Abstract. Glow discharge cleaning (GDC) is a widely used technique for wall conditioning in fusion experimental devices. Though the cleaning effects of GDC are essentially related to the microscopic modification of the wall surface, there are few reports about it. In the present study, samples of wall materials were exposed to GDC plasma of hydrogen, helium and neon in the Large Helical Device (LHD) by using the retractable material probe transfer system and examined microscopic modification by transmission electron microscopy (TEM) to understand the underlying mechanism of GDC. Based on the results of the material probe experiments, GDC of LHD was successfully improved. Reduction of the impurities in the LHD vacuum vessel was drastically improved by using Ne-GDC. In the case of Ne-GDC, the specimen surface was covered with very thick re-deposited layer of Fe and Cr. Due to its high sputtering efficiency and very shallow penetration, it is likely that neon atoms effectively sputtered surface contamination without remaining serious damage and themselves in the sub-surface region. Retained Ne can be successfully removed by the following short H-GDC

M Tokitani

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EX/P5-34 
  
1 
Microscopic Modification of Wall Surface by Glow Discharge Cleaning and 
its Impact on Vacuum Properties of LHD 
  
M. Tokitani 1), M. Miyamoto 2), K. Tokunaga 3), T. Fujiwara 3), N. Yoshida 3), A. Komori 4), 
S. Masuzaki 4), N. Ashikawa 4), S. Inagaki 4), T. Kobuchi 4), M. Goto 4), J. Miyazawa 4), K. 
Nishimura 4), N. Noda 4), B.J. Peterson 4), A. Sagara 4) and LHD experimental group 4) 
  
1) Interdisciplinary Graduate School of Engineering Science, Kyushu University, Kasuga, 
Fukuoka 816-8580, Japan 
2) Department of Material Science, Shimane University, Matsue, Shimane, 690-8504, Japan 
3) Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, 
Japan 
4) National Institute for Fusion Science, Oroshi, Toki, Gifu 509-5292, Japan 
  
e-mail contact of main author: tokitani@riam.kyushu-u.ac.jp 
  
Abstract. Glow discharge cleaning (GDC) is a widely used technique for wall conditioning in fusion 
experimental devices. Though the cleaning effects of GDC are essentially related to the microscopic 
modification of the wall surface, there are few reports about it. In the present study, samples of wall 
materials were exposed to GDC plasma of hydrogen, helium and neon in the Large Helical Device (LHD) 
by using the retractable material probe transfer system and examined microscopic modification by 
transmission electron microscopy (TEM) to understand the underlying mechanism of GDC. Based on the 
results of the material probe experiments, GDC of LHD was successfully improved. Reduction of the 
impurities in the LHD vacuum vessel was drastically improved by using Ne-GDC. In the case of Ne-GDC, 
the specimen surface was covered with very thick re-deposited layer of Fe and Cr. Due to its high 
sputtering efficiency and very shallow penetration, it is likely that neon atoms effectively sputtered surface 
contamination without remaining serious damage and themselves in the sub-surface region. Retained Ne 
can be successfully removed by the following short H-GDC. 
  
1. Introduction 
LHD is the largest heliotron-type plasma confinement device, and is equipped with 
superconducting magnetic coils. The first wall panels and the divertor plates of LHD are 
made of stainless steel (SUS316L) and isotropic graphite, respectively. The former is the 
major material in LHD, and the graphite area is only about 5 % of the total plasma facing area. 
The vacuum vessel temperature is limited up to 95 ˚C due to the cryogenic capability for 
superconducting magnetic coils. Wall conditioning is conducted by mild temperature (95 ˚C) 
baking, GDC and titanium or boron coating. Working gases for GDC are hydrogen (H), 
helium (He) and neon (Ne). Until the experimental campaign in 2002, He-GDC was most 
frequently conducted.  
In large-size plasma confinement devices like LHD, GDC is applied as a convenient wall 
conditioning method. Helium is used for working gas in some cases. In case of the all 
stainless steel wall machine TJ-II stellarator, control of plasma density at medium and high 
power injection became difficult, because the helium implanted in the wall by He-GDC 
desorbed during main plasma discharge [1]. In case of LHD, in spite of pumping for 7 hours 
after He-GDC (for about 8 hours), the residual helium gas was still detected in the vacuum 
vessel [2]. Such prolonged desorption of helium gas made the ICRF heating condition 
unstable, and helium leak test difficult. It is well known that helium atoms implanted into 
metals are deeply trapped by lattice defects such as vacancies and bubbles formed by their 
own irradiation even if they do not have sufficient energy for knock-on damage [3]. 
In order to solve these problems, the behavior of implanted helium in metals (stainless 
steel) must be investigated from a viewpoint of material science. In the present work, 
EX/P5-34 
  
