J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Enhancement of neutral beam deposition in hydrogen
discharge using carbon pellet injection in LHD
Katsunori IKEDA, Masaki OSAKABE, Allan WHITEFORD1) , Kenichi NAGAOKA,
Mikirou YOSHINUMA, Katsumi IDA, Osamu KANEKO, Daiji KATO, Shigeru MORITA,
Haruhisa NAKANO, Yasuhiko TAKEIRI, Katsuyoshi TSUMORI, and LHD experiment group
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
1)
University of Strathclyde, 107 Rottenrow Glasgow, UK(Received: 20 November 2009 / Accepted: 26 February 2010)

The central ion temperature in the large helical device (LHD), as measured by charge-exchange
recombination spectroscopy, has been improved to a record 5.6 keV by combining 21 MW of neutralbeam heating with the injection of a carbon pellet. The intensity of the neutral beam emission of thehydrogen Balmer line (Hα : n = 3 → 2) was observed to weaken along the beam injection axis following
the carbon pellet injection due to the increased beam attenuation. The beam-emission intensity wasreconstructed by calculating the density distribution, and the beam-stopping coeﬃcients, along a beam
injection axis and was found to ﬁt well to the measured beam-emission for a mixed hydrogen andcarbon target plasma. The dynamics of the neutral beam deposition power and the carbon fraction
were estimated from the beam-emission measurements using data from ADAS. We conclude that thebeam deposition power in a carbon pellet discharge is enhanced over that of a pure hydrogen discharge.
Keywords: NBI, neutral beam emission, beam attenuation, ADAS, LHD

1. Introduction
In many magnetically conﬁned fusion devices,
neutral beam injectors (NBIs) are widely used to produce high performance plasmas [1–5]. In future devices, such as the International Thermonuclear Experimental Reactor (ITER), it is anticipated to heat theplasma with neutral beams which perform at higher
energies than most current systems [6]. The higher
energy beams will be based on negative ion sources.
In the large helical device (LHD), we have a
low energy perpendicular NBI (BL4) and three highenergy tangential NBIs (BL1, BL2, and BL3), as
shown in Fig. 1. In particular, two tanjential beamlines (BL1 and BL2) with the hydrogen negativeion sources have operated continuously for over ten
years [5]. One 180 keV tangential neutral beam passes
through the plasma and impacts carbon armor tiles on
the inner surface of the LHD vacuum vessel. We measure the beam deposition power with a thermocouple
placed on the beam armor tiles, and the measured
beam deposition eﬃciency is calculated by averaging
over a single discharge [7].
In a recent LHD experiment, with four neutralbeam injectors and the injection of a carbon pellet,
the central ion temperature was found to be significantly higher than in NBI-only heated discharges.
The beam deposition in this discharge was investigated by observing the hydrogen Balmer emission(Hα : n = 3 → 2) from one of the injected neutral
beams.
In this article we present the dynamics of beam-

BL1

A
B

neutraL(m)plasmaion source

l beam

BL2

BL3

LHD

62.4 134.7

6-o
pivot
position upstream

downstream
5-o
BL4
A

B

Fig. 1 Schematic view of LHD equipped with three tangential NBIs and a perpendicular NBI. Two opticalsight lines set along the BL3 injector allow observation of the Hα spectrum of the beam-emission. BL3
has two ion sources (3A and 3B), the beams extracted from the 3A (beam-3A) and the 3B (beam3B) are tilted small angles from the beam line axis
to pass through the beam injection port. L is the
distance from the ion source.

emission evolution in a NBI/carbon pellet heated discharge. We calculate the beam-emission intensity using the beam stopping coeﬃcients and the targetplasma density along the beam injection axis. Finally,
we discuss the beam deposition power and the carbon
fraction in a high ion-temperature discharge.

author’s e-mail: ikeda.katsunori@lhd.nifs.ac.jp

88©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

K. Ikeda et al., Enhancement of Neutral Beam Deposition in Hydrogen Discharge Using Carbon Pellet Injection in LHD

ne (1019/m3) , Ti , Te (keV)

7

ECH
BL1 (6.7MW)
BL2 (4.4MW)
BL3 (4.4MW)
BL4
(6MW)

1.5

2

2.5

3

Beam emission (1019 photons/m2s)

Time (s)Fig. 2 Time sequence of a carbon pellet discharge heatedby electron cyclotron heating and four NBIs. BL4 isalso used with CXRS to obtain the ion temperature
proﬁle.

