22P151-P
Numerical Simulation of Negative Ion Extractionin Negative Hydrogen Ion Sources
水素負イオン源における負イオン引き出しの数値シミュレーション
S. Okuda1), N. Kameyama1), T. Fukuyama1), K. Miyamoto2), S. Kuppel1), S. Wada1), A. Hatayama1)
奥田晋 1)、亀山伸彦 1)、福山俊茂 1)、宮本賢治 2)、Sylvain Kuppel1)、和田將平 1)、畑山明聖 1)
1)Faculty of Science and Technology, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan慶應義塾大学理工学部, 〒223-8522 横浜市港北区日吉 3-14-1
2)
Naruto University of Education,
748 Nakashima, Takashima, Naruto-cho, Naruto-shi, Tokushima 772-8502, Japan鳴門教育大学, 〒772-8502 徳島県鳴門市鳴門町高島字中島 748 番地
The modeling and analysis of a negative ion source has been done by using a 2D particle-in-cellsimulation. The effect of the H- ion surface production process on the plasma grid (PG) surface isinvestigated. It is shown that with the increase of H- ion particles from the PG surface, the H- ioncurrent density is enhanced, while the electron current density decreases. These results agree wellwith the experimental results observed in typical negative ion sources. Moreover, it is found thatplasma quasi-neutrality holds mainly by both H+ and H- ions in the bulk plasma around the PG.

1. INTRODUCTION
Negative ion based neutral beam injection system(N-NBI system) is one of the promising candidates
for plasma heating and current drive of magnetic
fusion reactors. The negative ion source which can
produce negative ion beams with high power andlong pulse is the key component for the N-NBIsystem. It is essential for the development of such anegative ion source to suppress heat loads of theacceleration grids and beamline components1. Tosuppress heat loads, it is important to understandthe formation the mechanism of meniscus and Hextraction process in negative ion sources, because
the meniscus of source plasma-ion beam boundary
largely affects the H- ion beam optics.
The meniscus is determined from the sheathpotential near the plasma grid (PG), and the electricfield for H- ion extraction. In order to produce highcurrent H- ions, Cs vapor is seeded into most of
tandem-type negative ion sources [2-3]. The Cs
layers absorbed on the surface of plasma grid (PG)
result in low work function of the PG surface,followed by the surface production of H- ions. Thus,
there exists a large amount of H- ions near the PG
surface. Under this physical situation, formation
mechanism of meniscus and H- extraction process is
not clearly understood.
We have developed Particle-cell (PIC) code to
study the extraction mechanism of H- ions for theH- volume production [4]. The purpose of this studyis 1) to improve this 2D PIC model by taking into
account H- surface production, and 2) to perform asimulation under the case where the strong surface
production takes place.
2. SIMULATION MODEL
The motion of the charged particles (electrons,H+ ions, and H- ions) is solved by using the PICmethod. The trajectories of each particle arecalculated with the equation of motion:dv(1)
m= q (E + v × B )dt

where m, v, q, E, and B are mass, velocity, electriccharge, local electric field, and magnetic field. The
equation of motion is solved by using the BorisBuneman version of the leap-frog method.
The charge density is obtained at each meshpoint from the particle location by using a linear
interpolation. The Poisson equation is then solved
at each mesh point.
(2)∇ 2ϕ = q (nH − nH − ne ) / ε 0+−

where n and ε0 are the density of each particles andthe permittivity of vacuum, respectively.
Physical quantities are practically normalized.
The detail definitions are given in the Ref. [5].
Figure 1 shows the model geometry used in thepresent PIC modeling. In our study, we model theextraction region of the source with a simple 2D
geometry. The x-axis is taken parallel to thedirection of the extracted beam, while the y-axis isparallel to the PG surface. Normalized potentials at~
x = 0 and ~x =~xmax are 0 and 180, respectively.
~At t = 0 , H+ ions, electrons, and H- ions areloaded in the source region at the rate of NH+: Ne:
NH-= 10: 9: 1. When H+ ions are passed throughthe PG or ~x = 0 boundary, a pair of H+ ion andelectron is reloaded in the source region. The otherparticles passing through these boundaries areremoved. All the particles passing through ~x = ~xmaxboundary are reloaded in the source region. Allthe particles passing through yɶ = 0 or yɶ = yɶmax
boundary are reflected into model region. The
surface produced H- ions are modeled to belaunched at the surface of the PG with the initialenergy of a few eV.

Fig.1. The schematic view of the simulation model.

3. RESULTThe time evolutions of the extracted H- ioncurrent and the electron current are shown in Fig. 2.
The number of launched H- ions from the PG
surface is (a) 1 particle, and (b) 5 particles per time
step, respectively. In both cases, H- injected fromthe PG surface is started at tɶ = 1.0 ×104 in Fig. 2.
In Fig. 2, H- current density with surface
production increases from the one without surface
production, while electron current density decreases.
Moreover, the H- ion current density in the case (b)is twice higher than in the case (a), while theelectron density in the case (b) is about half as highas in the case (b). This means that the H- ion current
density is enhanced as the Cs effect, and theelectron current density decreases as the Cs effect.

Fig.2. The time evolutions of the extracted H- ion current
and the electron current.

Fig.3. The plot of the density profile of each particles inthe x- direction.

Figure 3 shows the density profiles of electron,H+, H-. These profiles are obtained along the lineyɶ = 7.5 in Fig.1 parallel to the x-axis. The profiles
have been obtained by averaging over the period of
2.8 × 10 4 ≤ tɶ ≤ 3.0 ×104 in Fig. 2 (b). As seen from Fig.
3, electrons do not exist in front of the PG surface
while H+ and H- ions equally exist (especially30 ≤ xɶ ≤ 40 ). This means that the plasma
quasi-neutrality is hold by a pair of ions in front ofPG surface. In addition, it should be noted that thethickness of the H+ and H- ion layer is relativelylarge ( ≥ 10λD ). These results agree well with thetypical experimental results in seeding Cs into
negative ion sources [6].
4. CONCLUSION
The modeling and analysis of a negative ion
source has been done by using a 2D PIC modelwith taking account of H- surface production. It isshown that with the increase of surface produced Hions, the H- ion current density is enhanced, while
the electron current density decreases. Moreover, it
is shown that plasma quasi-neutrality holds mainly
by H+ and H- ions in front of the PG surface. These
results agree with the typical experimental results.
Although the model is still under the improvement,
the present results show that the model is useful for
analyzing plasma transport, electrostatic potential
structure closed to the PG, and H- extraction under
the strong H- surface production.
References[1] M. Hanada, et al., Rev. Sci. Instruments, 69, 947(1998).
[2] K. N. Leung, et al., Rev. Sci. Instrum., 60, 531(1989).
[3] Y. Okumura, et al., Rev. Sci. Instrum., 71,1219(2000).
[4] S. Kuppel, et al., J. Appl. Phys., 109, 1 (2011).
[5] K. Miyamoto, et al., Proceedings of the 14th
International Conference on Ion Sources, Giardini
Naxos, Italy 2011.
[6] K. Tsumori, et al., Super CCBN 2010, Hida Earth
Wisdom Center, 21 Nov. 2010.