22P164
Production Property of Hydrogen Negative Ions in Multiaperture Magnetron Source

多孔マグネトロン方式の水素負イオン生成特性
Kiichi Kobayashi and Wataru Oohara
小林貴一, 大原 渡
Department of Electronic Device Engineering, Yamaguchi University
2-16-1, Tokiwadai, Ube, Yamaguchi 755-8611, Japan
山口大学大学院理工学研究科 〒755-8611 山口県宇部市常盤台2-16-1
A honeycomb-structure negative-ion source with new concept consisting of many coaxial magnetronplasma sources and a negative-ion production section using a plasma-assisted catalytic ionization method isdeveloped. One component of the negative-ion source can produce negative ions even though the production current is not high.

1. Introduction
The research and development of negative-ion
sources have been extensively performed in connection with neutral beam injection (NBI) heating
for fusion-oriented plasmas. A small admixture ofcesium vapor in a hydrogen discharge significantly improves negative-ion production and decreases the current of coextracted electrons [1].
However, the use of cesium complicates the ion
source operation and requires the careful stabilization of cesium injection and discharge parameters. There have been many attempts to develop
negative-ion sources with acceptable negative-ion
beam emittance but without a cesium admixture.

Fig. 1. Cross-sectional position relation among coaxial magnetron sources, apertures, and porous
cylinders.

We have proposed a plasma-assisted catalytic
ionization method for production of H+ and H− ions[2,3]. Hydrogen positive ions produced by discharge are irradiated to a porous catalyst, positive
and negative ions are produced from the back of theirradiation plane. In this work, the porous catalyst iscylindrical, not tabular. A tabular porous plate issuitable for fundamental investigation of the plasma-assisted catalytic ionization [3]. Backstreaming
positive ions, accelerated in the opposite direction,enter the negative-ion sources for the NBI systems.
If the tabular porous plate is used for negative-ion
production, the backstreaming strike and cause
damage to the porous plate. Therefore, the porous
catalyst should be cylindrical so that the backstreaming can pass through it. A honeycomb structure plasma tied many small-volume plasmas inbunch is used for large area irradiation. A small
volume plasma is produced for irradiation underconditions of filament free.

Fig. 2. Schematic diagram of experimental apparatus.
The negative ion source consists of a magnetronsource and an ion production section.

2. Experimental Apparatus
The negative ion source consists of double-stepprocess, a magnetron source and an ion production
section. The magnetron source is a honeycomb
structure of cylindrical anodes of 3.6 cm diameterand coaxial cylindrical cathodes of 1.5 cm diameter.
The ion production section consisting of many porous cylinders is connected with the magnetron3. Results
A hydrogen plasma generated by magnetron discharge is supplied through four apertures to aroundthe porous cylinder grounded. Positive and negativeions are produced inside the porous cylinder. The
ion current extracted Iex is measured using the extraction electrode and the collector biased at Vex,
and the current (Iex)-voltage (Vex) characteristics areshown in Fig. 3, where the discharge voltage Vd ischanged, the discharge current Id (the discharge
power Pd) is maximized by adjusting the magnetic
field B. The positive current is much higher than thenegative current, indicating the property of ionicplasmas. The currents increase with Vd (Pd), corresponding the irradiation current of positive ions. In
the other works relevant to the plasma-assisted catalytic ionization, the positive current becomes
higher than the negative current in the case of using
a porous plate and the positive-ion irradiation in adirection perpendicular to the porous surface. Thentransmitted positive ions through the porous plate

are detected. Therefore, positive ions irradiated appear to be partially transmitted through the porouscylinder.
12Vd (Pd)Iex (μΑ)

83 kV (150 W)

42 kV (76 W)

0 1 kV (14 W)−200

0Vex (V)

200Fig. 3. Current-voltage characteristics of extraction electrode and collector.

The electric field drift of the guiding center inmagnitude E/B is independent of charge, mass, andvelocity of electrons. Electrons gain energy fromthe electric field and produce positive ions in themagnetron discharge. Dependences of positive- andnegative-saturation currents of Iex, I+ and I−, on E/B
normalized are shown in Fig. 4, where I+ and I− areobtained at Vex = −200 V and +200 V, respectively.
Id (Pd) is maximized by adjusting B in the same asFig. 3. I+ and I− increase with E/B, depending on theproperty of magnetron discharge.
One component of the honeycomb-structure negative-ion source can produce negative ions eventhrough the produced current is not high. It is necessary for the future that the discharge section isimproved and the irradiation current to the porous
cylinder is increased more.

6I+, I− (μΑ)

source through apertures in a separate plate. A
cross-sectional position relation among the coaxial
magnetron sources, the apertures, and the porous
cylinders is shown in Fig. 1. Hydrogen plasma generated by magnetron discharge is supplied throughfour apertures of 0.45 cm diameter to around theporous cylinder. Positive and negative ions areproduced inside the porous cylinder by the plasma-assisted catalytic ionization process. The porous
cylinders of 1.5 cm diameter are positioned at center-to-center spacing of 1.8 cm.
In this work, production property of negativeions for a set of the coaxial magnetron source andthe porous cylinder is investigated, because the total
current of negative ions increases proportionally tonumber of the set. Eight dummy cylinders of 1.5 cm
diameter made of SUS304 are set around the porous
cylinder as alternatives to the other porous cylinders.
The experimental apparatus is illustrated in Fig. 2.
Currents of positive and negative ions extracted by
an extraction electrode of 1.5 cm inner diameter aremeasured using a collector at z = 6.5 cm separated
0.5 cm from the extraction electrode. The extraction
electrode and the collector are biased at Vex. The
anode is positively biased at Vd and the other electrodes are grounded. The porous cylinder of 3 cm
length is made of a porous nickel plate rolled with aporous body of a pore size of 0.45 mm, a thicknessof 1.4 mm, a specific surface area of 5,800 m2/m3,
and a porosity of 96.6 %. The hydrogen pressure in
the source during operation is about 0.2 Pa. A uniform magnetic field is applied to the apparatus bysolenoid coils.

I+42I−01.2E/B (normalized)

Fig. 4. Dependences of the positive- and negative- saturation currents of Iex on E/B normalized.

References[1] K. N. Leung, K. W. Ehlers, Rev. Sci. Instrum.
53, 803 (1980).
[2] W. Oohara, O. Fukumasa, Rev. Sci. Instrum. 81,023507 (2010).
[3] W. Oohara, T. Maeda, T. Higuchi, Rev. Sci. Instrum. 82, 093503 (2011).