22C05
Ion-Ion Plasma in a Large-Scaled Hydrogen Negative Ion Source
大型水素負イオン源内のイオン性プラズマ	 Katsuyoshi Tsumori, Haruhisa Nakano, Masashi Kisaki, Ken-ichi Nagaoka, Katsunori Ikeda,Masaki Osakabe, Yasuhiko Takeiri, Osamu Kaneko, Masayuki Shibuya, Eiji Asanoand NIFS NBI group
津守克嘉, 中野治久，木埼雅志，永岡賢一，池田勝則，長壁正樹，竹入康彦，金子	  修，渋谷真之，浅野英児，NIFS-NBIグループ
National Institute for Fusion Science322-6, Oroshi Toki Gifu 509-5292, Japan
核融合科学研究所 〒509-5292	  岐阜県土岐市下石町322-6
Ion-ion plasmas including quite low electron density are observed in a caesium (Cs) seeded negativehydrogen ion (H-) source. The voltage-current curve (V-I curve) monitored with Langmuir single probe
shows quite symmetric with respect to the inflection point of the curve. The symmetric feature breaks
during beam extraction, and electron density increases. That change indicates the negative charge densityof extracted H- is compensated with electron density. The spatial distribution of the changes suggests theextraction field affects deep region of ion-ion plasma.

1. Introduction
Negative-ion-based neutral beam injectors(NBI) have contributed to improve the performances of recent plasma confinement devices in
the nuclear fusion research [1,2]. To enhance theH- current, small amount of Cs is seeded in theplasma chamber of the negative ion source.
Although intensive R&D have been carried out toincrease the H- current in some laboratories, there
are some unclear parts in the mechanisms of theproduction and extraction of the H- ions. In order
to investigate the mechanism, we started themeasurement of the particle dynamics in thebeam extraction region using a diagnostic system
combining with a Langmuir single probe, a CRD
system, a millimeter-wave interferometer (MWI),and optical emission spectroscopy [3-6]. The
experimental setup, the results on the ion-ion
plasmas, the response of the H- and electrons
with and without applying the beam extraction
field, and on the spatial distribution of thesaturation currents and their changes during beam
extraction in the following sections.
2. Experimental Setup
A Cs seeded negative ion source is applied forthis experiment, and the source is a multi-cusp
one characterized with a pair of filter magnets asshown in Fig. 2. The source plasma is generated
with filament-arc discharge, and the primaryelectrons decrease their energies via diffusion in
the filter magnetic field. Near the plasma grid
(PG), diffusing electrons are trapped with anotherstrong magnetic field of ~ 10 mT induced by anarray of the permanent magnets embedded in theFig.1. Negative ion source and diagnostic configuration

second electrode called extraction grid (EG). An
insulator spacer is inserted to apply positive bias
potential to the PG with respect to the sourcechamber. The spacer is called bias insulator, anddiagnostic ports are installed through theinsulator. The Langmuir probe and the CRD unit
are time resolved system and the systems are able
to scan in the horizontal and vertical directions tothe PG plane.
3. Experimental Results
3.1 Positive-Ion and Negative-Ion Plasmas
The electron saturation current (ESC)
measured with the Langmuir probe is much largerthan the ion saturation one in a pure hydrogen
discharge. The ESC drastically decreases after Cs
seeding, and quite symmetric voltage-current(V-I) curve is observed as shown in Fig. 2. The
ion-ion plasmas are reproducible in relatively
high filling gas pressure and bias voltage. Theelectron density is crossly checked with the MWI[3], and the electron density can be quite small of
the order ~10-2 to that of H -.

3.3 Spatial Distribution of Electron Increments
As the next step, the partial distribution of theincrements of the compensating electron isinvestigated. Figure 4 shows the distribution of theFig. 4. Spatial distributions of the increments ofnegative ( ) and positive ( ) probe signals with andwithout beam extraction.

Fig.2. Ion-ion plasma observed in a negative ion source

3.2 Response of H- and electron with and withoutBeam extraction.
With and without beam extraction, the densitychanges of the charged particles in the ion-ion
plasma have been compared. The Langmuir probe
and CRD measurement, which can estimate theabsolute H- density [4], are applied for themonitoring. As shown in Fig. 3, the probe signal

Fig. 3. Waveforms of (a) negative probe current and (b)H- density with and without beam extraction.

jumps up (a) during beam extraction, and the Hdensity steps down (b) at the same time. There is no
negative charge except for H- and electron, so thatthe jumping-up of the probe signal is interpreted toindicate the electron charge compensating thepositive ion background remaining after Hextraction.

increments of the positive (ion) and negative(electron) probe signals with and without extraction.
The z direction is set in the direction separatingfrom the PG surface. The distribution suggests theextraction field affects even deep region of theion-ion plasma.
Acknowledgments
The authors would like to express their acknowledgements to the Director Generals Prof. Dr. A. Komori, forcontinuous support and encouragement of the development and the operation of our neutral beam injectors.
They also would like to express their sincere
acknowledgement to the LHD experimental group forexcellent execution of the LHD experiments. This work
is supported by National Institute for Fusion Sciencegrant administrative cord, NIFS-ULRR702.
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