Proceedings of ITC/ISHW2007

H/He ratio as a function of wall conditioning and plasma facing
material during past 9 years in LHD
Shigeru MORITA, Motoshi GOTO, Suguru MASUZAKI, Naoko ASHIKAWA and LHD exp. group
National Institute for Fusion Science, Toki 509-5292, Gifu, Japan(Received 15 October 2007 / Accepted 15 October 2007)
A simple method for monitoring the wall conditioning is presented for analysis on the flux ratio of hydrogen tohelium ions, which is determined from intensity ratio of Hα (6563Å) to HeI (5876Å) visible spectral lines. Thedensity and temperature dependences of the emissions, which are calculated with collisional-radiative model, canbe mostly deleted by taking into account the ratio. The H/He ion flux ratio in LHD has been thus calculated fromtoroidally integrated Hα and HeI signals. All discharges during past 9 years since the LHD experiment wasstarted have been analyzed. As a result, the effect of the discharge cleaning as wall conditioning, especially H2and He glow discharges, on LHD experiments becomes clearly visible. The history of the wall conditioning inLHD is presented with the analysis of the H/He ion flux ratio and results are discussed as a function of wallconditioning method and plasma facing materials.
Keywords: Hα, HeI, H/He ratio, wall conditioning, plasma facing material, visible spectroscopy, LHD

1. Introduction
Wall conditioning in fusion devices is one of important
experimental techniques in order to improve the plasma
performance. The edge temperature rise based on thereduction of the particle recycling is the main purpose of
the wall conditioning. In LHD the wall conditioning
becomes more important because the baking temperatureof the plasma facing components is limited to 95° due toa narrow space between the superconducting helical coils
and the vacuum vessel. In LHD, therefore, a variety of
the wall conditioning techniques such as Ti gettering, lowmagnetic field ECR discharge, boronization and H2, He
and Ne glow discharges have been attempted until now
and the He glow discharge has been finally selected as
the main method. Although the He glow discharge wasvery effective to remove the hydrogen from the wall anddivertor plates, a large amount of He was unexpectedly
appeared as the influx in the H2 discharge. It becomesthen important to study the status of the plasma facing
components after glow discharge cleaning.
Toroidal distributions of Hα and HeI visible emissions
have been measured in LHD to monitor the uniformity of
the hydrogen and helium influxes. In order to determine
their influxes the edge ne and Te profiles are always
required for the analysis in addition to the absolute valuesof the Hα and HeI emissions. However, it is reallydifficult to check all discharges in shot-by-shot basis.
author’s e-mail: morita@nifs.ac.jp

Therefore, a new method with more simple technique wasrequired to monitor the status of the wall conditioning.
The Hα/HeI intensity ratio was then adopted for thepurpose. The discharge conditions over past 9 years
have been analyzed using ion flux ratio of hydrogen to
helium evaluated from the Hα/HeI ratio. In this paper
the history of the LHD discharge condition is reported asa function of the wall conditioning method and plasma
facing component in addition to the description of theH/He ion flux ratio measurement technique.

2. Flux ratio calculation of H+/He+
The emission rates of the Hα and HeI lines are
generally functions of Te and ne, and then the populationof excited levels in neutral hydrogen and helium iscalculated using a collisional-radiative model [1]. We
need the ionization events per photon for calculating thehydrogen and helium influxes in addition to the emission
intensity. Here, the ionization events per photon givethe conversion rate from the emission to the ion flux.
Result for the Hα (2p2P-3d2D: 6563Å) is shown in Fig.1
as a function of electron density. In the figure thecalculation is done for four different electrontemperatures as a parameter. The radial profile of theHα emission has been measured in LHD. It is thenconfirmed that the electron temperature at the location
where the Hα line is emitted generally ranges inProceedings of ITC/ISHW2007

50
Te=100eV

4030
200.4

30eV

0.2

Te=100eV(a) 5876Å
3eV

10eV

10eV

101ne(1013cm-3)

10Fig.1 Ionization events per photon for Hα emission
as a function of electron density. Calculation isdone for four different electron temperatures.

0.1

30eV
Te=100eV

00.1(b) 6678Å
110ne(1013cm-3)

100Fig.3 Ratio of hydrogen ion flux to total ion flux as afunction of electron density; (a) HeI 5876Å and (b)HeI 6678Å. Calculation is done for four different
electron temperatures.