2. Beam-emission Measurement
Figure 1 shows the arrangement of two sight lines
through an outer port of LHD that are used for beamemission measurements. The upstream sight line andthe downstream sight line are tilted at 62.4˚and 134.7˚respectively with respect to the beam injection axis of
the third beam line (BL3) [8]. Each neutral beam
forms Gaussian distribution in horizontal, and thestandard deviation σ is 12 cm at the armor position in a typical operation. The divergence angle ofthe single beamlet is less than 10 mrad. The neutral beam becomes maximum strength on the beamaxis. Since beam emission intensity is proportional
to the product of the residual beam density and theelectron density of the target plasma, the beam emission signal also becomes maximum intensity at theposition of beam center. About 70% of the beamemission signal is generated from the 2σ area aroundthe beam center. Optical emission from the neutral
beams and the plasma discharge are corrected by asmall lens, and transferred by a quartz optical ﬁber toa 25 cm Czerny-Turner spectrometer with a gratingof 1800 grooves/mm. The spectrometric system wascalibrated using an Urbricht sphere with a standard
lamp.
The wavelength of the beam-emission Hα spectrum is shifted in the observed spectrum due to the
Doppler eﬀect induced by the beam speed and theobservation angle. Hence, we can observe the beamemission Hα spectrum separated from the dominant
Hα emission (656.3 nm) emitted from the backgroundplasma. The weak beam-emission signals are detectedusing an intensiﬁed charge-coupled device (ICCD) detector cooled to -20 ˚C which is positioned at the
focal plane of the spectrometer. We can simultaneously acquire the beam-emission spectrum of both the
upstream and the downstream positions because thespectra shift in opposite directions.

Open mark: #90985(without C-pellet)
T i :CXS

6n e :FIR

5(R=3.655m)

43

carbon2 pellet

Te :Thomson(R=3.651m)

1

n e :FIR

0
(b)

3

2

#90985 without C pellet#90982 with C pellet

open marks: upstream

1closed marks: downstream

0
1.5

2

2.5

3

Time (s)Fig. 3 (a) Dynamics of the averaged electron density ne ,the electron temperature and the ion temperaturein the carbon pellet discharge of shot 90982. The
waveforms in discharge 90985 without a carbon pellet is also plotted. (b) Dynamics of the intensity ofthe beam-emission Hα spectrum at the upstream
and downstream positions. Circles and squares givethe beam-emission intensity in the discharge 90982
and 90985, respectively. The open symbols aremeasured at the upstream position, and the solid
symbols are measured at the downstream position.
4

6
(b)

#90982(a)

2.1s
2.233s2.4s

ne (1019 m-3)

3

2

1

5
Te (keV) , Ti (keV)

Carbonpellet(a) Close mark: #90982(C-pellet)

4#90982(c)#909822.1s
2.233s
2.4s

2.11s
2.23s
2.41s

3
2
1

ne (Thomson)

0
3.6

4

4.4
R(m)

4.8

Te (Thomson)
0
3.6
4
4.4
R(m)

Ti (CXS)
3.6

4

4.4
R(m)

4.8

Fig. 4 Outer half radial proﬁles of electron density (a),electron temperature (b), and ion temperature (c)for the LHD sequence time t = 2.1 s, t = 2.23 s,
and t = 2.4 s in the carbon pellet discharge.

