22C07
Diagnostics of Negative Hydrogen Ions in a RF Ion Source by Using YAG Laser
高周波負イオン源内における水素負イオンのレーザー計測
Nozomi Tanaka1, Tetsuro Matsuno1, Takuya Funaoi1, Akira Ando1,Yasushi Tauchi2, Katsuyoshi Tsumori3, Yasuhiko Takeiri3
田中のぞみ1, 松野哲郎1, 船生拓矢1, 安藤 晃1, 田内康2, 津守克嘉3, 竹入康彦3
1

School of Engineering, Tohoku University, 6-6-05, Aramaki, Aoba, Sendai 980-8579, Japan1
東北大学大学院 〒980-8579 仙台市青葉区荒巻字青葉6-6-05
2

School of Engineering, Yamaguchi University, 2-16-1 Ube, Yamaguchi 755-8611, Japan2
山口大学工学部 〒755-8611 山口県宇部市常盤台2-16-1
3National Institute for Fusion Science, 322-6, Toki, Gifu 509-5292, Japan
3核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6

Negative hydrogen density measurements are required to clarify the characteristics of negative ion
production. Two laser measurement techniques, one is laser photodetachment and the other is Cavity Ring
Down (CRD) measurement, have been adopted in a RF FET-based ion source. The density ratio ofnegative hydrogen ion to the electron was successfully measured by laser photodetachment and effect ofmagnetic filter field on the negative hydrogen ion production was confirmed. The calculated CRD signal
showed that the designed CRD system is suitable for the H- density measurement in the RF ion source.

1. Introduction
Negative hydrogen or deuterium (H- or D-) ion
sources are required for neutral beam injection(NBI) in next generation fusion devices. The
negative ions are produced near the plasma grid
(P.G.) by volume production process with the helpof a magnetic filter field near the P.G. It is enhanced
in surface production process on a low workfunction P.G. surface with cesium (Cs) seeding. The
evaluation of the density of negative ions near theP.G. is one of the most important ion source plasma
diagnostics. Two laser measurement methods
utilizing photodetach process have been proposed
for negative ion measurements. The additional
electron is detached by photon irradiation with
sufficient energy. Then, a negative ion density canbe evaluated from (i) detached electrons detected byLangmuir probes (laser photodetachment), (ii) theabsorption of the light used in the photodetachment
by using sensitive absorption technique (cavity ringdown (CRD) measurement).
Recently, radio frequency (RF) ion source ischosen as the reference source for ITER NBI. For
high-density and effective plasma production, lower
frequency of around 1 MHz is feasible because oflarge skin depth. We have been developing a FET(field-effect-transistor) based RF H- ion source andsuccessfully produced high-density plasma withlower RF frequencies (0.3-0.5 MHz) [1]. The basic
characteristics of the source were presented in [2].
Efficient production of H- ions in such ion source is

expected and H- measurements are required.
This study aims to apply the laser measurement
techniques to the FET-based RF ion source andevaluate the characteristics of H- density near theP.G.
2. Experimental apparatus
The laser measurement systems were equipped in
the expansion region of the FET based H- ionsource. The expansion chamber has two laser ports
and two probe ports ~40 mm upstream of theplasma grid (See Fig. 1). A set of permanent
magnets that forms a magnetic filter field wasattached ~20 mm upstream of the laser ports.
The principle of laser photodetachment can befound in Ref. [3]. The ratio of H- density nH- to theelectron density ne is proportional to that ofphotodetach signal ΔIes and electron saturation
current Ies.
(1)
A Nd-YAG laser (pulse length ~8 ns, pulse energy
10-50 mJ) was injected into the expansion chamberfrom a fused quarts laser port. The schematics oflaser photodetachment experiment are shown in Fig.
1. An L shaped Langmuir probe was installed fromthe bottom probe port. The photodetach signal wasdetected with a high pass filter. Another Langmuirprobe was installed from the top probe port forelectron density measurement and both probes arevertically movable.

Fig.1. Schematic of laser photodetachment experimentand electrical circuit for signal detection.

A CRD measurement is an advanced laser
absorption techniques [4]. Sensitive laser absorption
measurement can be achieved with multi-passconfiguration using an optical cavity between two
highly reflective mirrors facing to each other. Lineaveraged absolute H- density can be obtained bymeasuring a decay time of the photo signal andcomparing between with and without plasma.
Nd-YAG laser of ~20 mJ in laser energy wasinjected into the optical cavity (mirror reflectivity R> 99.999%). The cavity length d was long enough(~1.3 m) to avoid overlap of the round-tripping
laser light. The CRD signal was measured by aphoto-detector set outside of the optical cavity.

Fig.2. Expansion region pressure dependence of electronand H- densities.

3. Results and Discussions
The measured nH-/ne ratio by the laser
photodetachment increased to ~10% when themagnetic filter field was adopted. Figure 2 showsthe source pressure dependence of ne and nHcalculated by using eq. (1). The nH- increased when
the filter magnet is used showing the effect ofmagnetic filter field on H- production.
In the CRD measurement, the photo signal I(t)gradually decreases according to the followingequation.

Fig.3. Calculated CRD signal.

,(2)

Where c is the speed of light, d is the optical cavity
length, L is the length of the plasma, here defined as
the width of the Gaussian distribution of plasma, σis cross section of photodetach process at laser
wave length of 1064 nm.
Figure 3 shows calculated CRD signal using eq.
(2), with the following parameters: the measured
plasma width L ~0.1 m, mirror reflectivity R =99.999%, H- density obtained from laser
photodetachment n = 1016 m-3, and σ = 3.5 x 10-21
m2 [5]. The decay times in the empty cavity (τ0) andin the cavity filled with plasma (τ) were estimatedto be 433 µs and 321 µs and τ was enough smaller
than τ0. The calculation result shows that thedesigned CRD system is suitable for the H- density
measurement in the FET-based ion source.
4. Summary
Laser photodetachment and CRD measurement
techniques were applied to a FET-based RF
negative ion source. The measured density ratio
was ~10%, and the effect of the magnetic filter field
was confirmed. The CRD system was designed andconstructed, and it was estimated that the H- density
was larger than the lowest detectable density in thedesigned system by calculations. The CRD
measurement is going to be started.
References[1] A. Ando, et al.: Rev. Sci. Instrum 81 02B107 (2010).
[2] A. Ando, et al.: AIP Conf. Proc. 1390, 322-328(2011).

[3] M. Bacal: Rev. Sci. Instrum. 71 (2000) 3981.
[4] E. Quandt, et al.: Europhys Lett. 45 32.
[5] C.F. Barnett, et al.: “Atomic Data for ControlledFusion Research”; Vol. I ORNL-5206 (1977)