S5-3

Production and Measurement of Hydrogen Negative Ionsin Low Pressure Hydrogen Discharge Plasmas
水素負イオンの体積生成と計測
Osamu Fukumasa
福政 修
Department of Electrical and Electronic Engineering,, Faculty of Engineering,
Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan
山口大学工学部電気電子工学科, 〒755-8611 宇部市常盤台 2-16-1

Hydrogen negative H- ion volume production is studied in a rectangular arc chamber. Axial distributionsof H- ion densities in the source are measured directly using a laser photodetachment method.
Relationship between H- production and plasma parameter control with using a magnetic filter (MF) isdiscussed. Extracted H- currents depend directly on negative ion densities in the source.

1. Introduction
Negative ions play important roles in thegeneration and maintenance of low pressure
discharge plasmas and their technologicalapplications, such as ion sources and materialsprocessing.
In pure hydrogen (H2) discharge plasmas, mostof the H- ions are generated by the dissociative
attachment of slow plasma electrons es (electrontemperature Te ~ 1 eV) to highly vibrationallyexcited hydrogen molecules H2(v”) (effectivevibrational level v” ≥ 5-6). These H2(v”) aremainly produced by collisional excitation of fastelectrons ef with energies in excess of 15-20 eV.
Namely, H- ions are produced by the following two
step process [1, 2]:collision frequencies for production and destructionof H- ions based on measured plasma parameters.
Estimating negative ion densities in the source withusing laser photodetachment technique, we discuss
the relationship between H- ions in the source andextracted H- ion currents.
2. Experimental Set-up
Figure 1 shows a schematic diagram of the ionsource. The rectangular arc chamber is 25 cm ×25 cm in cross section and 19 cm in height. Theline cusp magnetic field is produced by permanent
magnets which surrounded the chamber. Theexternal MF is composed of a pair of permanent
magnets in front of the plasma grid (PG). Details
are described in elsewhere[5,6].

H2 (X1Σg, v”= 0) + ef → H2*(B1Σu, C1Πu) + ef’ (1a)H2*(B1Σu, C1Πu) → H2 (X1Σg, v”) + hν,(1b)H2(v”) + es → H + H(2)Production process of D- ions is believed to be
the same as that of H- ions. This process is also
applicable for production of O- and Cl-.
Knowledge of spatial and temporal distributionsof negative ions is very important. One of theimportant diagnostic tools for studying negative ion
plasmas is based on the laser-photodetachment
technique [3], in which one rapidly destroys thenegative ions in a certain region in the plasma by anintense laser light, and studies the plasma responseby probe measurements[4].
In this paper, plasma parameter control byvarying the magnetic field intensity of the magneticfilter(MF) is presented [5,6]. Influence of theseplasma parameter distributions on H- production isdiscussed with estimated rate coefficients andNd-YAG Laser
1064nmWindowFilament

End Flange(cathode)Probe

Ion Analyzer

To PumpPlasma GridCusp
Magnets

Magnetic Filter

Laser Power MeterFig. 1 Schematic diagram of the ion source. Theprobe, the laser path, and power meter used in photodetachment experiments are also shown.

H- densities in the source are measured by thelaser photodetachment method[3]. A light pulse
from a Nd:YAG laser (wavelength 1064 nm,
duration of laser pulse 9 ns, repetition 10 Hz) isintroduced from the side wall window of the50
4030Filament

M.F.

(a)
150G
120G
100G
80G
60G

20
100

16 14 12 10 8 6 4 2 0 -2
Distance from M.F., z (cm)Electron temperature, Te (eV)Electron density, ne ( x 1010 cm-3)

32.5
2Filament

M.F.

(b)
150G
120G
100G
80G
60G

1.5
10.50

16 14 12 10 8 6 4 2 0 -2
Distance from M.F., z (cm)Fig. 2 Axial distributions of plasma parameters (a) neand (b) Te in H2 plasmas. Experimental conditions are
as follows: Vd = 70 V, Id = 5 A, p(H2) = 1.5 mTorr.
Parameter is the magnetic field intensity of the MF.