3. High-Temperature Discharge using
Carbon Pellet Injection
Figure 2 shows the sequence of events for the carbon pellet discharge in LHD shot number 90982. The

89

K. Ikeda et al., Enhancement of Neutral Beam Deposition in Hydrogen Discharge Using Carbon Pellet Injection in LHD

magnetic ﬁeld axis is 3.6 m long and the magnetic
ﬁeld strength is 2.9 T. The plasma is initiated byelectron-cyclotron heating, which is then sustained by
two tangential NBIs (BL1 and BL2), giving a low lineaveraged electron densities of ne = 0.4 × 1019 m−3 , asmeasured by a far-infrared laser interferometer (seeFig. 3(a)). A small carbon pellet, with a diameterof 1.4 mm, was injected into the plasma at t = 1.8
s causing the electron density to increase rapidly as
much as an order of magnitude. Additional tangential NBI heating (BL3) was introduced 100 ms after
the carbon pellet injection : this system was used formeasurements of the beam-emission. Just over 100 ms
later the low-energy NBI heating (BL4) was started tofurther increase the ion heating : this system is used
for the charge-exchange recombination spectroscopy(CXRS) to measure the ion temperature proﬁle.
At t = 2.4 s, the electron density decreased tone = 1.3 × 1019 m−3 and the ion temperature increased to Ti = 5.6 keV. In a similar discharge in hydrogen, without the carbon pellet injection (shot number 90985), the electron density built up smoothly asa result of the NBI injection. The electron density ofthis pellet-free discharge reached the same density asthat for the carbon pellet discharge at t = 2.4 s. The
central ion temperature in pellet-free discharge is 4.1
keV, which is slightly higher than the electron temperature measured with a Thomson-scattering system.
The beam-emission intensities in the discharge
with and without carbon pellet are plotted in Fig.
3(b). We clearly observed the beam-emission spectrum at both upstream and downstream positions at
the beginning of the beam injection in the pellet-freedischarge of shot 90985. However, for the carbon pellet discharge 90982, the beam-emission at both theobservation positions was not seen in the 100 ms following the BL3 beam injection. We posit that strong
beam attenuation occurs during this period of beam
injection in the carbon pellet discharge. The beamemission intensity does increase with time after t =1.9
s and we suggest that the beam particle penetration
increases then because beam-emission intensity is proportional to the residual beam density. After t =2.4 s both discharges exhibit almost the same beamemission, and the ion temperature is maintained at 5
keV in the carbon pellet discharge.

5ne(1019/m3), R(m)R(m)
4Beam-3A3
ne(L)t=2.100s2

t=2.233s1

t=2.400s

0

15

16

17
18
L(m)

19

20

Fig. 5 Electron density distribution of the target plasmaalong the beam injection axis for the LHD sequence
time t = 2.1 s (circles), t = 2.23 s (triangles) andt = 2.4 s (squares). The bold line is the major
radius R from LHD center.

decreases to ne = 1.3 × 1019 m−3 with a ﬂat proﬁleat t = 2.4 s. The electron temperature proﬁles vary
little over this period, with a central temperature of
Te = 3.8 keV, as shown in Fig. 4(b). At t = 2.1 s,
when the density distribution is peaked, the ion temperature distribution adopts the same shape as the Te
distribution, and the central Ti grows to 5.6 keV untilt = 2.4 s, as shown in Fig. 4(c). Understanding of
the dynamics of the ion heating power is required to
explain the increase in ion temperature.
If the radial density distributions along the beam
injection axis is known, we can estimate the beam attenuation. The intensity of the Hα beam-emission is
proportional to the electron density, the beam density, and the beam-emission coeﬃcient [9]. In our experimental conﬁguration, the local beam-emission ﬂux
Φ(L) at the position L along the beam injection axis
is expressed as
Φ(L) ∝ ne (L)ϵcr (L)Nb (L),(1)

where ne (L), ϵcr (L), and Nb (L)are the local electron density, the emission coeﬃcient, and the neutralbeam density, respectively, at the point L. In thepresent work, we used beam-stopping coeﬃcient, Scr ,from the Atomic Data and Analysis Structure (ADAS)
database [10–12]. The local beam density Nb (L) is
given by the exponential function√∫ Lmdl),(2)ne (l)Scr (l)Nb (L) = Nb (P )exp(−
2EP

4. Estimating the Neutral Beam Attenuation and Carbon Fraction

Figure 4 shows the proﬁles of the electron density ne , the electron temperature Te , and the ion temperature Ti for the outer half of LHD at three times
following carbon pellet injection. The electron density proﬁle is measured by Thomson scattering andforms a peak at t = 2.1 s after carbon pellet injection,
as shown in Fig. 4(a). The central electron density

where the Nb (P ) is the neutral beam density paththrough the beam injection port at port position P ,ne (l) is the local electron density, Scr (l) is the local

