Plasma and Fusion Research: Overview Articles

Volume 11, 2505038 (2016)

Recent Studies of Hydrogen Negative Ion Source and Beam
Production for NBI in Large Helical Device∗)
Katsunori IKEDA, Shaofei GENG1) , Katsuyoshi TSUMORI, Haruhisa NAKANO, Masashi KISAKI,
Kenichi NAGAOKA, Masaki OSAKABE, Osamu KANEKO and Yasuhiko TAKEIRI
1)

National Institute for Fusion Science, 322-6 Oroshi, Toki 509-5292, JapanSOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi, Toki 509-5292, Japan(Received 7 December 2015 / Accepted 25 February 2016)

The state of hydrogen plasma in the extraction region in a hydrogen negative-ion (H− ) source for NBI has
been investigated. We clearly observe an improvement of H− density owing to the surface production eﬀect with
Cs seeding. H− ions are widely distributed in the extraction region which is obtained by movable cavity ring
down (CRD). We confirm a negative ion rich plasma with a few electrons in the extraction region, which state isimportant for reduction of electron contamination in extracting beam. An extraction area is reached 30 mm fromthe PG surface, which is measured by a 2D imaging diagnostic for Hα emission. We find the insensitive area for
H− extraction at the PG surface between the apertures. Negative ions produced at the surface are considered tohave been supplied in the extraction region. The flow velocity of H− ions is obtained by a four-pin Langmuirprobe using a photodetachment technique with an Nd:YAG laser. H− ion flows from the plasma grid surface, andits direction drastically changes at 20 mm from the production surface. This flow behavior is considered to be an
important characteristic for improving H− density in the extraction region.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: NBI, negative ion source, extraction region, negative-ion rich plasma, particle flow
DOI: 10.1585/pfr.11.2505038

1. Introduction
Neutral beam injector (NBI) is an indispensable heating device for production of high-density and hot plasmas
in fusion development. We have installed five NBIs in the
Large Helical Device (LHD) [1], and have achieved 27
MW beam injection. We use a positive-hydrogen ion (H+ )source in two beam lines (BL) arranged radial injection.
High-power (6 MW/BL) with lower energy (40 ∼ 60 keV)
beam injection allows eﬀective ion heating [2] and profile
measurement of ion temperature [3]. These systems are
already upgraded for deuterium operation. We will inject
9 MW deuterium beams from each BL with higher beamenergy (60 ∼ 80 keV) in the future deuterium experimentin LHD.
We also use negative-hydrogen ion (H− ) source in thethree other BLs, which has been continuously operated
since 1998 [4]. These systems inject high-energy neutralbeam (180 keV) to LHD with tangential direction. The
nominal injection beam power is 5 MW for each BL. We
have achieved 6.9 MW beam injection in the first BL using
a slot type grounded grid (GG) [5]. The H− current density has reached 340 A/m2 , which exceeds the target current density for NBI system in ITER (International Thermonuclear Experimental Reactor). According to the ChildLangmuir Law, a current density of H− ion is proportional
author’s e-mail: ikeda.katsunori@lhd.nifs.ac.jp∗) This article is based on the invited presentation at the 25th International
Toki Conference (ITC25).

√
to 1/ mi by space-charge limit. Here, mi is the mass ofion. We expect the extracting current density of D− ions to
be 70% of H− beam current using the same configuration
for the beam accelerator and the same extraction voltage.
This is the main problem for D− operation in negative ion
source. For this reason, improvement of the negative hydrogen ion source is required for the deuterium beam operation.
On the other hand, it is considered that H− ions produced on the PG surface release to the extraction regionof the source. The direction of the extraction of H− ionsis opposite to the initial direction of H− ions. The understanding of this extraction mechanism for H− ions is the
key issue to improve beam current and beam divergenceangle. However, this is an unclear region. We have installed many diagnostic devices in the NIFS-RNIS(NIFSR&D Negative Ion Source) [5] to investigate the state ofsource plasma and the behavior of H− ions in the extraction region.
In this paper, we will introduce the the configurationof the NIFS-RNIS, and arrangement of the diagnostic devices. We find the negative-ion rich plasma in the extraction region, and the distribution of H− density is obtainedby movable cavity ring-down (CRD) system. Extraction
behavior of H− ions measured by 2D imaging diagnostic
is also shown. Finally, we will discuss the charged particle flows and H− ion temperature estimated by the four-pin

