29E27P
Comparison of plasma parameters of with and without caesium
seeding in beam extraction region of a negative ion source for NBI
K. Tsumori, H. Nakano, M. Kisaki, K. Ikeda, M. Osakabe, Y. Takeiri, O. Kaneko,
M. Shibuya, E. Asano, T. Kondo, M. Sato, A. Sekiguchi and S. Komada
National Institute for Fusion Science, 322-6 Oroshi Toki Gifu 509-5292, Japane-mail: tsumori@nifs.ac.jp

Production and transport mechanism of hydrogen negative ion (H-) in caesium (Cs) seeded
negative ion source for NBI is one of the important issues to improve the H- production efficiency,
which leads the stability of high power and long pulse beam injection.
We have investigated the plasma in beam extraction region using multi-diagnostic system toimprove the negative-ion-based NBI system for LHD. Ionic like plasmas including quite low
electron densities have been produced in the extraction region by optimizing the Cs condition in a
R&D negative ion source. In those plasmas, voltage-current curve obtained with single Langmuirprobe shows symmetric characteristics, and the symmetry breaks by applying electrostatic field forbeam extraction. This symmetry breaking is interpreted due to compensation of extracted Hcharge with electrons coming from driver region and is observed in long range of more than 20 mm
at the extraction region by measuring Langmuir probe and cavity ring down [1, 2]. Change ofplasma parameters before and after applying the extraction field has not been analyzed in purehydrogen plasmas in our ion source so far. In this presentation, we compare the plasma
parameters to applied external field in pure hydrogen and Cs seeded discharges.
Electron and ion saturation currents and H- density as functions of bias voltage is shown in Fig.
1, and the arc power and H2 gas pressure are 30 ± 2 kW and 1.3 Pa, respectively. The electron
saturation current and H- density are of quite similar dependence as extraction and acceleration
currents. Waveforms of electron and ion saturation currents are indicated in Fig. 1A and 1B,
which correspond to the cases with the bias voltages of 2 V and 8 V, respectively. Beam
extraction field is applied from the time scale of 0 to 1 sec, and the jump-up feature of electron
saturation current observed in Cs seeded discharges does not appears in those hydrogen discharges
with relatively high gas pressure of 1.3 Pa. Negative charge compensation with electrons in Cs
seeded plasmas is not observed in the hydrogen plasma. This indicates the response of ionic-likeplasmas to electrostatic field is different from usual electron-ion plasma. Furthermore, electron
density plays an important role for electrostatic shielding in ionic-like plasmas. Anothernoticeable feature is that electron density in the plasma decreases very low in high H2 pressure andhigh bias voltage case. We will present about the response to gas pressure and electron polar
distribution in this presentation, additionally.
[1] K. Tsumori et al, Rev. Sci. Instrum, 83, 02B116-1 - 5 (2012)[2] H. Nakano et al, AIP Conf. Proc. 1390, 2010, pp. 359-366

10010010-1

10-1n(H -) [x10 16 m -3]
Ies(A) [mA]
Iis(A) [mA]

10-2-20246Vbias [V]81010-2
1243Beam• Bias Voltage: 2 V extraction• Arc power: 30 kW• H2 pressure: 1.3 Pa• without Cs4A3221

10-2.0-1.00.0
time [s]

1.0

0.20

02.0

Ies and Iis [mA]101

Ies and Iis [mA]

Ies and Iis [mA]B

H- density [x1016 m-3]

A1010.20

Beam• Bias Voltage: 8 V
extraction• Arc power: 30 kW• H2 pressure: 1.3 Pa
0.15 • without Cs

B
0.15

0.10

0.10

0.05

0.05

0.00
-2-1Fig. 1. H- density, ion and electron saturation currents with respect to bias voltage.
voltages of 2 V and 8 V are shown on right hand side.

0time [s]120.00The waveforms at bias