90

K. Ikeda et al., Enhancement of Neutral Beam Deposition in Hydrogen Discharge Using Carbon Pellet Injection in LHD

stopping coeﬃcient, m is the mass of the beam particles, and E is the beam energy. In addition, we deﬁne
the beam attenuation factor η = Nb (L)/Nb (P ).
The neutral beam passes through the plasma,with a path length through plasma of nearly 5m. Figure 5 shows the major radius of LHD and the electron
density distribution, for the three times of Fig. 4,
along the beam injection path where the horizontal
axis L is the distance from the BL3 ion source. At
L = 17.5 m along the beam path it comes closest to
the center of LHD. The electron densities of the target plasma reach a maximum near this closest point
at t = 2.1 s and at t = 2.233 s. At t = 2.4 s, the target plasma density is almost constant along the beam
injection path.
Figure 6(a) shows the reconstructed beamemission intensities at both the upstream and downstream positions. In this analysis, we have added
the beam-emission intensities at two positions on the
beam center of two beams (beam-3A and beam-3B),
since the beam-emission is maximum intensity at thebeam center due to its Gaussian proﬁle. If we assume a pure hydrogen discharge for shot 90982 (recall
it has a carbon pellet injection) strong beam-emission
appears at t = 2.1 s in the calculated beam-emission
due to the increase in penetration of the beam particles, as shown by the narrow lines in Fig. 6(a). However as these calculated intensities are not consistent
with the measurements, we need adopt a mixed hydrogen and carbon model for the target plasma. The
carbon fraction is deﬁned as nC /(nH + nC ), wherenC and nH is the carbon density and the hydrogendensity, respectively. Figures 6(b) and 6(c) show thereconstructed beam-emission intensities obtained byassuming a ﬁxed carbon fraction at both the upstream
and downstream positions, respectively. With an increasing carbon fraction, the beam-emission intensity
decreases at both positions due to greater beam attenuation. For a 20% carbon fraction, the beam-emission
intensity decreases by 30% over that for a pure hydrogen discharge in the upstream position at t = 2.1
s. We expect the signal strength to be unobservable
at the downstream position with this carbon fraction.
From this result, we can estimate the dynamics of the
carbon fraction, which are shown by the cross marks
in Fig. 7. we ﬁnd that the plasma is composed ofabout 28% carbon particles at t = 2.03 s, which decreases to 0.5% at t = 2.4 s when the maximum Ti isachieved. As shown in Fig. 3(a), the electron density
rapidly increases by as much as an order of magnitude after the carbon pellet injection. If the carbon
particles fully ionize, the plasma carbon fraction increases to approximately 60%. Thus, the carbon density decreases faster than the electron density while
Ti rises. From this analysis, the reconstructed beamemission intensities, calculated in a mixed hydrogen-3(a) Continuous line : upstream#90982

Dashed line : downstream

upstream862

H(100%)

4
downstream

Carbon mix(change fraction)

0

3

2
0

(b)

Carbon pellet discharge #90982

8

upstream

6
2
4
0%
0.5%
1%
5%
10%
20%

1
Carbonfraction0(c)#9098230%
0.5%
1% Carbon
5% fraction
10%
20%

2

0

8

downstream

6
4

1

0
1.8

2

Reconstructed beam emission (104 a.u.)

Beam emission (1019 photons/m2s)

1

2
2

2.2
2.4
Time (s)

2.6

0

2.8

Fig. 6 (a) Thin lines show the reconstructed beamemission intensities calculated with ADAS coeﬃcients and under the assumption of a pure hydrogenplasma with the same density proﬁle as shot 90982.
Bold lines are the result of ﬁtting the reconstructedbeam-emission intensities for the mixed hydrogenand carbon plasma. Continuous lines and dashedlines give the estimated values at the upstream anddownstream positions, respectively. Squares andcircles give the measured value at the upstream anddownstream position, respectively. Panels (b) and(c) show the reconstructed beam-emission intensities calculated from the constant carbon fraction
at the upstream and downstream positions, respectively.