2 
therefore, microscopic modification and retention properties in metals exposed to He-, H-, 
Ne-GDC in LHD were studied and its impact on vacuum properties of LHD is discussed by 
using several kinds of analytical techniques complementally. Based on the experimental 
results more efficient GDC method in LHD was proposed. 
  
2. Experimental Procedures 
To examine the surface modification and retention properties in metals due to the He-, H- 
and Ne-GDC, GDC exposure experiments carried out in the LHD. Pre-thinned vacuum 
annealed stainless steel (SUS316L) disks of 3mm in 
diameter and stainless steel (SUS316L) plates of 0.1 
mm thick were used as specimens. All specimens were 
electrochemically polished. The specimens mounted on 
the retractable material probe system attached to the 
LHD, which is the same electric potential as the 
vacuum vessel, were placed on a position similar to the 
first wall surface through the 4.5 lower port (4.5L) as 
shown in Fig. 1. The GDC-plasma was sustained with 
two electrodes inserted into the vacuum vessel from 
the 4.5 upper port (4.5U) and the 10.5 upper port 
(10.5U). Discharge parameters of the GDCs were 
summarized in table 1. Total fluence was roughly 
estimated at 3.7x1022 He/m2, 4.1x1022 H/m2 and 
3.2x1022 Ne/m2 respectively, based on the total 
plasma-facing area of 780 m2. The temperature of the 
probe head during the GDCs was measured by 
thermocouples at the positions just beneath the 
specimens. It stayed almost constant near room 
temperature. 
  
Table 1. Discharge parameters of He, H and Ne-GDC. 
  
After exposing to GDCs, microscopic damage, chemical composition and retention 
properties were examined by means of scanning electron microscopy (SEM), atomic force 
microscopy (AFM), transmission electron microscopy (TEM), energy dispersive spectroscopy 
(EDS) and thermal desorption spectroscopy (TDS). 
Additional deuterium irradiation to the specimen pre-exposed to He-, H- and Ne-GDC was 
carried out in order to confirm the change of deuterium retention properties due to the GDCs. 
Samples of SUS316L (10x10x0.1 mm3) exposed to GDCs were irradiated with 2 keV-D+ at 
room temperature up to dose of 1x1022 D+/m2. And then the specimens were transferred into a 
TDS apparatus, where the thermally desorbed deuterium gas was measured with a 
high-resolution quadrupole mass spectrometer by heating up to 1400 K with a ramping rate of 
1 K/s. This device makes it possible to distinguish the small difference in mass of helium 
(4He: m=4.0026 amu) and deuterium gas (D2: m=4.0282 amu) [4]. Desorption rate of 
deuterium was quantitatively calibrated by comparing with helium standard leak with specific 
relative ionization efficiency. 
  
Vacuum Vessel
4.5L-Port
Probe Head
Helical Coil
Helical Coil
Material
Probe
System
 
Fig. 1. Schematic view of the 
experimental position. 
  Total discharge time Voltage Current 
He-GDC 65h 200V 20A 
H-GDC 71.5h 300V 20A 
Ne-GDC 55h 200V 20A 
EX/P5-34 
  
