J. Plasma Fusion Res. SERIES, Vol.7 (2006) 50-53

Observation of Dissociative Recombinationin the Hydrogen Sheet Plasma
ONO Masataka, TONEGAWA Akira, KUMITA Kentaro, YAZAWA Hiroyuki,SHIBUYA Takehisa1 and KAWAMURA Kazutaka1
Department of Physics, School of Science, Tokai University, Hiratsuka 259-1292, Japan
1 Department of Electro-Photo Optics, School of Engineering, Tokai University,Hiratsuka 259-1292, Japan（Received: 4 October 2004 / Accepted: 17 March 2005）

Abstract
The experimental observation of dissociative recombination in MAR for hydrogen detached plasma havebeen studied in the linear divertor plasma simulator, TPD-SheetIV (Test Plasma produced by Directed current for Sheet plasma). Measurements of the relative concentrations of the molecular and atomic ions(nH + , nH2 + , nH3 + ), and the heat load to the target plate (Q) were carried out in hydrogen detached plasma
with hydrogen gas puﬀ at various discharge current. An omegatron mass-analyzer, situated behind asmall hole of the endplate with the diﬀerential pumping system, is used for analyzing ion species. It is shown
from these results that the dominant molecular process is dissociative recombination process via H 2 + , H3 + in
the plasma over the range of low discharge current and low hydrogen pressure.
Keywords:
dissociative recombination, molecular assisted recombination, sheet plasma, mass-analyzer
1．Introduction
The knowledge of atomic and molecular processes indetached divertor regions has become more important,
as a dynamic gas target regime is considered to be oneof the most favourable conditions for divertor plasmasof fusion reactor to control the heat load on the divertor
targets [1,2]. In the detached divertor conditions withlower plasma temperature, vibrationally excited hydrogen molecules H2 (v) persist in dissociation and ionization processes of the plasma volume recombination.
Thus, the plasma volume recombination associated withvibrationally excited hydrogen molecules H2 (v), thatis, Molecular Assisted Recombination (MAR) is effective in the divertor plasma to enhance the reduction of ion particle flux [2-5]. The rate coeﬃcient for
MAR is much greater than that one for the radiative andthree-body recombination, that is, Electron-Ion Recombination (EIR) at relatively high electron temperatures
above 1.0 eV [6]. The vibrationally excited molecules
H2 (v) contribute to plasma volume recombination due
to the following chains of reactions: (1) H2 (v) + e → H−+ H (dissociated attachment) followed by H− + H+ →H + H (mutual neutralization), and (2) H2 (v) + H+ →H2 + + H (ion conversion) followed by H2 + + e → H + H(dissociative recombination) or H2 + + H2 → H3 + + H.
However, the role of the MAR in fusion experiments

is still under discussion and various conclusions are derived from the analysis of diﬀerent experiments [7-12].
One of the experimental results of the spectroscopy inthe linear plasma simulator has provided the evidence ofdissociative recombination in MAR, showing the reduction of the ion flux in a plasma with a hydrogen/helium
mixture [7]. Author presented the observation of themutual neutralization in MAR via negative ions formation of hydrogen atom for hydrogen detached plasma[8,9]. On the other hand, it has been pointed out thateven the contribution of recombination is not critical
for the reduction of the ion flux [10]. Recently it isreported that in a typical situation of detached plasma,vibrationally excited hydrogen molecules H2 (v) dissociate (MAD) and assist ionization (MID) before they contribute to recombination [11]. For detached plasma conditions, i.e. high density and low temperature plasma,
it turned out that the MAD dominates always over theMAR. It is, thus, required that the experiments which
will be essential to an understanding of the role of theMAR come out. In MAR the dissociative recombination with vibrationally excited hydrogen molecules
H2 (v) have not reported clearly in the detached plasma
which is observed in diverted tokamaks and divertor
C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchCorresponding author’s e-mail: tone@keyaki.cc.u-tokai.ac.jp50

Observation of Dissociative Recombination in the Hydrogen Sheet Plasma

simulators.
In this paper, we have been studied the experimental
observation of the dissociative recombination in MAR
for hydrogen detached plasma in the linear divertor
plasma simulator, TPD-SheetIV (Test Plasma producedby Directed current for Sheet plasma) [12,13]. Measurements of the relative concentrations of the molecular and atomic ions (nH + , nH2 + , nH3 + ), and the heat loadto the target plate (Q) were carried out in hydrogen detached plasma with hydrogen gas puﬀ. It is also intended to show that the observed emission intensity (theLyman-bands B1 Σg + → X 1 Σg + ) of VUV (vacuum ultraviolet) wavelength region from electronic excited hydrogen molecules H2 (B1 Σu + ) by electron impact could
be explained by MAR.