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c 2016 The Japan Society of Plasma
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agnostics are installed on the bias insulator. The centerof the diagnostic port is 11 mm far from the PG surface.
We use the two segments of molybdenum PG mounting
on the copper support frame. Multi-apertures with the diameter of 12 mm open in the PG in an area of 250 mm ×
500 mm. All of the lines of sight from the diagnostic port
pass through this area. A cavity ring-down (CRD) [6], a 4pin Langmuir probe [7], and a surface wave probe [8] use
a movable stage to obtain the spatial distributions of target
parameters in the extraction region.
H− ions are extracted through PG apertures, and areaccelerated between the extraction grid (EG) and the acceleration grid (GG). The specification of applied voltage of Vex and Vacc is 15 kV and 90 kV, respectivey. We
also observe the extracted beam current by an ammeter ineach power supply and the thermal beam calorimeter array
which is located 5 m downstream of the ion source.

3. Observation of Negative-Ion Rich
PlasmaFig. 1 (a) Schematic drawing of the cross-section of the negative hydrogen ion source (NIFS-RNIS). (b) Arrangementof the diagnostic devices in the extraction region of thesource viewing from the back plate.

probe with photo-detachment technique.

2. Configuration of Hydrogen Negative Ion Source and Diagnostic Positions
Figure 1 (a) shows a schematic drawing of the crosssection of the NIFS-RNIS source. The inner dimensions of
the arc chamber are 350 mm in width, 700 mm in height,
and 230 mm in depth. A hydrogen gas feeder is installedon the side port of the source, and the feeding hydrogen gas
pressure is 0.3 Pa in standard operation. We produce hydrogen plasma using arc discharge, which is confined bycusp magnetic field surrounding the arc chamber. A pairof permanent magnets is installed into the filter flange to
produce an external magnetic field in front of the plasma
grid surface. This horizontally traverse magnetic field isperformed to separate the source plasma into two parts
called the driver region and the extraction region. We introduce Cesium (Cs) vapor into the arc chamber from theback plate during discharge to produce H− ions on the PG
surface using the surface conversion process. We apply the
positive bias voltage (2 ∼ 3 V in usual operation) on the
PG, which performs to reduce electrons in H− beam.
Figure 1 (b) shows the position of the diagnostic devices viewing from the back plate of the source. All di-Figure 2 (a) shows the typical time trend of the H−
density measured by CRD and the electron saturation current (Ies ) measured by a Langmuir probe. The hydrogengas pressure (0.3 Pa) and the arc discharge power (50 kW)
are fixed in the experiment. We start Cs seeding with theoven temperature of 185 ◦ C from shot number 75570. The
H− density begins to increase after 15 shots of opening a
Cs valve. The H− density is greatly enhanced to 5 times
due to the surface production. We have measured large decrease of the signal intensity of Ies into the probe, which indicates that the electron density drastically decreased shotby shot. We also measured the optical emission intensities
from hydrogen neutral (Hα : 656 nm, Hβ : 486 nm) and theCs neutral (Cs0 : 852 nm) as shown in Fig. 2 (b) by the survey spectrometer [9]. The signal intensity of Cs emission
which reflects the amount of supplied Cs atoms is also delayed by 15 shots, which is the same as the H− trend. The
time trend of Hα intensity is also similar to the H− density
caused by the excitation process of mutual neutralization
which is discussed below.
Figure 3 shows the typical wave form of the currentvoltage(I-V) curve measured with a Langmuir probe after
Cs conditioning. We have observed a large electron saturation current in the pure volume operation of the source, but
it rapidly decreases with Cs seeding. The I-V curve forms
to be quite symmetric in the optimal Cs condition for the
beam current, which indicates the negative hydrogen ion
rich plasma in the extraction region [10]. This symmetric I-V characteristic has been observed in ion-ion plasmas
which used chlorine and the C60 [11, 12]. We consider the
charge neutrality to be conserved with H+ and H− ions witha few electrons lower than 1% in the extraction region of
the negative hydrogen ion source. We expect that such asmall electron content plays an important role in inhibiting
the electron contamination of extracting H− beam.

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Fig. 2 Typical time trend of H− density by CRD and the electron saturation current by a Langmuir probe (a) and theoptical emission intensity of hydrogen and Cs (b) during
Cs conditioning.

Fig. 4 Typical distribution of H− density in the pure volume discharge without Cs (a) and the discharge with Cs (b) measured by CRD.

Fig. 3 Typical I-V curve on a Langmuir probe in Cs seeded
plasma in the extraction region. The curve has rotational
symmetry with respect to the origin.

