated

Fig. 1. Model geometry

at each mesh point by using the analytical solution
based on the magnetic charge model [5].
Additionally, the ratio of the Larmor radii ofcharged particles and the characteristic decay lengthof the magnetic field amplitude is adjusted to be in
the same ordering with that in the simulation andexperiments.
In the numerical calculation, two extraction holes
are modeled to take into account the periodicity ofthe electron deflection magnets. The Dirichletboundary condition is imposed on ~x = 0 , PG, andEG. The periodic condition is applied to the lower
boundary ( ~y = 0 ) and upper boundary ( ~y = ~y max ).
For the right-hand side boundary ( ~x=~x max ), theabsorbing boundary condition is adopted. If
particles reach the right boundary, they are removed
from the calculation domain.
~At t = 0 , 2×105superparticles are loaded in thesource region. The initial ratio of the superparticle
numbers for H+ ions and electrons assumed to be
NH+:Ne = 1:1. Electrons and H+ ions are assumed tobe launched initially as Maxwellian distributions
with the initial temperature of 0.6eV and 0.5eV,respectively. Surface produced H- ions are loaded
one particle per one time step at PG surface, andassumed to be launched as a cosine distribution
with the temperature of 1eV. For simplicity, some of
the collision processes are not taken into account.
3. Results and Disucussion
The beam extraction has been started at thenumber of time steps N=105. Before beam
extraction (N<105), the potential profile has reacheda quasi-steady state after N~8×104 time steps.
Figure 2 shows the 2D density profiles of (a)electrons and (b) positive H+ ions before beam
extraction. These density profiles have beenobtained by averaging the calculated density overthe last 5×103 steps before beam extraction. As seenfrom Fig. 2(a), the electron density becomes verysmall and almost zero close to the extraction holeand the PG. This is mainly because electrons aretrapped by the magnetic field. They cannot
penetrate deeply into the extraction region due totheir small Larmor radii. The positive H+ ion
density also decreases towards the extraction holeand PG surface, but H+ ion density is still relatively
large compared with the electron density. As aresult, electron poor plasma is formed in thevicinity of the extraction hole and the PG.
The electron density in the vicinity of theextraction hole after beam extraction becomeslarger than that before beam extraction.
These initial results shown above supports thefollowing
qualitativeexplanationoftheexperimental results in NIFS R&D source proposedin Ref.[1] :(1) since H- ions are produced on PG
surface, bulk electrons are prevented to penetrate

Fig. 2. The 2D density profiles of (a) electrons and(b) positive H+ ions before beam extraction [6]Fig. 3. The potential profile near the PG with EG voltage

the beam extraction region. Consequently,electron-poor plasma is generated in the extraction
region without extraction. (2) when the H- ions areextracted from the region, charge neutrality breaks
down and bulk electrons transport from bulk tocompensate excess positive ions.
These initial simulation results also give someinsight into the H- ion emissive surface. Figure 3
shows the potential profile near the PG with EGvoltage. The shielding of the EG voltage isrelatively small due to the small density of thenegative charge (mainly H- ions) in the region
80< x% <100. Relatively deep penetration of the EG
voltage can be seen in Fig. 3. This makes it possible
for the H- ions to be extracted from the volume evenin the Cs-seeded surface H- production case.
References[1] K. Tsumori, et.al., Super CCNB 2010 (Hida Earth
Wisdom Center) (2010).
[2] S. Kuppel, D. Matsushita, A. Hatayama, and, M.
Bacal, J. Appl. Phys., 109, (2011)1.
[3] C. K. Birdsall and A. B. Langdon, Plasma Physics
via Computer Simulation (McGraw-Hill, New York,1985).
[4] H. A. Van der Vorst. SIAM J. Sci. Stat. Comput., 13,(1992)2.
[5] Y. Ohara, M. Akiba, H. Horiike, H. Imai, Y.
Okumura, and S. Tanaka, J. Appl. Phys., 61,(1987)1323.
[6] N. Kameyama, et.al., Proceedings of the 14th
International Conference on Ion Sources, GiardiniNaxos, Italy, 12-16 September, 2011 (submitted)