An omegatron mass-analyzer, situated behind asmall hole (Φ 1.0 mm) of the endplate with the diﬀerential pumping system, is used for analyzing ion species[14]. This analyzer yields signals due to the cyclotron
resonances of ion species. The peaks appear in a collector current of the analyzer when a frequency of applied
radio frequency (RF) electric field is equal to the ion cyclotron frequencies. The relative concentrations of themolecular and atomic ions (nH + , nH2 + , nH3 + ) were determined from the collector current of the mass-analyzer.
The Lyman Rydberg series lines of neutral hydrogen were measured at an axial distance of 3 cm in frontof the target with a CCD camera-equipped VUV spectrometer coupled with a diﬀerential pumping system toa side port on the target vacuum chamber. The brightness of the high-n Lyman series lines, such as Lγ , aredirectly related to the recombination rate of EIR. Therefore the ratio of Lγ (4-1) to Lα (2-1) line intensities
can be used as an indicator of EIR. The vibrationallyexcited hydrogen molecules H2 (X1 Σg + (v > 4)) result from the deexcitation of electronic excited hydrogen molecules H2 (B1 Σu , C1 Πu ) by electron impact [15].
When this process occurs in plasmas, vacuum ultraviolet lights are emitted. Thus it is an eﬀective approach tomeasure the spectra of VUV light emission of H2 (B1 Σu ,C1 Πu ) in production of the highly vibrationally excitedmolecules. The plasma through a viewing port installedat the sidewall of the experimental region is observedby the VUV spectrometer with the diﬀerential pumping
system and a CCD camera. The spectra of VUV lightemission from electronic excited hydrogen molecules
H2 (B1 Σu , C1 Πu ) were detected 3 cm apart from the target plate.

2．Experimental apparatus and method
The experiment was performed in the linear divertor plasma simulator TPD-SheetIV as shown in Fig. 1.
Ten rectangular magnetic coils formed a uniform magnetic field of 1.0 kG in the experimental region. The
hydrogen plasma was generated at a hydrogen gas flowof 70 sccm, with a discharge current of 30 − 100 A. The
neutral pressure PDiv in the divertor test region was controlled between 0.1 and 20 mtorr with a secondary gas
feed. Electron temperature and electron density weremeasured by a planar Langmuir probe located 3 cm infront of the target. The heat load on the target plate Q
was measured by a calorimeter. At a discharge currentof 100 A, the value of Q reaches about 1 MW/m2 .

3．Experimental results and discussion
Figure 2 shows the eﬀect of hydrogen neutral gas
pressure, PDiv , on the heat to the target plate, Q, the hydrogen Lyman spectrum ratio, Lγ /Lα , and the intensityof VUV emission spectrum (133.4nm) at the Lymanband (0,4) system (B1 Σu + → X1 Σg + ) at various discharge current Id . The Id changes ( ● ) 50, and ( △) 70 A. For PDiv less than 4.0 mtorr, Q is found to decrease rapidly under 0.1 MW/m2 with an increase in
PDiv and a value less than 30 % of the initial value inthe attached plasma. The intensity of VUV emission
spectrum at the Lyman-band system (B1 Σu + → X1 Σg + )rapidly decreases with increases in P Div , while the hydrogen Lyman spectrum ratio Lγ /Lα of VUV emission
remains nearly constant. With an increase of P Div to ∼ 4mtorr, Lγ /Lα is observed in front of the target plate. Theratio Lγ /Lα rapidly increases above P Div ∼ 6 mtorr. At
the same time, the intensities of the hydrogen Lyman

Fig. 1 Schematic diagram of the magnetized plasma
simulator (sheet plasma device) TPD-SheetIV anddetection system.

51

Observation of Dissociative Recombination in the Hydrogen Sheet Plasma

rapidly until PDiv ∼ 3 mtorr with increasing in PDiv .
Above PDiv ∼ 3 mtorr nH + gradually increases and obtains the maximum value with increasing P Div . On theother hand, nH + is larger than that of nH2 + and nH3 + forthe discharge current of 70 A. Therefore, it is shown
from these results that the dominant molecular process
is dissociative recombination process via H2 + , H3 + in
the plasma over the range of low plasma density andlow hydrogen pressure. For high density and low temperature plasma, the eﬀect of the dissociative recombination process in MAR is smaller than that of molecularassisted dissociation.
4．Conclusion
We have been studied the experimental observationof dissociative recombination in MAR for hydrogen detached plasma in the linear divertor plasma simulator,TPD-SheetIV (Test Plasma produced by Directed current for Sheet plasma). Measurements of the relativeion concentrations of H+ , H2 + , H3 + (nH + , nH2 + , nH3 + ) of
the molecular and atomic ions, the intensity ofVUV emission spectrum at the Lyman-band system(B1 Σu + →X1 Σg + ), and the heat load to the target plate(Q) were carried out in hydrogen detached plasma withhydrogen gas puﬀ. An omegatron mass-analyzer,situated behind a small hole of the endplate with thediﬀerential pumping system, is used for analyzing ion

