J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)457–460
000–000

Control of H– Ion Density in the Sheet Plasma
by an Electron Emitter
ONO Masataka, KUMITA Kentaro1, TONEGAWA Akira1,
SHIBUYA Takehisa2 and KAWAMURA Kazutaka
Tokai University, Hiratsuka 259-1292, Japan
Department of Physics, School of Science, Tokai University, Hiratsuka 259-1292, Japan2
Department of Electro-Photo Optics, School of Engineering, Tokai University, Hiratsuka 259-1292, Japan1(Received: 9 December 2003 / Accepted: 25 April 2004)

Abstract
The effect of the current IE and bias voltage VE of electron emitter on the negative ion density of hydrogen atom
nH- in the circumference of the hydrogen magnetized sheet plasma was studied. The value of nH- were determined bya probe-assisted laser photodetachment method. Under a secondary hydrogen gas puffing into a hydrogen plasma, nHhad a maximum value (nH- = 2 × 1017 m-3) at the gas pressure of 3 mtorr and the peak position of nH- is localized at
the circumference of the sheet plasma. When IE increases from 0 to 25 A, nH- increases and becomes eight times
larger than of the initial condition. On the other hand, nH- decreases with applying VE of the electron emitter against
the plasma potential.
Keywords:
negative ion density, hydrogen plasma, magnetized sheet plasma, electron emitter

1. Introduction

current for SHEET plasma), crossed with a vertical gas-flow
and an electron-emitter made of tungsten filaments [4-6]. The
magnetized sheet plasma is well suited for the production ofnegative ions because the electron temperature in the central
region of the plasma is as high as 10–15 eV, while in thecircumferential region of plasma, a low temperature of 1 eVwith obtained. After the secondary hydrogen gas is fed to thehydrogen sheet plasma through the gas injector, the densityof H– is controlled by changing the emission current and theapplied voltage of the electron-emitter. Also, the geometry ofthis system is estimated to be nearly one-dimensional (1-D)
based on the scale of plasma thickness. Therefore, aquantitative analysis of negative ions of hydrogen atom canbe achieved in the magnetized sheet plasma crossed with thevertical gas-flow and equipped with an electron-emittersystem.

The volume production of H– ions using H2 discharges
is being pursued due to the need for intense H– ion beams forparticle accelerators and for magnetic fusion energy research.
Magnetic multipole plasma source have been found to beefficient sources of negative ions of hydrogen atom [1]. The
most probable production process of negative ions is thedissociative electron attachment to the vibrationally excitedhydrogen molecules. The negative ions of hydrogen atom(H–) are formed by dissociative attachment of cold electrons
to the vibrationally excited molecules which are attributed toelectron-impact excitation of molecules by the fast primaryelectron in the plasma [2,3]. These volume production
processes of the negative ions depend on the density of thevibrationally excited molecules and the cold electron in theplasma. However, the control of negative ion density isdifficult for the commonly used plasma source, such as,microwave, dc, and rf discharges. Because the collisional
processes usually involve plasma with low electron
temperature far from thermodynamic equilibrium temperature
in its plasma source, it is important to produce and control ofplasma with the region of two electron temperature in volumeof plasma. In order to produce the hydrogen negative ions,
we have proposed a designed system of a magnetized sheet
plasma, TPD-SheetIV (Test Plasma produced by Directed

The experiment was performed in the linear plasma
simulator TPD-SheetIV [4-6]. The plasma in TPD-SheetIV
was divided into two regions: the sheet plasma source region
and the experimental region. The hydrogen sheet plasma wasproduced by a modified TP-D type dc discharge. The anodeslit was 2 mm thick and 40 mm wide. Ten rectangularmagnetic coils formed a uniform magnetic field of 0.7 kG in

Corresponding author’s e-mail: tone@keyaki.cc.u-tokai.ac.jp©2004 by The Japan Society of Plasma
Science and Nuclear Fusion Research

