22P163
Effect of Catalyst Materials on Plasma-Assisted Catalytic Ionization

プラズマ支援触媒イオン化機構における触媒材質効果
Tokuaki Hibino, Tuyoshi Higuchi and Wataru Oohara
日比野徳亮，樋口剛史，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
Production properties of positive and negative ions by a plasma-assisted catalytic ionization method areinvestigated using a porous nickel plate. Kinetic energy distributions of positive and negative ions emitted
from the plate surface are obtained using the energy difference among the ions. Positive current is higher
than negative current. The positive current is found to consist of the positive-ion currents of transmitted
through the porous plate and produced on the plate.

1. Introduction
The research and development of negative-ion
sources have been extensively performed in connection with neutral beam injection heating for fusion-oriented plasmas and ion guns for proton accelerator. A small admixture of cesium vapor in ahydrogen discharge significantly improves negative-ion production and decreases the current ofcoextracted 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 without acesium admixture.
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, the positive and negative ions are produced from the backof the irradiation plane. In this work, dependencesof production quantities of the ions on the irradiation energy and current are investigated attracting
attention to energy distributions of the ions.

charge between filament cathodes and a wall anodein a cuboidal chamber with a cross section of 25 cm
×25 cm, i.e., a bucket plasma source. The cathodes
are four horseshoe tungsten filaments of 0.7 mm
diameter and 15 cm length, which are biased at adischarge voltage of Vd = −70 V, at which the plasma density is maximized. The plasma generated ina field-free region is surrounded by line-cusp magnetic fields near the grounded chamber wall. Figure
1 shows a schematic diagram of the experimental
setup. A commercially available porous nickel platewith a porous body of a pore size of 0.45 mm, athickness of 1.4 mm, a specific surface area of
5,800 m2/m3, and a porosity of 96.6 % is used as acatalyst. The porous plate is negatively biased at adc voltage of Vpc. The positive ions are irradiatedand the irradiation current Iir is measured. Since thepositive ions are irradiation area is 12.6 cm2 (diameter of 4 cm) and the other electrode is covered witha mica plate, the irradiation current density Jir canbe obtained.

2Jex (mA/cm )2Vpc–500 V–400 V–200 V

10–800–400

0Vex (V)400

Fig. 2. Current-voltage characteristics of ion analyzer.

Fig. 1. Schematic diagram of experimental setup.

2. Experimental Setup
A hydrogen plasma is generated by a dc arc dis-The porous plate is located at z = 0 cm, the discharge section corresponds to the region z < 0 cm.
Plasma parameters in the region are measured usinga Langmuir probe at z = −3 cm. The hydrogenpressure in the source during operation is about 0.1

(a)

(b)

Pd =700 W

Vpc = –600 V

PdVpc

700 W

–600 V

600 W

f(K)

f(K )

–500 V–400 V

500 W
400 W

–300 V

300 W–200 V

200 W
100 W

–100 V0200

400

6000e(Vex–Vpc) (eV)

200 400 600e(Vex–Vpc) (eV)

20–10J+pc

10J–pc200400e(φs−Vpc) (V)1600

Vpc = –600 V

32(b)J+t (mA/cm )32

Pd = 700 W

J+tJ+tJ+pc, J–pc (mA/cm )

(a)

22

1J+t (mA/cm )Fig. 3. Dependences of kinetic-energy distributions of
the ions on (a) the irradiation energy (Vpc) and(b) the irradiation current (Pd).

2

3. Results
Plasma potential φs in the discharge section remains approximately constant at +4 V, independently of Vpc. Positive ions produced by discharge are irradiated to the porous plate which isnegatively biased at Vpc. They are accelerated up toe(φs − Vpc) (eV) in the sheath in front of the porous
plate. Positive and negative ions are produced by aplasma-assisted catalytic ionization from the backof the irradiation plane. The irradiation current density of positive ions Jir can be varied by adjusting
the discharge power Pd because the plasma density
in the discharge section depends on the power.
Since the extraction aperture area is 1.3 cm2 (innerdiameter of 1.3cm), the ion current density extracted Jex can be obtained using the ion analyzer. The
current density (Jex)- voltage (Vex) characteristics ofthe analyzer are shown in Fig. 2. The positive current is much higher than the negative current. Thecharacteristics depending on Vpc have precisely twoinflection points. One is at Vex ∼ Vpc and the other isat Vex ∼ 0 V. There are two components of the ionsin the kinetic energy distributions, taking the potential profiles into consideration. The kinetic energy
distributions are calculated by differential from theJex- Vex characteristics, as shown in Fig.3. The reference potential at z > 0 cm is Vpc because all ionsare emitted from the plate surface. The energy distributions in the cases of the constant irradiation
current (Pd = 700 W) and the constant irradiationenergy (Vpc = −600 V) are shown in Figs. 3(a) and(b), respectively. The kinetic energy positive andnegative ions produced by desorption ionization
appears to be less than several eV, and the former
component at e(Vex − Vpc) ∼ 0 eV will consist ofthem. On the other hand, the latter component ate(Vex − Vpc) ∼ −eVpc (eV) consists of the transmitted
positive ions which are a part of the irradiated positive ions. The transmitted quantity of positive ionsis about independent of the irradiation energy (Fig.
3(a)), but the quantity increases proportionally with
the irradiation current (Fig. 3(b)).
The produced current densities of positive andnegative ions, J+pc and J−pc, and the transmitted current density of positive ions J+t are separatelymeasured. Fig. 4 shows the dependences of J+t, J+pc,and J−pc on (a) the irradiation energy e(φs − Vpc)
(eV) and (b) the irradiation current density Jir. J+pcand J−pc increase both with the irradiation energy
under the constant irradiation current and with theirradiation current under the constant irradiationenergy. J+t is independent of the irradiation energy.
The production quantity of negative ions is greaterthan that of positive ions. Electronegativity difference between hydrogen atom and nickel atom is
considered on desorption ionization in the plasma-assisted catalytic ionization process. The electronegativities of H and Ni are 2.20 and 1.91 in
Pauling units, respectively. H tends to attract electron from Ni in desorption of H. Therefore, negativeions will be produced easily. It is found that thehigher current of positive ions is caused by the superimposed current of the transmitted positive ions.

J+pc, J–pc (mA/cm )Pa. Current densities of positive and negative ionsfrom the back of the irradiation plane are measured
using an ion analyzer, some type of Faraday cup.
The analyzer consists of a grid (SUS316 coatedwith a film of Cu, 635 mesh, wire diameter of 0.02mm) and a collector separated 1 mm from the gridat z = 0.8 cm.

1J+pc
0–100J–pc

5102Jir (mA/cm )15Fig. 4. Dependence of currents of the produced positive
and negative ions and the transmitted positive
ions on (a) the irradiation energy and (b) the irradiation current density.

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)．