3 
3. Results and Discussion 
3.1. Microscopic modification and impurity depositions by GDCs 
Fig. 2 shows TEM images of the pre-thinned SUS316L specimens exposed to He-, H- and 
Ne-GDC for 65h, 71.5h and 55h, respectively. The incident energy of these GDCs was at 
most about 200 eV, 300 eV and 200 eV, respectively. The temperature of the specimen holder 
during GDCs stayed almost constant near room temperature.  
Special features of He-GDC were rather 
strong sputtering erosion and heavy damage at 
the subsurface region; formation of dense 
bubbles with size of 2-20 nm, dislocation 
loops and cracks connecting the bubbles. Such 
a heavy damage structure was formed in spite 
of low incident energy. It is known that 
despite of insufficient irradiation energy for 
knock-on damage, injected helium atoms 
aggregate by themselves without any pre-existing vacancies and form dense helium bubbles 
[3]. Surface morphology obtained by a high resolution micrograph of AFM observation of the 
SUS316L specimen after the exposure is shown in Fig. 3. The surface is covered with dimples 
of about 200-400 nm in size. They are probably formed by exfoliation of blisters. Such large 
dimples and large bubbles have not been observed for an irradiation with 2keV-He+ at room 
temperature with comparable fluence [5]. In the case of helium ion irradiation experiment at 
B
rig
ht
 fi
el
d 
im
ag
es
Ne-GDCH-GDCHe-GDCUn-irradiated
D
ar
k 
fie
ld
 im
ag
es
20nm  
Fig. 2 TEM images after three GDCs, bright field images at large deviation parameter condition 
(upper series). White dot contrast in dark field images show dislocation loops (lower series). 
 
Fig. 3 Surface morphology of SUS316L after 
exposed to a He-GDC as observed by AFM. 
20 30 4010 500
depth [nm] 
a) LHD He GDC
b)  He+ irra. exp.
(2keV-He+,1x1022)
Fr
ac
tio
n 
of
 H
e 
bu
bb
le
 [a
.u
.] 
 
Fig. 4 (a)Depth distribution of helium bubbles in 
SUS316L exposed to He-GDC and (b) 
irradiated with 2keV-He+ at room temperature 
to a fluence of 1×1022 He/m2. Depth distribution 
of the helium calculated by TRIM91-code was 
also plotted together. 
EX/P5-34 
  
4 
room temperature, very dense helium bubbles of about 1-2nm and dislocation loops were 
observed. Formation of dense helium bubbles, exfoliation of blisters and cracks lead to the 
increase of the effective surface area of the materials. It is expected that the increase in surface 
area causes adsorption of impurities gas existing in the vacuum vessel. 
The depth distribution of helium bubbles formed in SUS316L exposed He-GDC is plotted 
in Fig. 4 together with the depth distribution of the helium calculated by TRIM91-code for 
200eV-He+. The data for irradiation experiment with 2keV-He+ at room temperature is also 
plotted in the Fig. 4 for comparison. In case of He-GDC, the helium bubble was distributed 
deeper than the injected range. In contrast, in case of helium ion irradiation, distribution of a 
helium bubble agrees well with the injected range. This difference seems to be the result of 
the concentration of vacancies formed by knock-on process. In the case of 2keV-He+, almost 
all the injected helium is trapped by the 
radiation induced vacancies which are formed 
in the helium injected range. In He-GDC case, 
since radiation induced vacancies are not 
formed, injected helium atoms can diffuse far 
deeper than the injected range, until it form 
their stable clusters. It is likely that some part 
of helium trapped in the heavily damaged 
region may release gradually through the 
nano-cracks connecting the bubbles and the 
surface. The prolonged helium release 
currently observed at LHD after helium-GDC 
can be explained from this mechanism [2]. As 
shown in Fig. 5, desorption of helium is also 
affected by hydrogen discharge; in spite of no 
helium supply from outside, helium content in 
plasma increased during hydrogen discharges 
performed just after He-GDC. 
On the other hand, accumulation of defects 
such as bubbles was not observed in the 
specimens exposed to H-GDC and Ne-GDC 
(see Fig. 1). After exposure, impurity deposits 
were also identified on the specimens. Fig. 6 
shows the electron diffraction pattern and the 
dark field image of deposition layer formed 
on SUS316L specimens exposed to GDCs. 
White contrasts show individual crystal grains 
of deposits. Especially, in case of Ne-GDC, 
the contrast of diffraction pattern is the 
clearest. This means that the specimen surface 
was covered with thick deposition layer. Due 
to its high sputtering efficiency, about ten 
times higher than that of helium, and very 
shallow penetration, it is likely that neon 
atoms effectively sputtered surface 
contamination without remaining serious 
damage and themselves in the sub-surface 
region. The element of deposition layers 
detected by EDS is mainly Fe and Cr, which 
0
3
6
P
NBI
 (MW)
n
e,bar
 (1019m-3)
0
1
2
0 2 4 6 8
time (sec)
Hα
HeI
gas-puffing
(a.u.)
 
