22P150-P
Development of a 14 GHz microwave driven negative hydrogen ion source
14 GHzマイクロ波駆動水素負イオン源の開発
Tomoaki Nishida1, Toshiro Kasuya1, Syuichi Maeno2, Masaki Nishiura3,Katsuhiro Shinto4, Motoi Wada1
西田睦聡1，粕谷俊郎1，前野修一2，西浦正樹3，神藤勝啓4，和田 元1
1 Graduate School of Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan
2 Novelion Systems Co. Ltd , Kyotanabe, Kyoto 610-0394, Japan
3 National Institute for Fusion Science, Toki, Gifu, 509-5292, Japan
4 Japan Atomic Energy Agency, Rokkasho, Aomori,039-3212, Japan
1 同志社大学大学院工学研究科 〒610-0321 京都府京田辺市多々羅都谷1-3
2 ノベリオンシステムズ株式会社 〒610-0394 京都府京田辺市興戸地蔵谷1番地
3 核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
4 日本原子力研究開発機構 〒039-3212 青森県上北郡六ヶ所村大字尾駮字表舘2-166
Development of a negative hydrogen ion (H-) source for neutral beam injection (NBI) heating of future nuclear fusion
device requires operation with RF power for simplification of the maintenance. We have been developing a compact
H- source driven by 14 GHz microwave power coupled to an elecroton cyclotron resonance (ECR) magnetic field.
Preliminary beam extraction experiments had revealed beam deflection due to strong magnetic field at the extractor, andthe modification to reduce magnetic field intensity at the extractor proved better extraction efficiency of the ion beam.
A design to further reduce the magnetic field intensity at the extractor is proposed.

1. Introduction
High frequency microwaves can excite plasma
with good efficiency as they are coupled tomagnetic field intensity strong enough to realize
electron cyclotron resonance (ECR) condition.
However, magnetic field structure of an ECR ion
source often becomes very complicated as the fieldstructure have to match with efficient extraction ofions from produced plasma. Thus the size of theion source together with the weight often becomeslarge.
Reduction in microwave wavelength
requires increased intensity of the ECR magnetic
field, and we have chosen 14 GHz as themicrowave frequency that gives a proper
combination between the magnetic field intensity,and the wavelength of the electromagnetic
radiation.

mean energy of electrons in the plasma. Thisgeometry can effectively produce H- throughelectron volume process. However, the strong
magnetic field intensity near the ECR zone should
bend the H- ions during beam extraction. In orderto mitigate the beam deflection due to the field, ironmagnetic shielding structure has been employed in
the beam extraction structure.
The adverse effectdue to transverse magnetic field has been improved,
and the ion source produces beam of negativehydrogen ions (H-) with reasonable efficiency of 10
A/cm2-W.
Iron yoke

Ne-Fe permanent Magnet
Extraction electrodes

Gas Inlet

2. Dipole field configuration
Figure 1 shows the structure of the ion source thathas a pair of permanent magnets forming enoughintensity to realize ECR condition at 14 GHz
microwave frequency. This ion source has thedimensions of 14 cm X 14 cm X 14 cm, and themicrowave excites a hydrogen plasma in a 2 cm
diameter 9 cm long alumina discharge chamber.
The direction of the magnetic field to realize ECR
condition is perpendicular to the beam extraction
axis, and the region between the ECR zone and theextractor can work as a magnetic filter to reduce theMicrowave window

Alumina discharge chamber

Iron magnetic shield

FIG.1.Schematic of double pole structure microwave ionsource.

3. New concept magnetic field design
To further increase the H- extraction efficiency,
the ion source is being modified to form ECR fieldwith a single permanent magnet. The ion source
design is shown in Fig. 2. About 5cm diameter 4
cm long Nd-Fe magnet is attached to a cylindricaliron bases, which forms a closed magnetic field
circuit realized by iron yokes. A 56 mm outside
diameter 44 mm high alumina cup serves as adischarge chamber.
Figure 3 shows the calculated magnetic flux
intensity distribution near the ECR zone on acentral axis of the ion source. The field intensity
distribution
has
been
calculatedbya
general-purpose three-dimensional magnetic field
analysis program TRI-Comp®. The zero point on
the abscissa indicates the position of the plasma
electrode of the extractor. This diagram describesthat the ECR point is formed at 13 mm from theextraction hole.
Figure 4 shows the magnetic flux density
distribution near the ECR zone along themicrowave propagation axis. The figure showsthat the ECR point is formed at about 15 mm fromthe axis of the ion source. This size is far muchlarger than the size of the extraction hole of 4 mmdiameter.
Microwave power is launched into the dischargechamber from a rectangular waveguide thatintersects the axis of the ion source at the end of thedischarge chamber. A plunger structure to control
microwave field intensity distribution around thedischarge chamber has been formed at the opposite
side of the power introducing waveguide across thedischarge chamber.
Iron yoke
Ne-Fe permanent Magnet

Insulator

FIG.3. Simulation result of Magnetic flux density
distribution on axis direction inside of ECR chamber.

FIG.4. Simulation result of Magnetic flux density in aradial direction inside of ECR chamber.

4. Performance test
The source will be put into operation as H- source.
The magnetic field intensity of the dipole structure
ion source could have been too strong to produce Hions efficiently, and the beam extraction currents
were nearly the same for positive and negative ionextractions. This may be attributable to smaller
electron drain current, but it also suggests thesmaller positive ion extraction efficiency. This
will be confirmed with the extractor structure withnegligible intensity of the transverse magnetic field.
Acknowledgments
This work is performed with the support andunder the auspices of the NIFS Collaboration
Research program (NIFS10KOAR011).

Microwave window
Alumina discharge chamber Extraction electrodes

FIG.2.Schematic of single pole structure microwave ion
source.

References[1] M. Wada, Y. Demura, T. Kasuya, T. Kenmotsu, S.
Maeno, T. Nishida, M. Nishiura, K. Shintoh, H.
Yamaoka, Plasma Fusion Res. 6, 2406100 (2011).