4. Distribution of H− Density
The H− density is measured by the CRD system [6],which is one of the laser absorption diagnostics. We introduce an Nd:YAG laser pulse (λ = 1064 nm) into the plasma
between two high-reflective cavity mirrors set at the bias
insulator. The line averaged H− density is evaluated by the
decay time of the laser light attenuated by H− ions. The
accuracy of this measurement depends on the alignmentof the laser path and the position of the two cavity mirrors. To measure the distribution of H− density, we haveinstalled movable drive units with cavity mirrors which aresynchronized with the laser path position [13].
Figure 4 (a) shows the typical distribution of the H−
density in the pure volume discharge without Cs. The arc
discharge power is 50 kW with 0.3 Pa hydrogen gas pressure. The horizontal axis is the distance from the PG surface (z), and the scannable area is 0 mm < z < 27 mm.
The H− density is 4.4 × 1016 m−3 close to the PG surface,which increases to 5.2 × 1016 m−3 at z = 15 mm. This value
increase slightly to the upstream direction. Main contribution of H− production is considered as the volume production process in the extraction region.
On the other hand, the H− density improves to 2 ×
17 −3
10 m close to the PG surface by Cs seeding as shownin Fig. 4 (b). The distribution in Cs-seeded plasma has adiﬀerent property for pure volume discharge. The density
slightly increases to 2.4 × 1017 m−3 at z = 15 mm, thenit decreases to 2.0 × 1017 m−3 at z = 25 mm. These ions
are widely distributed in the extraction region of the negative ion source, which is evidence of the production ofnegative-ion rich plasma in the source. However, this flat
distribution is a surprising result because we expected theH− ions would concentrate close to the PG surface owing
to the small Larmor radius of H− (∼ 2 mm). It is expectedthat there is a flow of H− ions toward the upstream direction and a mechanism of confining the H− ion at aroundz = 15 mm around from the PG surface. Understanding of
the negative ion distribution formation mechanism requires
more detailed study by considering the distribution of electrons and positive hydrogen ions and their charge balance.
We also need to know how large is the area for extracting
H− ions in the this region. That will be important information for improving H− beam current and reducing electron
contamination.

5. 2D Distribution for Extraction of
Negative Hydrogen Ions

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Figures 5 (a) and 5 (b) show the time trace of arc dis-

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Fig. 5 (a) Time trace of the arc discharge power in H− source
operation. Extraction voltage is applied 1 s at t = 8 s. (b)Time traces of H− density (open square) and Hα emission
intensity (open circle) decrease during beam extraction.

charge power, H− density, and the intensity of Hα emission
after the Cs conditioning. The time duration of the arc discharge is 9 s which power remains constant during 1 s forH− extraction at the end of the discharge. The temperature of the PG surface varied in the discharge between 180
and 230 ◦ C. The temperatures of the two Cs ovens were
controlled at 185 ◦ C in the experiment; the Cs evaporation
rate is estimated at 0.17 mg min−1 for each oven. The H−
density measured by CRDS at 2 mm from the PG surface
increases to 1.25 × 1017 m−3 by Cs conditioning from 0.3
× 1017 m−3 at pure volume operation. The time trace ofthe H− density has a similar wave form with arc discharge
power during power increasing phase, and it suddenly decreases during beam extraction as shown in Fig. 5 (b). Thisis also similar to the sudden decrease found on the Hαemission measured at 11 mm from the PG surface.
According to the study of atomic and molecular processes in hydrogen plasma [14, 15], the dominant excitation mechanisms for Hα emission in low temperature(∼ 1 eV) hydrogen plasmas are the dissociative recombination between an electron and a H+2 , and mutual neutralization between a H+ and a H− . Since the excitation
cross-sections for two reactions is of the same order, thedominant process depends on the population of H− ions toelectrons in the plasma. The fraction of H− ions is muchlarger in the extraction region of H− source after Cs conditioning. Therefore, the reduction of Hα emission resultsfrom the decrease in the excited-state hydrogen population
created by the mutual neutralization processes, which inturn results from a decrease in the H− density. Here the reduction value of the Hα intensity owing to beam extraction
is defined as ΔHα , which is the key value to understand theH− extraction.

Fig. 6 (a) Field of viewing of the imaging diagnostic for Hαemission. (b) ΔHα distribution close to the PG surface.
Increasing zone, constant zone and decreasing zone isrepresented in white, gray, and black color. Reproducedwith permission from New Journal of Physics, Volume
15,103026 (2013). Copyright 2013 IOP Publishing and
Deutsche Physikalische Gesellschaft.

