J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 25V254

Plasma Stainless Steel Wall Interaction of the LHD
SUZUKI Hajime*, OHYABU Nobuyoshi, KOMORI Akio, MOTOJIMA Osamu
and LHD exPerimental grouP 1,2National Institute for Fusion Science, Toki-shi 509-52, Japan(Received: 18 January 2000 / Accepted: 8 June 2000)

Abstract
For steady state plasma operation, the study of plasma wall interaction is very important. The Large

Helical Device (LHD) has a plasma vacuum vessel, which is made of stainless steel. The particle balanceof plasma experiments was studied by a fast ionization gauge (FIG), which is operational in a highmagnetic field. Plasma discharge experiments of the LHD have been carried out using hydrogen andhelium gas. In the case of helium plasma discharge experiments, no or very few missing particles could
be detected. However, in the case of hydrogen plasma discharge experiments, almost half of the input
particles have disappeared. After a hydrogen plasma discharge, gas desorption from the wall has beenmeasured. The gas desorption has two kinds of time constants. Although the particles from the gas
desorption have been added to the balance, still missing particles exist. These stay in the wall for a longtime. This result indicates that two kinds of trapped state of hydrogen particles exist in stainless steel.
Two kinds of diffusion constants for steel below 500 K have been reported. The pressure curves of theLHD experiments consists with this report.

Keywords:
hydrogen, helium, stainless steel, plasma-wall interaction, wall pumping effect, diffusion, lattice defect,hydrogen embrittlement.

steel. However, the hydrogen particles ofthe studies are1. lntroduction
Stainless steel is often used for ultra high vacuum

naturally-exist hydrogen in the steel or introduced byvessels. However, it is known that steel containshydrogen particle in the bulk. Thus, if a stainless steel
vacuum vessel would be used for a plasma vacuum

chemical method, electric method and abrasion method.
It must be studied whether hydrogen particles in steel,which is implanted by plasma, have same feature or not.
The LHD plasma experiment has been carried out
in a stainless steel vacuum vessel using hydrogen or
helium working gas [1]. Wall-pumping effect has beenvessel, characteristics of hydrogen stainless steel
interaction must be studied. From a point of view ofplasma control, absorption and desorption of hydrogenparticles from a stainless steel wall are important. And

studied by investigating the defects in the particle

operation under high energy and high particle flux

balance. The characteristics of trap site of hydrogen in
the steel has been studied by analyzing the pressure
curve of desorption. This study shows where and how
the hydrogen particles are trapped in the stainless steelcondition.

wall of the LHD.

from a point of view of material, a mechanism oftrapping of hydrogen and hydrogen embrittlement areimportant. Especially, they are important for steady state

There have been several studies on hydrogen in*C o rre sp ondin g aut ho r' s e - mail : h s uzuki @ LH D. nifs. ac.j p250

@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchSuzuki H. et al., Plasma Stainless Steel Wall Interaction of the LHD

2. Apparatus

1.4 10-3

3.5 xl0re

The plasma vacuum vessel of the LHD is made of

1.2 l0-3J

stainless steel SUS316L (Fe:.lOVo, Cr:l7Vo, Ni:ll7o,Mo:2Vo), with a thickness of 15 mm. The volumecontained in the vacuum vessel is 210 m3, and the total
area including ports is about 730 m2. Carbon armor tiles
for the neutral beam injectors (NBI) with a surface areaof about 3 m2 were installed. It was only carbon area inthe vacuum vessel during the second campaign of theLHD experiment. The temperature of the vacuum vessel
is kept to about 300 K by cooling water. The plasma
volume is about 30 m3. The ratio between the plasma
volume and the vacuum vessel volume is largecompared with other plasma devices.
The vacuum vessel is pumped out by two cryopumps and four turbo molecular pumps. The total andI l0-3)<e El04
p oro'

1.5 _JF

4 104

I

2 lo4

0.5

0 100

05 10-3

5xlOle4 l0-3$* rrooJ2J!

2 ro-'

I l0-31o looo[00.5 | 1.5.2.2,53 3.5 4trme(sec)Fig. 1 Typical pressure curves of hygrogen shot(a) andhelium shot(b) with line density curves.