For the MF with 150 G (BMF = 150 G), not only
Te but also ne are decreased far from the MF, i.e. z =8 cm, in the source region. On the other hand,decreasing points of ne and Te are shifted todownstream region for the case of 80 G and 60 G.
In this source, due to the external MF, width of thehalf-maximum of magnetic field intensity is wider(about 16cm in this case) than the case of rod filter.
Thus, the external MF has the merit of control ofplasma parameters gradually. Te control can be done
precisely with keeping ne high in the extraction
region. The grid method is also applicable tocontrol Te[7].
H- density distributions across the MF aremeasured and its dependence on plasma parameters
are studied. Figure 3 shows axial distributions of
H- ion densities, where BMF = 150 G and 80 G,
respectively. Plasma parameters corresponding tothese H- ion densities are shown in Fig. 2. Spatial
distributions of H- densities are varied by changing
plasma parameters. When BMF = 150 G, H- density
distribution decreases toward to the extraction hole(i.e. z = -2 cm). On the other hand, when BMF =80
G, H- density distribution remains nearly constantvalue although ne decreases toward to the extraction
hole. In front of the extraction hole (i.e. plots at z= -1.5 cm), H- density with 80 G is higher than thatwith 150 G by a factor about 2. Extracted H--3cm )M .F.

20103. Experimental Results and Discussion
On H- volume production, desired condition forplasma parameters is as follows: Te in the extraction
region should be reduced below 1 eV with nekeeping higher. To realize above-mentioned plasma
conditions, the MF is used[5,6].
Figures 2 shows axial distributions of plasma
parameters (ne and Te) in H2 plasmas. By varying
the intensity of the MF, axial distributions of ne andTe in H2 plasmas are changed strongly in thedownstream region (from z = 8 to -2 cm).

currents are also the same ratio. Extracted Hcurrents depend on H- densities in the source.
Negative ion density, n− ( x 10chamber and passes through the source plasmas.

150G80G

15105043210-1
D istance from M .F., z (cm)-2Fig. 3 Axial distributions of H- ion densities.
Experimental conditions are as follows: Vd = 70 V, Id = 5A, p(H2) = 1.5 mTorr. Parameter is the magnetic field
intensity of the MF. Corresponding plasma parameters
are shown in Fig. 2 (with BMF = 150 G and 80G).

A two-dimensional PIC particle simulation,
which can correctly treat the cylindrical geometryof the laser beam region, has been conducted todiscuss the physics related with the photodetachedelectron dynamics and to estimate quantitatively
various values related with it in the timescale of 20- 60 ns after the photodetachment [8,9].
4. Summary
Production and control of plasma parameters in
H2 plasmas are performed by varying the intensityof the MF. Extracted H- currents are mainly
determined by H- densities in the source. According
to the discussions based on estimated rate
coefficient and collision frequencies of dissociative
attachment and electron detachment processes[6],for enhancement of H- production, it is reconfirmedthat Te in the extraction region should be reduced
below 1 eV with ne keeping higher. For further
studying enhancement of H- production,
simultaneous measurements of VUV emission andnegative ion density in the source is necessary.
Acknowledgments
The author would like to thank Prof. H. Naitou,
S. Mori and Y. Sakurai for their useful discussions.
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[3] M. Bacal and G. W. Hamilton: Phys. Rev. Lett. 42(1979) 1538.
[4] L. Friedland et al.: Phys. Rev. E49 (1994) 4353.
[5] O. Fukumasa et al.: Contrib. Plasma Phys. 44 (2004)
516.
[6] O. Fukumasa and S. Mori: PNNIB-10 (2004).
[7] K. Kato et al.: Appl. Phys. Lett. 65 (1994)816.
[8] H. Naitou et al.: Plasma Phys. Control. Fusion 46(2004) 1217.
[9] Y. Sakurai: private communication (2004).