(a) HeI 5876Å
Ionization/photon

10eV

00.2

00.1

80
60

Te=100eV
30eV

40
10eV
203eV

0400(b) HeI 6678Å
Ionization/photon

3eV
0.6

30eV

3eV

300

Te=100eV

200
30eV
10eV

100

3eV
00.1

1ne(1013cm-3)

10Fig.2 Ionization events per photon for HeI emissionsof (a) 5876Å and (b) 6678Å as a function of electrondensity. Calculation is done for four different
electron temperatures.
Results calculated for two neutral helium transitions of
2p P-3d3D (5876Å) and 2p1P-3d1D (6678Å) are shown in
Figs.2 (a) and (b). The ionization events per photon of30.8

Hα/Hα+HeI

Ionization/photon

Hα 6563Å

HeI 5876Å have a large density and temperaturedependences compared to HeI 6678Å. In particular it isemphasized exceeding the density of 1x1013cm-3.

Hα/Hα+HeI

10-100eV except for the recombining phase after turning
off the heating devices. Seeing Fig.1 it is understoodthat the temperature dependence of the ionization events
per photon is really weak in the 10-100eV range.
However, the density dependence becomes considerablylarge, especially at higher densities greater than
1x1013cm-3, whereas the ionization events per photons forHα have been frequently used to be constant [2].

The ratio of hydrogen ion flux to the total ion flux(≡sum of the hydrogen and helium ion fluxes) is shown inFigs.3 (a) and (b) for HeI5876Å and 6678Å, respectively.
The calculation is done at the same photon number forboth of the Hα and HeI. It is understood that the edge
parameter dependence can be considerably reduced when
the ratio of Hα to HeI emissions is taken into account.
Especially, the disappearance of the parameter
dependence tends to be favorable to the HeI 6678Å case.
The low Te of 3eV as seen in the figure is not realistic innormal LHD discharges except for the recombining phase.
Thus, the analysis on the ratio can be practically done
without consideration of the edge parameters.
The triplet transition (HeI 5876Å) of neutral helium isreally strong compared to other singlet transitions likeHeI 6678Å. The intensity of HeI 5876Å is always 5-10
times stronger than other lines in LHD. However, theratio of hydrogen ion flux to total ion flux has a little
large density and temperature dependences compared toHeI 6678Å case. Taking into account only the ratio
itself, the use of HeI 6678Å is of course better thanProceedings of ITC/ISHW2007
is enough high and the temperature is considerably low
like 0.3-1.0keV. Then, the output signal from the HeIfilter involves a large amount of the continuum signal
whereas the HeI signal intensity is not changed.

60500.6400.430200.2

1005700

5800

λ (Å)

06000

5900

Fig.5 HeI 5876Å spectrum during H2 pellet injection.
Dashed line indicates filter response curve.

λ (Å)

6600

06700

70(b) HeI 5876Å

60
5040

130

λ0=5868ÅΔλ=23.6Å

201005700

5800

λ (Å)

5900

800
600
40006000

Fig.4 Transmission rates of interference filters andline spectra for (a) Hα and (b) HeI.
The full width at half maximum (FWHM) of the filter
is Δλ=26.6Å at Hα 6653Å and Δλ=23.6Å at HeI 5876Å.
It is difficult to adjust all the signals to the central
position of the filter response because of its
nonuniformity.
However, the signals keep thetransmission rate of, at least, 30-50% against the primaryline intensity from the optical fiber.
On the other hand, the present system becomes
unavailable to extremely high-density operation with H2pellet injection and Ne-seeded discharges. In case of theH2 pellet injection the increase in the backgroundcontinuum level mainly formed by the visible
bremsstrahlung emission is really large, since the density

550(b)500

NeI 6143.1Å

6500

210NeI 6074.3Å

06400

1000

CIII 5894.1Å

520

HeI 5875.6Å30

λ0=6550ÅΔλ=26.6Å

1200

I (counts/ch/100ms)

10(a)HeI 5875.6Å
NeI 5881.9Å

40151400

NeI 5852.5Å

50

NeI 5656.7Å

6020

I (counts/ch/100ms)

70

Transmission rate (%)(a) Hα 6563Å

Transmission rate (%)

I (103counts/ch/40ms)

HeI 5876Å

0.88030

I (103counts/ch/40ms)

I (103counts/ch/40ms)

The Hα and HeI emissions have been measured using
a combination of optical fibers, an interference filter andphotomultiplier tubes. Ten optical fibers installed oneach toroidal section, which correspond to m=10 toroidal
pitch number in LHD, are set on the interference filter
with focusing lenses. Since a large interference filter
with diameter of 10cm is used, the transmission rate of
the line is a little different for each fiber due to thespatially nonuniform transmission rate.
Typicaltransmission rates of the filters for Hα and HeI are shownin Figs.4 (a) and (b), respectively.
2570

1.0

3. Measurement of Hα/HeI ratio

Transmission rate (%)

5876Å. In the wavelength range near 6678Å, however,other weak emission lines exist in addition to the Hαemission. The HeI 5878Å is used in the present
measurement in order to increase S/N ratio of the signal.