2. Experimental apparatus and method

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Ono M. et al., Control of H– Ion Density in the Sheet Plasma by an Electron Emitter

like plasma with a large temperature gradient; (iv) since thegeometry of this system is estimated to be nearly 1-D basedon the scale of plasma thickness, the experimental results canbe compared with the result determined based on the 1-Dmodel; and (v) the density and energy of the electron at thecircumference of the sheet plasma are controlled by changing
the emission current and bias voltage of the electron emitter.
Therefore, this system is capable of treating complex
phenomena of the negative ion production as the simple
model. The electron temperature and the electron density weremeasured by a plane Langmuir probe. The plane Langmuirprobe was located 3 cm apart from the target plate. A
cylindrical probe made of tungsten (Φ 0.4 × 2 cm) was used
to measure the spatial profiles of H– by a probe-assisted laser
photodetachment method [7,8]. The maximum laser energy
at the fundamental wavelength (1064 nm) was 100 mJ. The
pulse length was about 10 ns and the diameter of the beam
was 8 mm. The negative ion density was determined fromthe photodetached electron current. The spatial profile of thenegative ions was measured by scanning the cylindrical probe
under the laser irradiation.

the experimental region. The sheet plasma was terminated by
the electrically floating and water-cooled target plate (stainlesssteel) axially positioned of z = 0.7 m away from the discharge
anode electrode. The hydrogen plasma was generated at ahydrogen gas flow of 70 sccm with a discharge current of 50
A. The neutral pressure PE in the experimental region wascontrolled by feeding a secondary gas from 0.1 to 20 mtorr.
The change of PE in the experimental region had no effect on
the plasma production in the discharge region because thepressure difference between the discharge and theexperimental regions extends to 3 orders of magnitude.
The magnetized sheet plasma crossed with the verticalgas-flow system and electron emitter system is shown in Fig.
1. The secondary hydrogen gas was fed perpendicularly tothe hydrogen sheet plasma in the low-pressure experimental
region (≤ 10-4 Torr). This gas feeder was placed in the gas
contact chamber located in front of the anode at a distance of
40 cm and below the sheet plasma at a distance of 2.0 cm. In
this condition, neutral-neutral collisional mean free path is~ 0.5 m in the gas contact chamber. Also, the electron emitter
was located above the center of the sheet plasma at a distanceof 1.2 cm. The four filaments of the electron emitter weremade of tungsten wire (φ 0.4 mm) put in parallel. The heating
current and bias voltage of the electron emitter were changed
from 0 to 25 A and floating potential from 0 to –15V,
respectively. The advantages of the magnetized sheet plasma
crossed with the vertical gas-flow and equipped with electron
emitter system are as follows: (i) the plasma parameters andconditions of the secondary hydrogen gas can be controlled
independently because this system is separated into the partconsisting of the plasma source and the part consisting ofnegative ion production; (ii) the plasma density and electron
temperature can be widely varied by changing both current
and gas flow rate during the discharge, independently; (iii)the secondary hydrogen gas is vertically fed in a narrow zone
with both high temperature and low temperature plasma
because the sheet plasma is as a two-dimensional boundary-

3. Experimental results and discussion
Figure 2 shows the spatial profiles of nH-, ne , and Te in
the y-direction at various PE without electron emitter. The ydirection is along the thickness of the sheet plasma. A small
amount of secondary hydrogen gas puffing into a hydrogenplasma (PE ~ 0.8 mtorr), both ne and T e have hill-shaped
profiles with half widths of about ± 2.5 mm and ± 5.0 mm in
the sheet plasma, respectively. The produced sheet plasma hasa steep electron temperature gradient over the narrow spaceof several centimeters: the hot plasma (~15 eV) in the central
region and the cold plasma (1–2 eV) in the circumferential
region. The value of nH- is localized in the outer region ( y =10–20 mm) where cold electrons (ne ~ 4 × 1017 m-3, Te = 0.5–
1 eV) come from the circumference of the plasma. The peakvalue of nH- is 4 × 1016 m-3 and is less than one order of that

Fig. 1 The magnetized sheet plasma crossed with the vertical gas-flow system and electron emitter system.

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Ono M. et al., Control of H– Ion Density in the Sheet Plasma by an Electron Emitter

Fig. 2Fig. 3

The characteristic of the negative ion density ofhydrogen atom nH- and the temperature of filament of
the electron emitter TF are plotted against the emission
current of the electron emitter IE in the circumferential
region of the sheet plasma (y = 10 mm).

Fig. 4The spatial profiles of nH- in the y-direction at variousemission current of electron emitter IE . The IE changes(■)0, (●)10, (▲)20, (▼)25 A.

