24P132
Control of ECR Plasma Potential for Hydrogen Pair-Ion Plasma Generation

水素ペアイオンプラズマ生成に向けたECRプラズマ電位制御
Tomoki Ohta and Wataru Oohara
太田智喜, 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
Generation of hydrogen pair-ion plasma is our target for investigating collective properties of pair plasmas.
Hydrogen atomic positive and negative ions (hydrogen pair ions) are produced by a plasma-assisted catalytic ionization method using a porous nickel plate as catalyst and an ECR plasma. Irradiation energy ofpositive ions to the porous plate can be varied by plasma potential in the ECR plasma. It is clear that theproduction quantity of pair ions depends on the irradiation energy.

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 the ordinary production methodof H+ and H− ions. We have proposed a new production mechanism of the ions as a plasma-assisted
catalytic ionization method [4,5]. Hydrogen positive ions produced by discharge are irradiated toporous nickel plate and the hydrogen pair ions areproduced from the back of the irradiation plane.
2. Experimental Apparatus
Figure 1 shows a schematic diagram of the experimental setup. Microwave (2.45GHz) are introduced into a cylindrical electrode via quartz windows and a horn antenna applied by a magnetic
field. Electron cyclotron resonance (ECR) point isin the cylindrical electrode of 12 cm inner-diameterand a hydrogen plasma is generated. A porous

nickel plate is set at z = 0 cm, the plasma generated
is irradiated. The cylindrical electrode and the porous plate are biased at dc voltages of Vcy and Vpc,respectively. Plasma parameters are measured using
Langmuir probes at z = −6 cm and 5 cm. A gridelectrode for electrostatic-wave excitation (z = 10cm) is biased at dc voltage of Vg, but the grid isused for collector to obtain the ion energy distributions along magnetic field lines here. The hydrogenpressure in the source during operation is about 0.1
Pa. The forward and reflected powers of microwaves are Pf = 500 W and Pr ~ 60W, respectively.
The porous nickel plate with a pore size of 0.5 mm,a thickness of 3.2 mm, a specific surface area of
3,750 m2/m3, and a porosity of 94.8 % is used as acatalyst.

Fig.1. Diagram of experimental setup.

3. Results
The dependences of the irradiation current Iir andthe plasma potential φus at z = −6 cm on Vcy are
shown in Fig. 2(a), where Vpc is maintained at 0 V.
φus is approximately proportional to Vcy. Positive
ions are accelerated up to eφus (eV) in the sheath
formed in front of the porous plate and the irradiation energy can be controlled by adjusting Vcy. Iir,30(a)(b)200

Ig+, I g– (μΑ)φus (V)Iir (mA)

10φus →

5000100200
Vcy (V)

0300Vpc
0VIg+–200 V–400 V00V

Ig––500(b)+300 V+200 V+100 V+50 V

P–200 VTVpc = –300 V

Vcy = 0 V+100 V

0+400 VdIg/dVg

Vcy+200 V–50–100400← Iir(a)50+300 V

Vpc = –300 V

10020face.

–Ig (μΑ)

which depends on the ECR plasma density, rapidly
decreases when Vcy decreases to below 20 V because of the superimposed current of electrons.
Electrons are electrostatically reflected in the sheath
when Vpc < −100 V, they cannot reach the porous
plate, not shown here. Therefore, positive ions are
only irradiated when Vcy > 20 V and Vpc < −100 V.

–4000Vg (V)+400 V
400–4000Vg (V)400

Fig.3. (a) Current-voltage characteristics of the grid. (b)Kinetic-energy distributions of the ions along
magnetic field lines. Reference potential is Vpc (=−300 V).

–400 V

200Vcy (V)400

Fig.2. Dependences of (a) the irradiation current of positive ions and the plasma potential at z = −6 cmand (b) the grid saturation-currents of positive
and negative ions on the dc bias voltage of thecylindrical electrode.

Positive and negative ions are emitted from thesurface of porous plate and their fluxes toward
downstream are measured using the grid at z = 10cm. The positive- and negative-saturation currentsof the grid, Ig+ and Ig−, are obtained at Vg = −520 V
and +520 V, respectively. The dependences of Ig+and Ig− on Vcy are shown in Fig.2(b). Fast electronsof corresponding to the tail component in the Maxwellian distribution can reach and pass through theporous plate without termination and Ig− increases,
when Vpc = 0 V and Vcy < 50 V; thus, the current
dependences are paid attention in Vcy > 50 V here.
Ig− increases proportionally with Vcy and Vpc (theirradiation energy); that is, the negative-ion flux
increases. On the other hand, Ig+ is almost constantand the positive-ion flux appears to be independently of the irradiation energy. We refer to thisprocess involving positive-ion irradiation, dissociative adsorption, surface migration, and desorption
ionization as plasma-assisted catalytic ionization.
The production mechanism of positive and negativeions is the same excepting desorption ionization.
Current-voltage (Ig-Vg) characteristics of the grid
are measured under positive-ion irradiation, asshown in Fig. 3(a), where Vpc = −300 V without
electron irradiation and transmission. The characteristics depending on Vpc and Vcy have two inflection points. One is at Vg ~ Vpc and the other is at Vg~ Vcy. Kinetic energy distributions of the ions arecalculated from the Ig-Vg characteristics, as shownin Fig. 3(b). The reference potential at z > 0 cm isVpc because all ions are emitted from the plate sur-

There are two components of the ions. One at Vg ~Vpc (= −300 V) is referred as component P and theother at Vg ~ Vcy as component T. Kinetic energy ofpositive and negative ions produced by desorption
ionization appears to be less than several eV, andthe component P will consist of them, because potential of the component P is close to the reference
potential. On other hand, the component T consistsof transmitted positive ions irradiated to the porous
plate since φus is close to Vcy. The quantity of thecomponent P increases proportionally with Vcy, thatis the irradiation energy, but the quantity of thecomponent T decreases. Ig+ of Vg = −520 V is sum
current of the transmitted and produced positive
ions, corresponding to Ig of Vg < −350 V, and Ig+ is
independently of the irradiation energy. Ig of Vg >Vcy corresponding to the negative-ion flux increases
proportionally with Vcy. Therefore it is concluded
that the production quantity of positive and negativeions excepting transmitted positive ions depends onthe irradiation energy of positive ions.
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