We install an imaging diagnostic for Hα emission[16] in the extraction region to obtain the 2-dimensional
distribution of H− reduction owing to beam extraction.
Figure 6 (a) shows the photograph taken from the viewing
port. An area of 35 mm from the PG surface can be observed by the imaging diagnostic. A row of the PG apertures appears as a quadrangle shape in the image becauseof the shallow angle between the PG surface and the lineof sight. Right side is the direction of the center of the ion
source, the Cs evaporator is located at 9 cm right from thecenter of the viewing port. Left side is the the directionof the chamber wall. We produce the ΔHα image by subtracting the image acquired before beam extraction fromthe image acquired during beam extraction as shown in
Fig. 6 (b). Here the decreased zone, the constant zone, andthe growth zone are represent in black, gray, and white,

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respectively. We find strong spot-like reductions in the region close to the PG surface (< 10 mm) above the apertures, and these reductions has expand to the inside of theplasma farther than 30 mm in the extraction region. We
consider that the distribution of Hα reduction results fromthe transport of H− ions owing to the beam extraction. This
result leads to the speculation that the negative ion richplasma are widely distributed in the extraction region because of the optimal Cs conditioning, which is consistent
with the flat density distribution measured by CRDS. We
also observe the insensitive area for H− extraction at thesurface area between the PG apertures. Negative ions produced at the surface are considered to have been suppliedin that region. The reduction distribution is asymmetric in
the horizontal directions. The right side of the image is the
direction of the source center nearly to the Cs evaporator,
where is the intense beam area. The left side of the image
is located close to the arc chamber wall where the small
beam current area is located. We consider that the asymmetric ΔHα distribution results from a fraction of the element of H− ions and electrons due to the diﬀerent Cs conditioning on the PG surface. This source non-uniformity
is considered to be an important issue to improve the H−
source performance toward to the deuterium operation.

Fig. 7 Dimension of the four-pin Langmuir probe for particle
flow measurement. Four probe tip using tungsten wire
are radially arranged on the ceramic stem.

6. Observation of Charged Particle
Flows
Figure 7 shows the configuration of a four-pin Langmuir probe [7] to obtain an electron flow and an ion flow.
The probe tips with the length of 2 mm made by 0.5 mm
diameter tungsten wire are set radial on a ceramic stem.
Angle of the probe of the next is 90◦ . The probe works
in electrostatic mode at measurement of electron and positive ion flows. We measure the probe current every 30◦
by rotating the probe. Figure 8 (a) shows the polar distribution of positive saturation current at z = 26 mm. The
setting gas pressure and bias voltage is 0.3 Pa and 2.4 V,
respectively, in the pure volume plasma with 50 kW arc
discharge. We do not apply extraction voltage in this experiment. The distribution of positive saturation current isshifted to the direction of 263◦ caused by positive ion flow.
The flow direction is 83◦ , which is nearly parallel to the
PG surface. The flow velocity in this direction is estimatedto be v = 3.8 × 103 m/s, as shown in Fig. 8 (b). Positiveion and electron have the same velocity with the same direction. The local magnetic field and the electric field is0.006 T and 25 V/m, respectively, at z = 26 mm. We calculate the velocity of 4200 m/s in y-direction which is thesame order for the measured value. The y component of theflow velocities are the same in three positions. Therefore,
we consider that the y component of the flow velocity is
due to the crossed field drift. The z component of the flow
velocity increases near the plasma grid surface. The observed value of is velocity is −470 m/s at z = 26 mm. The
flow velocity in z direction is estimated by collisional dif-

Fig. 8 (a) The polar distribution of positive saturation current
pure hydrogen discharge at 26 mm from the PG surface.
(b) Flow vectors of proton and electron at PG bias of
2.4 V and 0.3 Pa hydrogen pressure without Cs. The flow
velocities are also shown in three positions. Reproducedwith permission from Rev. Sci. Instrum. 87, 02B103(2016). Copyright 2016 AIP Publishing LLC.

fusion. The flow velocity in z component is to be −240 m/s
using the electron mobility of 4 m2 /(Vs) which is consistent of the measured value. Therefore we consider that thisflow in z direction is due to ambipolar diﬀusion.
The four-pin Langmuir probe works in photodetachnent mode at H− flow measurement. Figures 9 (a) and

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Fig. 9 (a) Illustration of the procedure of H− flow measurement
using photodetachment technique. (b) Typical time traceof the signal intensity of photodetachment electron withrecovery time definition. Reproduced with permission
from Rev. Sci. Instrum. 87, 02B103 (2016). Copyright2016 AIP Publishing LLC.