The spatial profiles of nH-, ne , and Te in the y-direction
at various PE without electron emitter. The y-direction isalong the thickness of the sheet plasma.

of the electron density in the circumferential region. At PE ~3.0 mtorr, the peak value of nH- is 0.9 × 1017 m-3 and theratio of nH-/ne in the circumferential region goes up to over20%. In this condition, the ion-neutral and electron-neutral
collision mean free paths are a few centimeters for this gas
pressure. Also, the dissociative-attatchment cross sections forthe metastable molecules at the vibrationally excited state υ= 4, are four orders of magnitude larger than those for thevibrationally ground state υ = 0 [9]. Therefore, the negative
ion production by dissociative electron attachment isenhanced when the hydrogen molecule is vibrationally excitedin the detached plasma. With further increase of PE, both Te
in the central region and nH- in the circumferential region of
the sheet plasma gradually decrease.
In Fig. 3, the characteristics of the negative ion densityof hydrogen atom nH- and the temperature of filament of theelectron emitter TF are plotted against the emission current of
the electron emitter IE in the circumferential region of thesheet plasma ( y = 10 mm). The bias voltage of electron
emitter is floating potential (VE = 0.6 V) and the gas pressurein the experimental region is ~ 3.0 mtorr. When the value ofIE changes from 0 to 25 A, TF increases to 1400 K. At thesame time, the value of nH- increases from 1.0 × 1017 m-3 to
8.0 × 1017 m-3 with increasing in IE . From the Langmuir probe
measurement, the cold electron density in the circumferential
region of the sheet plasma increases from 4 × 1017 m-3 to 5 ×1017 m-3 with the constant electron temperature (Te ~ 1 eV)
when the IE increases from 0 to 25 A. The negative ion densityof hydrogen atom depends on the temperature of the electron
emitter TF , that is, the density of the cold electron.
Figure 4 shows the spatial profiles of n H- in the y-direction at various emission current of electron emitter IE ,i.e., (■)0, (●)10, (▲)20, (▼)25 A. The bias voltage ofelectron emitter is floating potential (VE = 0.6 V) and the gaspressure in the experimental region is ~ 3.0 mtorr. The valueof n H- has hill-shaped profiles and is localized in thecircumference region ( y = 8–13 mm) with the cold electrons(Te ~ 1eV). With increasing in IE , the value of nH- increasesand its peak position slightly sifted to the center to the sheet
plasma. On the other hand, nH- decreases with applying
negative bias voltage VE against the plasma potential. Theenergy of emission electron is changed by applying negative
bias voltage. The negative ion is produced by attachment of
its emission electrons to vibrationally excited hydrogenmolecules. The cross section for dissociative attachment ofelectrons to vibrationally excited hydrogen molecules
decreases with increasing the energy of electron. Therefore
this characteristic of nH- against the applying VE is related tothe cross section for dissociative attachment of electrons tovibrationally excited hydrogen molecules [10]. The negative3459

Ono M. et al., Control of H– Ion Density in the Sheet Plasma by an Electron Emitter

section for dissociative attachment of electrons tovibrationally excited hydrogen molecules.

ion density of hydrogen atom in the circumference of thehydrogen magnetized sheet plasma is controllable bychanging the current IE and by the bias voltage VE against theplasma potential of the electron emitter.

Acknowledgments

4. Conclusion

This work is part of a program supported by the LHD
Joint Project, the National Institute for Fusion Science.

We demonstrated the control of negative ion of hydrogen
atom in the magnetized sheet plasma crossed with a verticalgas-flow and equipped with electron emitter system. The
effect of current IE and bias voltage VE of electron emitter on
the negative ion density of hydrogenatom n H- in thecircumference of the hydrogen magnetized sheet plasma iselucidated. Under these conditions, nH- increases when IE
increases from 0 to 25 A. On the other hand, nH- decreases
with applying VE of the electron emitter against the plasma
potential. The energy of emission electron is changed byapplying negative bias voltage. The negative ion is produced
by attachment of its emission electrons to vibrationallyexcited hydrogen molecules. The cross section for dissociative
attachment of electrons to vibrationally excited hydrogenmolecules decreases with increasing the energy of electron.
Therefore this characteristic of nH- is related to the cross

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