partial pressures are monitored by pressure gauges and aquadruple mass spectrometer. One of the pressuregauges is fast ionization gauge (FIG), which is installed

at7.5 U port, about 3 m far from plasma surface. Since
the FIG is operational in a high magnetic field [2], thepressure in the vacuum vessel can be measured.
CAMAC system starts recording the pressure data of theFIG after trigger signal with a sampling speed of 100

power decreases the particle confinement time. After
stopping the heating, the plasma disappears, and thepressure increases by recombination of ions. However

ps, and it stops recording after the plasma discharge isfinished. The ordinary measuring time is 13 sec. So theneutral gas pressure before, during, and after a plasma
discharge is measured with a very high frequency. Using

outgassing from the wall. Then the pressure decays by
pumping of the cryo pumps and the turbo molecularpumps. Fig. 1(b) shows a typical pressure curve of thehelium plasma experiment. In the case of helium, thepressure curve is almost the same as in the hydrogenthe pressure still increases after the plasma has
completely disappeared. This is explained by anthe pressure decay curve, the effective pumping speed
can be determined. The effective pumping speed ofhydrogen gas is 67 m3/s and the effective pumping
speed of helium gas is 17 m3/s.
The hydrogen and the helium gas are introduced
through piezo valves. The voltage of the piezo valve iscase before the heating stops. However the pressure
curve of the helium gas case is different from thehydrogen gas case after the heating stopped. The
pressure rises by recombination then drops a little. This

also recorded, and total amount of inlet gas is calculated

drop may be explained by absorption on a clean surface,which has been produced by helium bombardment

using these data. Therefore the particle balance can beduring the plasma discharge.

The most significant difference between thedetermined.

surface conditioning has been carried out by heliumhydrogen and helium discharge is difference in the gasbalance, which is the result of the wall-pumping. Fig.
2(a) shows pressure curves for the same hydrogen gas

glow discharge or ECR-DC before the plasma

puffing condition with/without plasma. Fig. 2(b)experiments at night.

displays pressure curves in the same helium gas puffing
condition with/without plasma. In the helium case, no orvery small missing particles exist. However in the caseof hydrogen, almost half of the input hydrogen particle

The plasma vacuum vessel is un-bakable, becauseof a protection for the super conducting magnets. A

3. Experimental
The experiments have been carried out usinghydrogen and helium gas. Fig. 1(a) shows a typicalhave disappeared. Fig. 3 is a plot of the total amount of
missing particles after plasma shots as a function of thepressure curve of the hydrogen plasma experiment.
When ECH heating starts, the pressure decreases byionization. The pressure increases a little by NBI
heating. This is the well known effect that heatingmaximum stored energy. There is an energy threshold ofwall pumping at 80 kJ. When the stored energy exceeds
80 kJ, hydrogen particles have always been absorbed in25r

Suzuki H. et al.. Plasma Stainless Steel Wall Interaction of the LHD

6l

1.5 l0-3

:t/

traA

I l0-3

()

E o.r
5 104

r:0.75sec (01,4oLr.r/bra./,* fitting)::i:l|t12;0 sec QilO sgg -fitting)li,lritlrl

t*'1.?...

O'.Qa()

:

E0.01

0 100

l0ti,n"1r."f
2 rc'3Fig. 4 Desorption rate of hydrogen gas from the wall ofthe LHD. f = 0 sec is a same time as f = 1.5 sec ofGt

I l0'30 100

Fig.1(a).

oI

1'5

implantation, and not by surface absorption. Highenergetic hydrogen atoms, which are produced byFig. 2 Pressure curves of hydrogen(a) and helium{b)charge exchange interaction, have been implanted intoexperiment with/without plasma heating underthe wall. The time constant of desorption is about a few

same gas puff conditions.

seconds. If a source of outgassing is the surface, thetime constant should be about l0-r2 sec [3]. This points
out that the source of outgassing is not the surface.
R.Calder and G.Lewin have pointed out that thedesorption rate of hydrogen gas from a stainless steel
could be explained by a diffusion to the surface from theinterior of the metal [a]. They have been calculated thedesorption rate as,

o'5time(sec)l0. | 3r''tr:fir::

O H2 shot 1.....,A Ho shot'''''r"ai

O i:3':ao=rf3"jl.^a ta?

'\dX )r=n4c.DSEtrA!
ET:2500 3000 3500 4000

L\?

\= ;_i-.*o\llr12n+l)l-^\\ o )r, l
4500 5000 5500

where c is the concentration of hydrogen and d is thedepth where hydrogen particles exist. For Dtld2 > 0.025,
which is the case in many practical situation, the desorp-6000 6500

shottion rate is written by,

Fig. 3 Total amounts of missing particles after hydrogenand helium shots as a function of maximum;4cnDt-\stored energy of the shots.

'\d'4 or\IL) =----=. exp 1-dthe wall. No saturation of the wall has been observed
during the second campaign of the LHD experiment.
The total amount of the implanted hydrogen particles

and the time constant is given by T = d2hE2D.