Fig. 2 The effect of hydrogen neutral gas pressure, PDiv ,on the heat to the target plate, Q, the hydrogen
Lyman spectrum ratio, Lγ / Lα , and the intensity ofVUV emission spectrum (133.4 nm) at the Lymanband system (B1 Σu + →X1 Σg + ) at various discharge
current Id . The Id changes ( ● ) 50, and ( △ ) 70 A.

series from n = 3 to 6 due to the EIR were observed in
front of the target (full detachment region), that is, theradiative and three-body recombination processes haveappeared.
Figure 3 shows the measured resonance frequency
fion as function of the magnetic field strength B at thedischarge current Id of 50 A and the PDiv of 4 mtorr.
The value of B changes from 0.5 to 1.0 kG. Mass spectroscopy is performed by applying a radio frequency (rf)field to the parallel plate of the omegatron and measuring the resulting current on the collector plate. Analysis of the ion trajectories in a uniform rf field applied perpendicular to a constant uniform magnetic fieldshows that the ions execute spiral orbits with a maximum excursion radius. From the typical plasma omegatron experiment, it can be seen that the peaks correspond to H+ , H2 + , H3 + . The peak identification is obtained by comparing the observed peak frequencies tothe cyclotron frequency fc ≡ eB/m to obtain the mass
charge ratio m/q. The relative ion concentrations of
H+ , H2 + , H3 + (nH + , nH2 + , nH3 + ) should be roughly equal
to the measured ratios of corresponding resonance peakheights.
Figure 4 shows the relative ion concentrations asfunction of PDiv at the discharge current Id of 50 and70 A at the magnetic field B of 0.78 kG. For the discharge current of 50 A, nH + , nH2 + and nH3 + decrease

Fig. 3 The measured resonance frequency fion as function of the magnetic field strength B at the discharge current Id of 50 A and the PDiv of 4 mtorr.
The value of B changes from 0.5 to 1.0 kG. Fromthe typical plasma omegatron experiment, it canbe seen that the peaks correspond to H+ , H2 + ,H3 + .
52

Observation of Dissociative Recombination in the Hydrogen Sheet Plasma

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

[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]

Fig. 4 The relative ion concentrations as function of PDivat the discharge current Id of 50 and 70 A at themagnetic field B of 0.78 kG.

[10][11]species. It is shown from these results that the dominant molecular process is dissociative recombination
process via H2 + , H3 + in center of the plasma over therange of low discharge current and low hydrogen pressure. For high density and low temperature plasma, theeﬀect of the dissociative recombination process in MAR
is smaller than that of molecular assisted dissociation.

[12]
[13]
[14]
[15]

53

References
G. Janeschitzs et al., J. Nucl. Mater. 220-222, 73(1995).
Post D E, J. Nucl. Mater. 220-222, 143 (1995).
S.I. Krasheninnikov et al., Phys. Plasma 4, 1637
(1997).
S.I. Krasheninnikov et al., Phys. Lett. A 314, 285(1996).
A.Y. Pigarov et al., Phys. Lett. A 222, 251(1996).
D. Reiter et al., J. Nucl. Mater. 241-243, 342(1997).
N. Ohno et al., Phy. Rev. Lett. 81, 818 (1998).
A. Tonegawa et al., J. Nucl. Mater. 313-316,1046 (2003).
H. Ogawa, K. Kumita, M. Ono, A. Tonegawa, T.
Shibuya and K. Kawamura, J. Plasma Fusion Res.
SERIES 6, 453 (2004).
P.C. Stangeby, The Plasma Boundary of MagneticFusion Devices (Institute of Publishig, Bristol,
2000), Chap.l6.
U. Fantz et al., J. Nucl. Mater. 290-293, 367(2001).
K. Sunako et al., Nucl. Instrum. Methods. B111,151 (1996).
A. Tonegawa et al., J. Advanced Science 11, 232(1999).
H. Sommer et al., Phys. Rev. 82, 697 (1951).
J.M. Ajello et al., Phys. Rev. A 25, 2485 (1982).