22P158-P
Measurement of electron density at extraction region in negative ion source bymeans of surface wave probe
表面波プローブを用いた負イオン源引き出し領域における電子密度計測
Masashi Kisaki, Katsuyoshi Tsumori, Haruhisa Nakano, Katsunori Ikeda, Masaki Osakabe, Kenichi
Nagaoka, Masayuki Shibuya, Yasuhiko Takeiri and Osamu Kaneko
木崎雅志, 津守克嘉，中野治久，池田勝則、長壁正樹、永岡賢一、渋谷真之、竹入康彦、
金子修
National Institute for Fusion Science322-6, Oroshi-cho 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
Surface wave probe was adopted for measuring the electron density near a plasma grid in alarge-scaled negative ion source and it was confirmed that the surface wave probe is highly available
for diagnostic of magnetized plasmas. In addition, it was found that applying the bias voltage reduces
the electron density while the H- ion density is almost constant and H- ions become the dominantnegative particles at high bias voltage.

1. Introduction
Recently, a cavity-ring down method [1] and amillimeter-wave interferometer [2] have beenutilized for measuring the H- density and theelectron density near a plasma grid in H- ionsources, respectively. The H- density and theelectron density have been successfully determined
during the beam extraction. However, the obtained
electron density with the interferometer isline-averaged and the local electron behavior,especially near the extraction hole, has not beenclarified. One of the simplest methods to determine
the local electron density is a Langmuir probe (LP),but it is quite difficult to determine the electron
density precisely by the LP due to the magnetic
field in H- ion sources.
A surface wave probe (SWP) is one of themost potential candidates for measuring theelectron density at an extraction region. The SWP iswidely used for diagnostic of reactive plasmas
where the LP is covered with an insulator layer [3].
The SWP is relatively simple, consisting of adielectric tube and a co-axial cable only. In addition,
the electron density can be obtained with highspatial resolution. As can be seen, the SWP has
many advantages for the electron densitymeasurement. But, the SWP has not been adopted
for studies of the magnetized plasmas.
To confirm the availability of the SWP for thestudy of H- ion sources, the SWP has been installed
into a 1/3-size negative ion source for LHD-NBI
and measurements of the electron density near theplasma grid were carried out.

experimental setup. In H- ion sources, filter magnets
are installed to form the transverse magnetic field
which produces the low electron temperature region
near the PG and electron deflection magnets areembedded in an extraction grid (EXG) to suppress
the acceleration of the co-extracted electrons. As
shown in Fig. 1(b), the magnetized direction of theelectron deflection magnet changes alternately. TheSWP is set over the aperture of a plasma grid (PG).
Distance between the central axis of the SWP andthe PG is 10 mm. Fig. 1(c) shows a cross-sectionalview of the SWP. The SWP consists of an alumina
tube with a diameter of 6 mm and a semi-rigid
cable whose inner conductor with a length of 8 mm
is exposed at the end. Radiofrequency power isapplied to the semi-rigid cable at the frequency of
50 MHz to 1.5 GHz and the ratio of the reflected
power Pref to the applied power Pin is detected by anetwork
analyzer.
When
theincident
radiofrequency power satisfies the resonantcondition, an electrostatic wave is excited at theinterface between the alumina tube and the plasma
at the region where the inner conductor of theco-axial cable is exposed and the incident power isstrongly absorbed.
3. Experimental results
The Typical absorption spectra (log10Pref/Pin) of
the applied radiofrequency power are shown infigure 2. The absorption signals are observed andthe resonant frequency increases with the discharge
power. The electron density ne is obtained from thefollowing equation.
⎛ ϖ ⎞2n e = 1.2 × ⎜ ⎟ × 1010 [cm-3],⎝ F ⎠

2. Experimental setup
Fig. 1 shows a schematic illustration of the€Fig. 3. Dependence of electron density measured
by the SWP and the LP on arc power (a) with and(b) without magnetic field.
Fig. 1. (a) Schematic illustration of experimental
setup, (b) positional relation of SWP with PG, (c)cross-sectional view of SWP.

where ω is the resonant frequency and F is aconstant derived from the dispersion relation anddepends on the geometrical configuration of theprobe.
To verify the validity of the SWP for H- ionsources, the electron density was determined by theSWP and the LP at the pure-volume production.
The dependences of the electron density on the arc
power with and without magnetic field are shown in
Fig. 3(a), (b), respectively. In the case of nomagnetic field, the electron density measurement
was carried out using a small test chamber [4]. In
Fig. 3(a), the obtained electron densities by theSWP and the LP increase monotonically as afunction of the arc power and correspond to eachother. On the other hand, applying the magnetic
field the variation of the electron density observedwith the SWP has the same tendency as that with
the LP, but the obtained electron density by the LP
is less than that by the SWP. This results from thestrong magnetic field formed in the negative ion
source. The electron cyclotron motion keeps theelectrons from flowing into the probe tip andresulting in underestimation of the electron density.
These results confirm that the SWP is highly
available for the study of H- ion sources.
To investigate the effect of the bias voltage on
the electrons near the PG, the electron density wasmeasured at various bias voltages. Fig. 4 shows thevariation of the electron density and the H- ion
density as a function of the bias voltage. Applying
the bias voltage reduces the electron density while
the H- ion density is almost constant with the bias
voltage and H- ions become the dominant negative
particles at the bias voltage of more than 6 V.

Fig. 4. Dependence of electron density and Hion density on bias voltage.

4. Conclusion
In this study, it was confirmed that the surface
wave probe is highly available for the study of theH- ion sources by comparison of the electron
densities obtained with the surface wave probe andthe Langmuir probe.
The electron density was measured withvarying conditions for the plasma production. As aresult, applying the bias voltage reduced theelectron density while the H- ion density was almost
constant with the bias voltage and the H- ionsreplace the electrons as dominant negative particles
at high bias voltage.
AcknowledgmentsAuthors would like to thank for technical staff ofLHD-NBI group for excellent operation of NBI teststand.
References[1] Haruhisa Nakano et al, AIP Conf. Proc. 1390, 359(2011).
[2] K. Nagaoka et al, AIP Conf. Proc. 1390, 374 (2011).
[3] Hikaru KOKURA et al, Japan J. Appl. Phys. 38,5262 (1999).
[4] M. Nishiura et al, Rev. Sci. Instrum., 69, 974 (1998).

Fig. 2. Typical absorption spectra.