22P157-P
Reaction of H- Ion Source Plasma to Beam Extraction
H-イオン源のビーム引き出しに伴うプラズマ応答
NAKANO Haruhisa, TSUMORI Katsuyoshi, KISAKI Masashi, SHIBUYA Masayuki, NAGAOKA
Kenichi, IKEDA Katsunori, OSAKABE Masaki, KANEKO Osamu, ASANO Eiji, KONDO
Tomoaki, SATO Mamoru, SEKIGUCHI Haruro, KOMADA Seiji, and TAKEIRI Yasuhiko
中野治久，津守克嘉，木﨑雅志，渋谷真之，永岡賢一，池田勝則，長壁正樹，金子修，
浅野英児，近藤友紀，佐藤守， 関口温朗，駒田誠司，竹入康彦
National Institute for Fusion Science, Natural Institutes of National Institutes
322-6, Oroshi-cho, Toki, 509-5292, Japan
自然科学研究機構核融合科学研究所 〒509-5292 土岐市下石町322-6
Negative-hydrogen-ion (H-) density in the vicinity of the beam extraction surface of H- source is one of theimportant parameters for the clarification of the particle dynamics concerning H- beam extraction. We
successfully measured the H- density and its profile with Cavity RingDown (CRD) method. The linear
relations were observed between the H- density and input arc power to plasma, and between the H- densityand extracted beam current. We observed that the H- density changes at the moment of the beam extraction.
Since variation of H- density at the moment of beam extraction depends on the bias voltage applied
between the arc chamber and the electrode of the beam extraction surface, the bias voltage can affects
dynamics of not only electron but also H-.

1. Introduction
Negative hydrogen ion (H-) sources are appliedand scheduled to be utilized for neutral beam
injectors (NBIs) with more than a few hundreds
keV of beam energy for nuclear fusion
experiment devices [1-3]. In the H- sources,
cesium is seeded for increasing H- beam current.
In the cesium seeded H- source, it is consideredthat H- is mainly produced on the electrode ofbeam extraction boundary, so-called plasma grid
(PG). In this case, the velocity direction of Hneeds to change from the inward of the discharge
area to opposite direction (extracted beamdirection). The particle dynamics including thisphenomenon in the vicinity of the PG has notbeen clearly understood. The clarification of theparticle dynamics is one of the important tasks forthe improvement of the H- source.
The H- density in vicinity of the PG is one ofthe most important parameters for theclarification of the particle dynamics. We adopted
Cavity RingDown (CRD) method [4] for themeasurement of absolute H- density in the large
scaled H- source which is used for developmentof NBI on LHD (Large Helical Device) in NIFS
(National Institute for Fusion Science) [5].
In this article, we discuss the H- density
measurement with CRD method in the H- source,
the relation among H- density, extracted H- beamand parameters of ion source plasmas, and theresponse of H- density in the beam extraction.

2. Experimental Setup
Figure 1(a) shows a horizontal cross section of(a)(b)

Fig. 1 (a) Short-side cross section of large-scaled Hsource. (b)Schematic view of the CRD system for Hdensity profile measurement.

the large-scaled H- source. The large-scaled Hsource consists of an arc chamber, PG, extraction
grid (EG) and grounded grid (GG). The size of arc
chamber is 350 mm of horizontal width, 700 mm ofvertical length, and 220 mm of depth. The H- source
has two PGs with round apertures with 12 mm ofdiameter. The typical H- beam energy and current
density are 50 keV and ~100 A/m2, respectively, inthis experiment.
In the CRD system (Fig. 1(b)), from intensity
decay of laser injected to an optical ring-down
cavity by photo detachment reaction, line-integrated
H- density along the laser path is evaluated. The
system consists of Nd-YAG laser (wave length:1064 nm, pulse width: 5 ns, repetition frequency:
50 Hz), high reflective mirrors for the optical cavity,
optical fiber for output light transmission, photodetector, analog digital converter, and dataacquisition system. The lengths of the optical cavity
and effective plasma along the cavity axis are 1570
mm and 180 mm, respectively. A base laser path isthe center of PG apertures and 9 mm from the PG.
To measure density profile, the optical cavity axis isable to move y-z plane by mounting mirrors for
optical cavity on 2D drive unit [6]. A measurable
range is 5 mm to 17 mm from the PG and -22 mm
to 22 mm in y direction from the base laser path.
The photo detachment cross section of H- by thelaser is 3.5x10-21 m2.
3. ResultsFigure 2(a) shows decay signals, so-called
ringdown signals, of the CRD system using baselaser path with and without the ion source plasma.
From these decay times (ringdown times), the Hdensity is evaluated as 3.2x1017 m-3. And the Hdensity is proportional to input arc power andextracted H- beam current measured with water
calorimeter equipped in beam line [5]. This can
indicate that measured density reflects both effectsof H- production and beam extraction.
Figure 2(b) shows time evolutions of the Hdensity, input arc power, and drain current ofacceleration power supply. The H- beam isextracted for 1 sec from 0 sec. The input arc power
enhances during beam extraction by the backstreaming which is a positive charged beam into thearc chamber produced in downstream of the PG.
The H- density drops at the moment of extracting
the H- beam. We found that an amount of thedensity drop depends on a bias voltage applied
between the arc chamber and the PG, and on
distance from the PG. The bias voltage, bycontrolling electron dynamic in the vicinity of thePG, has been mainly used for suppressing(a)Parc ~ 50 kW
with Cs(b)

Fig. 2 (a) Ringdown signals with and without theplasma. The waveform with ringdown times of smallerand larger numbers are with and without the plasma.
(b) Time evolution of H- density (n(H-)), input arc
power (Parc), and drain current of acceleration power
supply (Iacc).

simultaneous extracted electron with H-. However,
it is confirmed that the H- dynamics is affected by
the bias voltage.
Acknowledgments
One of the author (H N) thanks Prof. A. Andoand Prof. M. Sasao for fruitful discussions. This
study was supported by NIFS (NIFS10ULRR008
and NIFS10ULRR702).
References[1] Y. Takeiri, et al., AIP Conf. Proc. 1097, pp. 470-479,
2009.
[2] Ikeda, Y., et al, Fusion Eng. and Des. 82, pp.
791-797, 2007.
[3] Hemsworth R., et al., Nucl. Fusion 49, p. 045006(15pp), 2009.
[4] G. Berden and R. Engeln, Cavity Ring-Down
Spectroscopy: Techniques and Applications (JohnWiley and Sons Ltd., 2009).
[5] H. Nakano, et al., AIP Conf. Proc. 1390, pp. 359-366,
2011.
[6] H. Nakano, et al., 14th International conference onion sources, C28, Giardini Naxos in Italy, September,2011.