In the LHD case, the desorption rate is shown inFig. 4, which is calculated using the same data of Fig.
l(a) after 1.5 sec. It looks as if two kinds of constantexist with values of 0.75 sec and,22.0 sec, i.e. there aretwo kinds of trapped states of hydrogen in stainlesshave not reach the saturation level during theexperimental time of one day. The saturation level isdiscussed in the next section. The wall conditionings atnight have been effective for removing the hydrogenparticles in the wall, and the wall has been recovered.

steel.

R.C. Frank et.al. have reported two different
diffusion coefficients of hydrogen in steel (C:0.09Vo,Cr:0.01Vo. Mn:0.35Vo. Ni:0.057o. Mn:l.02Vo\ in atemperature range of 25'C-90'C for the same sample[5]. The first coefficient has been measured by a build

4. Discussion
Without plasma, wall pumping has not beenobserved. The wall pumping is obviously caused by252

Suzuki H. et al., Plasma Stainless Steel Wall Interaction of the LHD

up method and is
D = 5.0 x I o-hydrogen saturation level and hydrogen embrittlement.
These are especially important under high energy, highparticle flux steady state operation. Nagumo has pointed.fl3

expl3aWoJlueLl ,^, |
"r"
I.RI]

.

out that an annealing over 500 K is effective forand the second coefficient by a decay method and is
D = | .eox I o-2*pl- Qry-tL]*, r ,",recovering lattice defects [9].

5. Gonclusion.

tFeatures of the wall pumping effect of the stainlesswall of the LHD have been studied using FIG data. Thewall pumping effects have been observed after hydrogenplasma shots with a maximum stored energy over 80 kJ.
In the case of helium shots, no or few wall pumping

R.C. Frank et.al. have pointed out that the two sets ofdiffusion coefficients could not be attributed to an abrasive process. They have also pointed out that the twosets of diffusion coefficients have a temporal dependence. In general, the measured diffusion coefficients
below 500 K vary [5-7].

effects have been observed.
Two kinds of hydrogen trap sites in stainless steel
have been found by analyzing the desorption curves
after the hydrogen plasma shot. This result consists withthe old TDS experiments or measurements of diffusion

Thermal desorption spectroscopy (TDS) analysis ofhydrogen in steel shows a double peak structure [8,9].
The sample of Ref. 8 is SUS3I6L, and the sample ofRef. 9 is pearlite structure steel. M.Nagumo has reports

coefficient of steel.
The implanted hydrogen particles diffuse in thesteel with a low activation energy. Then they trapped ata site with a high activation energy

in Ref. 9 that the first peak decreases dramatically after
48 hour from stopping of solution of hydrogen, keeping
the sample at room temperature. This result also shows atime effect of two kinds of trapped state. Since stainlesssteel is alloy, SUS3I6L is not completely same asFrank's steel. And SUS3I6L is austentic steel inAcknowledgements
The author is grateful to the people in theexperimental group and operators of the LHD,
especially, to Dr. A. Sagara, Dr. Y. Hirooka, Dr. R.
Sakamoto, Dr. J. Miyazawa and Dr. S. Masuzaki forcontrast with Nagumo's pearlite structure steel. In spiteof these differences, characteristics of hydrogen particles
in steel reported by these references coincide with acharacteristic of hydrogen in the LHD case.
A variety of measured diffusion coefficients andsolution ratios of hydrogen in steel below 500 K havebeen reported [9]. These varieties and two kinds oftrapped state are explained by several kinds of trappingsites. The tight trapping site with high activation energy
is considered to be from lattice defects [9,10]. At first,
implanted hydrogen particles are trapped at loose
trapping sites, then diffuse with low activation energy.
Some part of the hydrogen particles reach the surface

useful discussions and help.

Appendix
A diffusion coefficient of SUS316L over 500 K has
been reported by Y. Ishikawa et al.

llll.If the diffusion

coefficient of 300 K would be estimated byextrapolating the data ofRef.ll, the diffusion constantof 300 K is D - l0-rr cm2lsec. It is much smaller thanand are released. The other parts of the hydrogen

Franc's steel. In general, Ti, Zr, Cr, Mn, Si and C makediffusion coefficient smaller. If Cr reaches to l2Vo, theactivation energy becomes two times higher [2]. Thisparticles reach the tight trapping site and stay there. Thismay be a reason why the diffusion coefficient ofis a scenario of hydrogen desorption from the stainlesssteel wall of the LHD
If the hydrogen has been trapped at lattice defects,
the total amount of pumped particle and desorption
curve should be changed historically. However, nosignificant change has been observed during the second
campaign of the LHD experiments. This study should becontinued for a longer term.

SUS3l6L is small.

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253

Suzuki H. er al., Plasma Stainless Steel Wall Interaction of the LHD

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