91

4

3

0.3

2

#90985without carbon pellet

10

Carbon fraction#90982
NBI-BL3
2

2.1

2.2

2.3 2.4
Time (s)

2.5

0.2
0.1

2.6

1
#90982
Beam attenuation factor η

0.6#90982 with carbon pelletmix H &C model
0.5#90982hydrogenassumption model 0.4

Carbon fraction nC/(nC+nH)

Beam deposition power (MW)K. Ikeda et al., Enhancement of Neutral Beam Deposition in Hydrogen Discharge Using Carbon Pellet Injection in LHD

00.6
t=2.400s0.4

t=2.233s
0.2
0Fig. 7 Crosses show the estimated carbon fractionnC /(nH + nC ) in the carbon pellet discharge.
Squares show the beam deposition power estimatedby beam-emission measurement using the mixedhydrogen-carbon target model in the carbon pellet discharge 90982. The data shown by the triangles are based on assuming a pure hydrogen target
model in 90982. Diamonds show the beam deposition power for shot 90985 with no carbon pellet,
estimated from the beam-emission measurement.

Beam 3A0.8

t=2.100s
15

16

17
18
L(m)

19

20

Fig. 8 Beam attenuation factors η along the beam injection axis at the LHD sequence times t = 2.1 s,
t = 2.23 s, and t = 2.4 s, plotted as the dashedline, the chain double-dashed line, and the solidline, respectively, in the carbon pellet discharge.

hydrogen-only discharge after carbon pellet injection.
The diﬀerence in the deposited beam power between
pure hydrogen discharge and discharge with 13% carbon with the same density proﬁle at t = 2.1 s is 0.8
MW, which we believe is the net eﬀect of carbon inthis discharge.
In conclusion, the central ion temperature has
been improved to 5.6 keV by combining 21 MW of NBI
heating with a carbon pellet injection. Strong beam
attenuation is observed with beam-emission Hα spectroscopy and the carbon fraction and beam deposition
power have been estimated by comparing the beamemission with an ADAS calculation. The beam deposition power for carbon pellet discharge is enhanced by
the carbon pellet until the ion temperature saturates.
The method of diagnosing beam attenuation using thebeam-emission has been shown to be useful for understanding the dynamics of the beam deposition power.

carbon model for both the upstream and downstream
position, ﬁt well to the measured beam-emission intensities (see bold lines in Fig. 6(a)).
Figure 8 shows the beam attenuation factors η
along the beam injection axis at the time t = 2.1 s,
t = 2.23 s, and t = 2.4 s. Because the target plasma
discharge contains 13% carbon impurities at t = 2.1
s, 80% of the neutral beam particles are deposited
before the point of closest approach to the center at
L = 17.5 m, and 97% of the neutral beam particles
injected are deposited into the plasma. At t = 2.4
s, when the density distribution is ﬂat with a 0.5%
carbon impurities, the beam deposition rate is reduced
to 55%. Here, we have estimated the evolution of
the beam deposition power (Pdep ), deﬁned as Pdep =(1 − η) × Pinp , where Pinp is the input beam power
passing through the beam injection port.
The square symbols in Fig. 7 show the beam
deposition power estimated from the beam-emission
measurement of the carbon pellet discharge of shot
90982. The maximum deposited power is 4.2 MW
at t = 2.1 s which decreases to 2.4 MW at t = 2.4
s, which is almost the same deposition power as forthe pellet-free discharge of the shot 90985 (diamondsymbols). Compared with a pellet-free discharge, theneutral-beam heating power improves about 2 MW
due to the injection of a carbon pellet at the beginning of the BL3 beam injection. The triangle symbols
in Fig. 7 show the beam deposition power assuming

ACKNOWLEDGMENTS

The authors thank the LHD and NBI staﬀ fortheir operational and diagnostic support. We also
thank Professor I. Murakami (National Institute for
Fusion Science) and Professor H. Summers and Dr.
M. O’Mullane (University of Strathclyde) for useful
discussions. This work is supported by the budget for
NIFS09ULBB517.

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K. Ikeda et al., Enhancement of Neutral Beam Deposition in Hydrogen Discharge Using Carbon Pellet Injection in LHD

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