22P162
Development of Beam Passing Hydrogen Negative-Ion Source by Plasma-Assisted Catalytic Ionization Method

プラズマ支援触媒イオン化法を用いたビーム通過型水素負イオン源の開発
Tsuyoshi Higuchi, Tokuaki Hibino and Wataru Oohara
樋口剛史, 日比野徳亮，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 cylinder. The production quantity of negative ions increases with irradiation energy and current of positive ions in a discharge plasma, which has two peaks at least and the irradiation energy of the peak shifts to high energy due to increasing the irradiation current.

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 thecurrent of coextracted electrons [1]. However, theuse 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 a cesium 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, the porous
catalyst is cylindrical, not tabular. Backstreaming
positive ions, accelerated in the opposite direction,enter the negative-ion sources for NBI. If the tabular porous plate is used for negative-ion production,
the backstreaming strike and cause damage to theporous plate. Therefore, the porous catalyst should
be cylindrical so that the backstreaming can passthrough it. At this stage, the production properties
are paid attention in the case using a capped porous
cylinder for eliminating the passing electrons andpositive ions through inside the porous cylinder.
2. Experimental Setup
A hydrogen plasma is generated by a dc arc discharge 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. The plasma issurrounded by line-cusp magnetic fields near thegrounded chamber wall. Figure 1 shows a schematic diagram of the experimental setup.
A porous cylinder of 4 cm length is made of aporous nickel plate rolled with a porous body of apore size of 0.45 mm, a thickness of 1.4 mm, a specific surface area of 5,800 m2/m3, and a porosity of
96.6 %. The porous cylinder is negatively biased ata dc voltage of Vpc. The positive ions are irradiatedand the irradiation current Iir is measured. An extraction aperture, an exit of the porous cylinder, islocated at z = 0 cm. Plasma parameters in the discharge section are measured using a Langmuirprobe at z = −12 cm. The hydrogen pressure in thesource during operation is about 0.1 Pa. Current
densities of positive and negative ions from the porous cylinder are measured using an ion analyzer,
some type of Faraday cup. The analyzer consists ofa grid (SUS316 coated with a film of Cu, 635 mesh,wire diameter of 0.02 mm) and a collector separated
1 mm from the grid at z = 1 cm.

Fig. l. Schematic diagram of experimental setup.

3. Results
Positive ions produced by discharge are irradiated to the porous cylinder which is negatively biasedat Vpc. Positive and negative ions are produced by aplasma-assisted catalytic ionization inside the porous cylinder. The ion current extracted Iex is measured using the ion analyzer. Since the extraction
aperture area is 1.3 cm2 (inner diameter of 1.3 cm),the ion current density extracted Jex can be obtained.
The current density (Jex) - voltage (Vex) characteristics of the analyzer are shown in Fig. 2. The negative current is mach higher than the positive current,
contrasting results to in the case of using a porous
plate. The characteristics depending on Vpc have
two inflection points. One is at Vex ~ Vpc and theother is at Vex ~ 0 V. There are two components of
the ions in the kinetic energy distributions, taking
the potential profiles into consideration. The kinetic
energy of positive and negative ions produced bydesorption ionization appears to be less than several
eV, and the former component at Vex ~ Vpc will consist of them. On the other hand, the latter component at40Vpc

2Jex (μΑ/cm )200–200 V–400 V–600 V–20–40–800–400
Vex (V)

02Fig. 2. Current density-voltage characteristics of ion analyzer

catalyst. Jex of Vex < Vpc is a sum current density ofthe transmitted and produced positive ions. Thecurrent density of transmitted positive ions J+t isobtained from current difference at Vex = 0 V and−25 V. The current density of produced positiveions J+pc is obtained from current difference between Jex of Vex = Vpc − 100 V and J+t, taking thesymmetry to the current density of negative ions
into consideration. The dependences of J−pc, J+t, andJ+pc on the irradiation energy e(φs − Vpc) (eV) and Pdcorresponding to the irradiation current of positive
ions are shown in Figs. 3, 4 (a), and 4 (b), respectively, where the plasma potential φs in the discharge section remains approximately constant at+20 V. J−pc, the production quantity of negative ions,tends to increase with the irradiation energy and theirradiation current (Pd) in Fig. 3. There are at least
two peaks at a given irradiation current. The peakenergies are found to increase with the irradiation
current. The peaks are a distinctive production
property of negative ions. Both J+t and J+pc increase
with the irradiation energy and current in Figs. 4(a)and (b). The transmitted quantity of positive ions,J+t, is found to be less than the production quantity,
J+pc, in contrast to in the case using the porous plate.
Since the irradiation flux of positive ions in a direction perpendicular to the porous surface decreases
and the flux in an oblique direction increases, thethickness of the porous cylinder is equivalently increased and the transmitted quantity will decrease.
(a)

68 μA/cm

Pd(b)1515350 W

Pd300 W300 W

200 W

22Pd

J+pc (μΑ/cm )J+t (μΑ/cm )2J–pc (μA/cm )1010250 W

300 W
5200 W

100 W
5150 W

200 W
100 W

200

400 600 800e(φs − Vpc) (eV)

100 W
0200

400 600 800e(φs − Vpc) (eV)0200 400 600 800e(φs − Vpc) (eV)Fig. 3. Current density of negative ions produced depending on the irradiation energy of positive ionsand the discharge power.

Fig. 4. Current densities of (a) transmitted positive ionsand (b) produced positive ions depending on theirradiation energy and the discharge power．

Vex ~ 0 V consists of the transmitted positive ions
irradiated to the porous cylinder, since the plasma
potential in the discharge section is close to 0 V.
Since Jex of Vex > 0 V is only the current densityof negative ions, the current density J−pc is measured at Vex = +100 V, where the subscript of J−pc
indicates the negative ions produced on the porous

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).