24P131
Production of Hydrogen Pair Ions using DuoPIGatron

DuoPIGatronを用いた水素ペアイオン生成
Yuichiro Matsushima and Wataru Oohara
松島祐一郎, 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
In order to generate hydrogen pair-ion plasma for investigating collective properties of pair plasmas, hydrogen atomic positive and negative ions are produced by a plasma-assisted catalytic ionization method
using a porous nickel plate or cylinder and a DuoPIGatron. Production quantities of positive and negativeions depend on irradiation current and incident angle of positive ions irradiated.

1. Introduction
Pair plasmas consisting of only positively andnegatively charged particles of equal mass have thecharacteristics of space-time symmetry in collective
phenomena, in contrast to ordinary electron-ion
plasmas [1]. Electron-system pair plasmas consisting of electron and positron of antimatter electron
have been tried to be generated experimentally inlaboratories [2], but it is not easy to maintain asteady-state electron-positron plasma. So an ionic-system pair plasma (pair-ion plasma), consistingof positive and negative ions of equal mass, hasbeen generated [3]. Collective phenomena in apair-ion plasma consisting of C60+ and C60− ionshave been investigated experimentally. Since thefrequency range of interesting collective phenomena is limited to low frequencies in the massive C60
pair-ion plasma, our attention is concentrated on thegeneration of a hydrogen pair-ion plasma consistingof H+ and H− ions, which are the lightest ions andhave high response frequencies to electromagneticfields. It is difficult to generate the hydrogenpair-ion plasma by ordinary production methods of
H+ and H− ions. We have proposed a new production mechanism of the ions as a plasma-assisted
catalytic ionization method [4,5].
2. Experimental Apparatus
The apparatus for producing the pair ions mainly
consists of a DuoPIGatron and an ion production
part. The DuoPIGatron consists of a sub PIG discharge part and a main PIG discharge part, and thetwo parts are connected tandem. In the sub PIG part,two cathodes are located on opposite sides of a cylindrical anode with an inner diameter of 6.4 cm
and a length of 10 cm, which is placed in a uniform
magnetic field (100 mT), and a tungsten ring filament of 0.5 mm diameter, grounded at the same
voltage as the cathodes, is set in front of the cathodeat z = −48 cm to supply thermionic electrons. The
anode is positively biased at Vas relative to groundvoltage. The electrons are accelerated in a sheath
formed in front of the filament cathode and injected
into the space between the two cathodes, and mostelectrons accelerated are reflected in the sheath infront of the opposite cathode (anticathode) atz = −26 cm because the cathode voltages are thesame. Since the accelerated electrons are electrostatically confined between the two cathodes along
the magnetic-field lines, neutral particles can be
efficiently ionized by electron impact, generating ahydrogen plasma (sub plasma) in this region. Theanticathode at z = −26 cm has a center aperture withan inner diameter of 2.2 cm, a part of electrons inthe sub plasma enter the main PIG part through theaperture and are accelerated. A cylindrical anodewith an inner diameter of 5 cm and a length of 10
cm is biased at Vam. The cathode in the main plasma
is used a porous plate or a porous cylinder at z = 0cm. The porous-plate cathode grounded can be rotated, the angle θ to the magnetic field lines can bevaried. On the other hand, the cylindrical porous
cathode, in the case of θ =180o, can be biased at Vpc.
The porous cylinder with an inner diameter of 2 cm
and a length of 6 cm is capped for preventing theplasma passage. A schematic diagram of the experimental setup in the case of using the cylindricalporous cathode is illustrated in Fig. 1.

Fig.1. Schematic diagram of experimental apparatus.

3. Results
Positive ions produced by the PIG discharge areirradiated to the porous-plate cathode grounded in a
direction perpendicular to the porous surface andthe irradiation current Iir is measured. Positive andnegative ions are produced by the plasma-assisted
catalytic ionization from the back of the irradiation
plane. Positive- and negative-saturation currents ofthe probe, I+ and I−, are obtained at probe bias voltages of −200 V and +200 V, respectively. Iir can bevaried by adjusting the discharge power. The dependences of I+ and I− on Iir are shown in Fig. 2.
Both I+ and I− increase with Iir, that is, the production quantities of positive and negative ions increase with the irradiation flux. I+ is almost thesame as I−, indicating the property of ionic plasmas.
20

porous cylinder is equivalently increased.
20θΙ+, Ι− (μΑ)

A grid for electrostatic-wave excitation (z = 5 cm) isgrounded here. The hydrogen pressure in the source
during operation is about 5×10−2 Pa. The porous
plate is made of nickel with a pore size of 0.5 mm,a thickness of 3.2 mm, a specific surface area of
3,570 m2/m3, and a porosity of 94.8 %. The porous
cylinder, on the other hand, is made of a porous
nickel plate rolled with 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 %. Plasma parameters are measured using a Langmuir probe at z
= 2 cm.

0–20I+
I––50r (cm)5Fig. 3. Radial profiles of the ionic plasma dependingon the incident angle to the magnetic fieldlines.

Positive ions produced are irradiated to the cylindrical porous cathode biased negatively at Vpc inan oblique direction close to a parallel direction to
the magnetic field lines. Probe characteristics aremeasured at z = 2 cm, as shown in Fig. 4. The negative current is much higher than the positive current,
contrasting results to in the case of using the porous
plate. It is known from the other works that atransmitted current of positive ions through the porous cylinder is relatively low in the case of using
the porous cylinder, where the positive current consists of the transmitted current and the produced
current of positive ions. The thickness of the porous
cylinder is equivalently increased and the transmitted current will decrease under the irradiation in an
oblique direction.
610I+0–10I––Ipu (μΑ)Ι+, Ι− (μΑ)

90°
100°
140°

–200150
Iir (mA)

300

Fig. 2. Probe saturation currents of positive and negative ions as function of the irradiation current of positive ions.

The radial profiles of I+ and I− are shown in Fig.
3, where dashed lines indicate the diameter ofplasma limiter. The porous plate can be rotated, theangle θ to the magnetic field lines can be varied.
When positive ions are irradiated to the porous
plate in a direction perpendicular to the plate surface, θ = 90o, the production quantities of positive
and negative ions are large. However, the production quantities drastically decrease when positive
ions are irradiated in an oblique direction, θ > 90o.
This seems to be the reason that the thickness of the3

Vpc0V–50 V–100 V–150 V–200 V

0–1–400 –200

0200
Vpu (V)

400Fig. 4. Probe characteristics in the case of using thecylindrical porous cathode.

References[1] N. Iwamoto, Phys. Rev. E, 47, 604 (1993).
[2] M. D. Tinkle, R. G. Greaves, C. M. Surko,
Phys. Plasmas 2, 2880 (1995).
[3] W. Oohara, R. Hatakeyama, Phys. Rev. Lett.
91, 205005 (2003).
[4] W. Oohara, O. Fukumasa, Rev. Sci. Instrum.
81, 023507 (2010)．
[5] W. Oohara, T. Maeda, T. Higuchi, Rev. Sci.
Instrum. 82, 093503 (2011)．