S4-4

Impact of Formation of Codeposition Layer
on Global Wall Pumping in the Fusion Experimental Device
核融合プラズマ装置における共堆積層の形成とグローバルな壁排気への効果
M. Sakamoto
坂本瑞樹

Advanced Fusion Research Center, Research Institute for Applied Mechanics, Kyushu University,Kasuga, Fukuoka, 816-8580, Japan
九州大学 応用力学研究所 炉心理工学研究センター，〒816-8580 福岡県春日市春日公園6-1
A global wall pumping is important from the viewpoint of the density control and the tritium inventory inthe steady state operation of fusion plasma. The wall pumping rate was estimated by using a particle
balance model. A transition of the wall role from the particle sink to the source was observed. It seems todepend on the wall temperature. When the increase in the wall temperature was suppressed, the net wall
pumping rate was enhanced to about 3.6 times higher than that of higher wall temperature. The continuous
wall pumping is attributed to the co-deposition of hydrogen with the sputtered metal. The codeposited
metal layer is very important similarly to the codeposition of hydrogen with carbon.

1. Introduction
Steady state operation (SSO) is one of the mostimportant issues for the future fusion reactor. As adischarge duration grows longer, phenomena withlong characteristic time such as current diffusionand plasma-wall equilibrium times play more
important role for the SSO. The particle balance inthe main chamber is one of those and it is importantfrom the viewpoint of the density control. The wall
plays both roles of the particle sink and the source.
The wall temperature is a key for the particle source
and the co-deposition is a key for the particle sink.
It should be noticed that the formation of theco-deposition layer changes the wall surface
properties such as particle absorption.
In Tore Supra, an uncontrolled density increase
due to outgassing from uncooled and poorly
conditioned in-vessel components had been
observed in the high power and long duration
discharge, but it has not been observed andcontinuous wall pumping has been observed after
upgrading the plasma facing components (PFCs)[1-3]. Also in TRIAM-1M, the importance of thewall temperature and the codeposition for the global
particle balance has been reported [4-7].
2. Particle Balance Model in the Main Chamber
A particle balance equation in the main chamberis written by the following equation:dN H0 dN Hp+= Γ fuel − Γ pump − Γwall ,dtdt(1)

Fig. 1 Schematic diagram illustrating the particle
balance flow.

where N H0 is the total number of hydrogen neutral
atoms in the main chamber, N Hp the total numberof hydrogen ions in the plasma, Γfuel the fueling rate,Γpump the pumping rate of the external pump-unitand Γwall the net wall pumping rate. A schematic
diagram illustrating the particle balance flow isshown in Fig.1. The net wall pumping rate means
the balance between the hydrogen absorption rate
Γab and hydrogen release rate Γre.
3. Transition of Wall Role from Sink to Source
Figure 2(a) shows the time evolution of the wallinventory in the ultra-long discharge in TRIAM-1M(stainless steel wall and molybdenum limiters anddivertor plates). The horizontal axis indicates thetime from the plasma initiation. In the first 30 min,the wall pumped the hydrogen, i.e. it played a roleof the particle sink. The averaged wall pumping rateis evaluated to be 2.4 x 1016 atoms m-2 s-1 from theequation (1). At t~30 min, the role of the wall

(a)

76675

210-1012Time (h)3104Temperature (oC)

Temperature (oC)

500064201234Time (h)12Time (h)35680103(b)100

4Fig.2 Time evolution of (a) wallinventory and (b) wall temperature.

228010315076675

(b)

80150

10(a)

Total retention[D/m2]

3

Wall inventory (10 21 H)

Wall inventory (10 20 H)

4

1021

● Mo+O on SS (R.T.)■ Mo+O on SS (473K)▲ Mo on SS (R.T.)◆ Mo Bulk (R.T.)▽ SS (R.T.)

0% o n10 nti
tere
101001020

1910201021

Fluence[D/m2]

102250001234Time (h)56Fig.3 Time evolution of (a) wallinventory and (b) wall temperature.
The wall temperature is suppressedby the insert of the movable limiter.

transited to the particle source. After that, the wall
released the hydrogen until the end of the discharge.
During the discharge, the wall temperature
increased due to the heat load from the plasma as
shown in Fig. 2(b). It is considered that thetemperature rise in the wall is attributed to thetransition of the wall role from the particle sink tothe source.
On the other hand, when the increase in the wall
temperature is suppressed by the insert of themovable limiter, the wall inventory continue toincrease until the end of the discharge (t ~ 5 h 16min) as shown in Fig.3. In this discharge, the net
wall pumping rate is ~8.6 x 1016 atoms m-2 s-1 and itis 3.6 times higher than that of Fig.2. It means that
Γre is suppressed due to the less temperature
increase and Γab does not saturate.
4. Impact of Codeposition on the Wall Pumping
As shown in Fig.3, the wall continues to absorb
the hydrogen during the ultra-long discharge. One
possible candidate for this continuous wall pumping
is codeposition of hydrogen with Mo, which is
sputtered by high energy neutrals. The
co-deposition layer is expected to be always formedon the wall surface during the discharge. Namely,
the wall surface can be refreshed and keep up thewall pumping capability due to the codeposition.
In practice, the deposition layer was observed on
specimens which were inserted in the scrape off
layer of the plasma and was exposed to theultra-long discharge. Moreover, a reasonableamount of hydrogen was also observed in thedeposition layer of Mo [8].
Figure 4 shows the fluence dependence of theFig.4 Fluence dependence ofthe deuterium retention for Modeposit in oxygen atmosphere.
Deuterium retention in bulk
Mo, Mo-deposit in highvacuum and SUS304 are alsoplotted [9].

deuterium for Mo deposit in the laboratory
experiment [9]. The Mo-deposit in oxygen
atmosphere has a capability of one order ofmagnitude stronger retention than that of Mo bulk.
The structure of Mo-deposit observed inTRIAM-1M is similar to that in oxygen atmosphere.
It is a fine fcc-like grain structure. It suggests thatoxygen impurity in the plasma affects on theformation of codeposition layer of Mo.
Codeposition of hydrogen with metal makes asubstantial wall pumping. Codeposited metal layer
is important similarly to codeposition of hydrogenwith carbon.
Acknowledgements
The author would thank members of the TRIAM
group and also thank Prof. Yoshida of Kyushu Univ.
and Dr. Miyamoto of Shimane Univ. for theirvaluable discussions and supports.
References[1] Equipe Tore Supra (presented by F. Saint-Laurent),Nucl. Fusion 40 (2000) 1047.
[2] J. Jacquinot, et al., Nucl. Fusion 43 (2003) 1583.
[3] D. van Houtte, et al., Nucl. Fusion 44 (2004) L11.
[4] M. Sakamoto, et al., Nucl. Fusion 42 (2002) 165.
[5] M. Sakamoto, et al., J. Nucl. Mater. 313-316 (2003)
519.
[6] Y. Hirooka, et al., J. Nucl. Mater. 313-316 (2003)
588.
[7] M. Sakamoto, et al., Nucl. Fusion 44 (2004) 693.
[8] M. Miyamoto, et al., J. Nucl. Mater. (to be published)[9] M. Miyamoto and N. Yoshida, J. Plasma Fusion
Research 77 No.9 (2001) 888.