Fig 5. A typical discharge after He-GDC with 
hydrogen gas-puffing. (#26500) 
  
He-GDC Ne-GDC
20nm
H-GDC
 
Fig. 6 Electron diffraction pattern and the dark 
field image of deposition layer formed on 
SUS316L exposed to GDCs. White dots show 
individual crystal grains of deposits. 
EX/P5-34 
  
5 
were probably sputtered from the first wall of LHD (SUS316L). In the case of H-GDC, 
because the energy is insufficient for knock-on damage, the irradiation damage was not 
formed. Furthermore, due to its low sputtering efficiency, re-deposition of impurities is also 
very little (see Fig. 6). 
  
3.2. Retention of deuterium after GDCs 
  Fig. 7 shows TDS of deuterium injected in the SUS316L specimens pre-exposed to 
He-(65h), H-(71.5h) and Ne-GDC(55h). Deuterium irradiation was performed subsequently at 
room temperature with 2keV-D+ up to dose of 1x1022 D+/m2 is plotted in Fig. 7 (a), and 
without pre-exposes case (fresh specimen) was also shown in Fig. 7 (b). In case of the fresh 
one, thermal desorption spectra of deuterium is broad, and desorption is continued from 300K 
to 500 K, but a sharp peak has appeared in the samples exposed to GDCs. The total retention 
is shown in Fig. 8. The fresh specimen has the highest deuterium retention. Namely, it was 
shown that the total retention of deuterium becomes lower by performing GDCs. Its 
mechanism is not understood well at the 
present, but it may have influenced that the 
oxide thin film of the surface was removed 
by performing GDCs. It was reported that an 
oxide film at the surface affects the retention 
and the diffusion of deuterium [6,7].  
He-GDC showed highest deuterium 
retention and Ne-GDC showed lowest 
deuterium retention among the three GDCs. 
In the case of He-DGC, most of the injected 
deuterium ions (2keV) are stopped in the 
heavily damaged layer of about 40 nm (see 
Fig. 4), because their projected range is 
about 20 nm. Therefore, it is considered that 
possible trapping sites for deuterium injected 
in the sample exposed to He-GDC is helium 
bubbles and strong stress field around them. 
It was reported that the stress fields around 
the highly pressurized helium bubbles in 
tungsten act the effective trapping sites of 
deuterium [8]. In contrast with He-GDC, 
Ne-GDC showed lowest retention of 
deuterium. Sufficient amount of trap sites for 
injected deuterium does not exist, because 
radiation induced defects are scarcely 
formed. 
In case of H-GDC, desorption temperature 
shifts to lower side comparing with He-GDC, 
Ne-GDC and even the fresh specimen case. 
Most of the retained deuterium detraps up to 
95 ˚C. We can say that this is one of a surface 
cleaning effect of H-GDC. This result 
indicates that it is possible in LHD to remove 
hydrogen effectively from the surface treated 
by H-GDC by the usual backing at 95 ˚C. 
  
 
300 350 400 450 500 550
0.0
5.0x1017
1.0x1018
1.5x1018
2.0x1018  Fresh specimen
 
 
Temperature [K]
0.0
5.0x1017
1.0x1018
1.5x1018
2.0x1018  After He-GDC
 After H-GDC
 After Ne-GDC
 
 
D
2 
de
so
rp
tio
n 
ra
te
 [D
2/m
2 s
]
(a)
(b)
2keV-D+, 1x1022 [D+/m2]
(2)
(1)
 
Fig. 7 Thermal desorption spectra of deuterium 
(a) with pre-exposes of GDCs and (b) without 
pre-exposes (Fresh specimen). 
  