450

400
600

650

700
750
Number of channel

800

Fig.6 Visible spectra near HeI 5876Å from (a)without and (b) with Ne-seeded discharges.
In case of the Ne-seeded discharges many lines appear
near the HeI 5876Å. The Ne-seeded discharges havebeen sometimes used in LHD for the diagnostic purpose,the increase in temperatures and resultant NBI-driven
toroidal current. It is impossible to measure the HeI line
using only the interference filter method.
The H/He flux ratio (accurately ion flux ratio of+
H /H++He+) is thus obtained by integrating 10 emission
intensity signals from toroidal array observing the Hαand HeI lines.

Proceedings of ITC/ISHW2007

References

3. H/He ratio during past 9 years in LHD

Acknowledgements
The authors would like to thank all the members ofLHD experimental group. This work was carried out
undertheLHD
projectfinancialsupport(NIFS06ULPP527).

[1]
[2]

M.Goto et al., Phys. Plasmas 9, 4316 (2002).
K.J.Behringer, J.Nucl.Matter. 145-147, 145 (1987).
Ratio of H/He (2nd Cycle: #1889-7132)

1.0

@1.0s

(a)

nH/nH+nHe

0.8
0.60.40.202000

3000

4000
5000
6000
#Shot
Ratio of H/He (3rd Cycle: #7133-17311)

1.07000

8000
@1.0s

(b)

nH/nH+nHe

0.80.60.40.208000

10000

12000
14000
#Shot
Ratio of H/He (4th Cycle: #17312-26207)

1.0

16000

18000
@1.0s(c)nH/nH+nHe

0.8
0.6
0.4
0.2018000

20000
22000
24000
#Shot
Ratio of H/He (7th Cycle: #41313-48822)

1.0

26000
@1.0s

(d)

nH/nH+nHe

0.8
0.6
0.4
0.2041000

42000

43000

44000

45000
#Shot

46000

47000

48000

49000

Fig.7 H/He flux ratio in the (a) 2nd, (b) 3rd, (c) 4thand (d) 7th experimental cycles. Repetitive H2pellet operation is very few in these cycles.
Ratio of H/He (10th Cycle: #66054-75951)

1.0

@1.0s

0.8
nH/nH+nHe

The H/He flux ratio has been analyzed in almost alldischarges from the 2nd cycle (1998) to the 10th cycle(2006). Typical results are shown in Figs.7 (a), (b), (c)and (d) for the 2nd, 3rd, 4th and 7th cycles. The
different wall conditionings were attempted during these
9 years;
2nd: daily He glow (stainless steel divertor)
3rd: daily H2 or He glow (carbon divertor)
4-5th: daily He glow (carbon divertor)
6-7th: H2 (He) glow before H2 (He) experiment
8-10th: He glow only when necessary
The 2nd cycle (1998) had no carbon divertor plates andthe discharges were operated with stainless steel wall.
In addition, the H2 and He discharges were repeatedalternately. As a result, most of discharges weredominated by He. Before the 3rd cycle (1999) carbon
divertor plates were installed on the vacuum wall. Bothof the H2 and He glow discharges were tried after
experiments.
In contrast to this only He glow
discharges were done in the 4th cycle (2000). The
difference is clearly visible in the two figures (see Figs.7(b) and (c)). After He glow discharges any pure H2discharge can not be performed, although the He ion flux
decreases according to accumulation of H2 discharges.
The additional He flux is mainly released from the carbon
divertor plates. In the 7th cycle (2003) the H/He flux
ratio was drastically changed as seen in Fig.7 (d). The
working gas used in the glow discharge was selectedaccording to the fueling gas in the next day's experiment.
Since in the 7th cycle the He experiment was not done sofrequently, almost pure H2 operation became thenpossible in the H2 gas fueled discharges.
Recent LHD operation (8th-10th cycles) excellently
maintains good H2 discharges with less He flux. On thecontrary the maintenance of the He discharge becomesdifficult by the enhanced hydrogen flux, since the numberof NBI beam lines (at present NBI#1-#4) increases withfurther enhancement of hydrogen flux. Then, theincrease in the H/He flux ratio is frequently appeared
even in the He fueled discharge. It is shown in Fig.8
obtained from the 10th cycle (2006).
Finally, it is summarized that the present diagnosticmethod using the ratio of Hα to HeI can give useful
information on the status of wall conditioning.

0.6
0.4
0.2066000

68000

70000

72000

74000

76000

#Shot

Fig.8 H/He flux ratio in the 10th cycle (2006) with H2discharges including H2 pellet (red), He discharges(blue) and Ne-seeded discharges (green).