9 (b) show the procedure of photo detachment mode andthe typical wave form of photodetachment signal, respectively. We use an Nd:YAG laser with the wavelength of1064 nm, and the beam diameter is 4 mm at the probe tip
position. H− ions are destroyed to a proton and a photo detached electron by irradiated pulse laser around one probe
tip at the first phase. In the second phase, the photo detached electrons in the laser column are evacuated by the
positive biased probe tip. The signal intensity of the probe
rapidly increases by these electrons. Then the surrounding
negative ions flow into the detached column to recover theoriginal charge state, the photodetachment signal decreases
as shown in Fig. 9 (b). Here we define the charge recovery
time as the zero-crossing point of the photodetachment signal. We change the laser position at third phase, and takethe photodetachment signal at all positions using the sameprocedure.
We assume the charge recovery speed is the summation of the thermal velocity and flow velocity of H− ion.
The temperature of H− ions is obtained by the averaged
value of the recovery times. Averaged ion temperature is0.12 eV at z = 24 mm, which is almost the same value estimated in another position. The flow velocity of y direction
and z direction is obtained by the subtraction velocity usinga pair of probe tips along the y axis and the z axis, respectively. We evaluate the flow vectors of H− as shown in
Fig. 10. The 187◦ flow direction is observed at z = 14 mm,
which is opposite to the beam direction. This result might
be attributed to the surface produced negative ions on theFig. 10 Flow vectors of H− at PG bias of 2.4 V and 0.3 Pa hydrogen pressure wit Cs. The flow direction turn in the extraction region and the velocity is ten times smaller than the
proton and electron. Reproduced with permission fromRev. Sci. Instrum. 87, 02B103 (2016). Copyright 2016
AIP Publishing LLC.

PG surface. The flow speed is ten times slower than positive ions and electrons. The flow velocities are almost
the same in three positions, but the flow directions turn to246◦ at z = 19 mm, and the direction change to 288◦ at z =26 mm. This conversion in the flow direction might be thereason for the distribution of H− ions shown in Fig. 4 (b)in the extraction region. Control of this flow direction will
be a key to improve negative hydrogen ion density in theextraction region and to improve the beam current.

7. Summary
In this study the main stress falls on the negative-ion
rich plasma which consists of mainly positive and negative ions with a small number of electrons in the extraction
region of the hydrogen negative ion source for fusion application. As described in section 3, this plasma is widelydistributed in the extraction region of the NIFS-RNIS. The
H− density is 4.0 × 1017 m−3 at the 15 mm from the PG surface, these ions are not concentrated close to the PG surface. H− ions producing on the PG surface by Cs seedingflows to the extraction region, the flow speed is ten times
smaller than the thermal speed of H− ion shown in section 6. We also find the changing of flow direction of H−
about z = 20 mm. This flow behavior is considered to beimportant role for concentrated H− ions in the extraction
region before the beam extraction. We also considered that
this H− flow phenomenon suppresses the electron inflow
from the discharge region by space charge compensation,and for keeping low electron contamination in the beamduring the beam extraction. As shown in section 5, we ob-

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tain the 2D distribution of H− reduction results from thetransport of H− ions owing to the beam extraction using
the reduction distribution of Hα emission. The extraction
area for H− ions is also expanded into the extraction regionin negative-ion rich plasma. The insensitive area for H−
extraction is observed at the surface area which result fromthe production of H− ions on the PG surface. This result is
consistent for the H− flow measurement.
However, we only measured three positions for H−
flow, and did not extract beams during this experiment. We
will apply these flow measurements for charged particle inother position on the PG aperture with beam extraction.
This result will be a variable output to know the flow direction of extracting H− ions and the charged balance on
the extraction region during beam extraction. We plan deuterium operation in negative hydrogen ion source in future
LHD experiment. We expected further improvement of thenegative beam current with low electron contamination in
D− operation by investing and controlling this flow phenomenon in the negative ion source.

Acknowledgements
The authors acknowledge the NBI staﬀ for their technical and operational support. We would like to give heartfelt thanks to Prof. A. Hatayama, Prof. M. Wada, Prof.
W. Oohara, Prof. G. Serianni, and Prof. Fantz for discussions and useful comments. This research is supported by
JSPS KAKENHI Grant Number 25249134 and the budget
for the NIFS No. ULRR702.

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