0.0
2.0x1019
4.0x1019
6.0x1019
8.0x1019
1.0x1020
1.2x1020
1.4x1020
1.6x1020
H-GDC
5.4x1019
1.4x1020
2.9x1019
8.8x1019
Fresh specimenNe-GDCHe-GDC
 
 
To
ta
l r
et
en
tio
n 
[D
2/m
2 ]
 
Fig. 8 Total desorption of deuterium in SUS316L, 
After GDCs and fresh specimen. 
EX/P5-34 
  
6 
3.3. Improvement of GDC 
  Based on the results of the material probe experiments mentioned above, improved GDC 
technique was proposed; a two-step GDC method, combination of a Ne-GDC for efficient 
sputtering of surface contamination and a successive short H-GDC to desorbs Ne retained 
during the first Ne-GDC. At the beginning of the experimental campaign in 2003, this 
two-step method was conducted to shorten the wall conditioning process and suppress the 
undesired desorption of the working gas of 
the GDC. Fig. 9 shows the reduction of 
partial pressure of M=28 (CO) measured by 
quadrupole mass spectrometer during 
He-GDC and Ne-GDC. The decay time for 
Ne-GDC is much shorter and the achieved 
partial pressure was one order lower than 
those for He-GDC. These results are due to 
the high sputtering yield and very shallow 
penetration of Ne-GDC. After the Ne-GDC 
phase, H-GDC was conducted to remove 
implanted Ne in the first wall panels and the 
divertor plates. The partial pressure of Ne 
was reduced to the initial level after about 12 
hours of H-GDC. 
  
4. Summary 
  The material probe experiments were carried out by exposing the SUS316L specimens to 
three different types of GDCs in LHD. Special features of He-GDC were rather strong 
sputtering erosion and heavy damage at the subsurface region; formation of dense bubbles 
with size of 2-20 nm, dislocation loops and cracks connecting the bubbles. The surface is 
covered with large dimples which seem to be formed by exfoliation of blisters. Such a heavily 
damaged structure leads to an increase of the effective surface area of a LHD vacuum vessel 
wall. It was suggested that He-GDC has a disadvantage as a wall conditioning. On the other 
hand, in cases of H-GDC and Ne-GDC, irradiation damages were not observed. However, 
thick deposition layers were observed on the sample after Ne-GDC. 
The influence of the GDCs on the deuterium retention was also examined. The sample 
exposed to He-GDC showed highest deuterium retention while Ne-GDC showed lowest. In 
case of H-GDC, most of deuterium desorbs up to 95˚C. 
  It is considered that Ne-GDC and H-GDC is more suitable for GDC. At the beginning of 
the experimental campaign in 2003, Ne-GDC was conducted for efficient wall conditioning, 
due to its high sputtering efficiency and very shallow penetration. After the Ne-GDC phase, 
short H-GDC was conducted to remove implanted Ne. 
It should be emphasized that understanding of characteristic feature of the microscopic 
processes of damage and surface modification for each GDC with different working gas was 
very effective for improvement of wall conditioning process of LHD. 
  
  
  
  
  
  
  
  
10-10
10-9
10-8
10-7
0 86400 172800
Partial pressure of m=28 (a.u.)
Total GDC time (sec)
He-GDC
Ne-GDC
 
Fig. 9 Partial pressure of m=28 versus total GDC 
time. 
EX/P5-34 
  
7 
Reference 
[1] D. Tafalla, F.L. Tabares, J. Nucl. Mater. 290-293 (2001) 1195-1198 
[2] H. Suzuki et al., J. Nucl. Mater. 313-316 (2003) 297-301 
[3] H. Iwakiri et al., J. Nucl. Mater. 283-287 (2000) 1134-113 
[4] S. Hiroki et al., J. Nucl. Mater. 224 (1995) 293-298 
[5] M. Tokitani et al., J. Nucl. Mater. 329-333 (2004) 761-765 
[6] Y. Ishikawa et al., Vacuum, 47, 6-8, 701-704 (1996) 
[7] T. Hino et al., J. Nucl. Mater. 329-333 (2004) 673-677 
[8] H. Iwakiri et al., J. Nucl. Mater. 307-311 (2002) 135-138 


Microscopic Modification of Wall Surface by Glow Discharge Cleaning and its Impact on Vacuum Properties of LHD

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Microscopic Modification of Wall Surface by Glow Discharge Cleaning and its Impact on Vacuum Properties of LHD

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