24P049-P
Hydrogen gas and plasma driven permeation in counter directions
through the first wall of a DEMO reactor
実証炉第１壁用低放射化合金の水素気体・プラズマによる双方向透過挙動① フェライト系低放射化合金のプラズマ照射透過挙動
Yoshi Hirooka, Haishan Zhou, Naoko Ashikawa and Takeo Muroga
廣岡慶彦, 周 海山，芦川直子，室賀健夫
National Institute for Fusion Science322-6 Oroshi, Toki, Gifu509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
The first wall backed by a breeding blanket structure of a DEMO reactor can be subject to hydrogenpermeation in two counter directions because on the one side it is exposed to hydrogen isotope plasmas in
the edge region but is in contact with tritium dissociated from the breeding coolant. As the first stage of theinvestigation of bi-directional hydrogen permeation, a reduced activation ferritic steel alloy (F82H), acandidate first wall material, is subject to hydrogen plasma-driven permeation in a laboratory-scale linear
plasma device: VEHICLE-1 at ion bombarding energies up to 100eV, and at temperatures up to 500 oC.
Resultant permeation behavior has been analyzed using the DIFFUSE code.

1. Introduction
In recent DEMO reactor studies such as FFHR
[1], the breeding material serves as a coolant aswell in the blanket structure. The first wall isthen defined as the plasma-facing surface of theseblankets. For efficient tritium recovery and heatexchange, the breeding blanket is required tooperate at rather high temperatures. However, it
is also true that to reduce thermal stress, the firstwall thickness is limited to a certain level, which
is often less than 1cm.
Under these conditions, the first wall can be
subject to bi-directional hydrogen permeation by
two separate mechanisms because on the one side
it is exposed to the edge and on the other side it isunder the pressure of dissociated tritium as highas ~10 Torr [2]. One predicts that bi-directionalhydrogen permeation will occur simultaneously
but in a yet-to-be explored manner. If
plasma-driven permeation dominates, deuterium
flowing into the blanket will hinder the recoveryof tritium and will probably necessitate isotope
separation as well. On the other hand, if theopposite is true, gas-driven permeation will resultin an increase in first wall recycling such that
R>1, which can lead to confinement instabilities.
In the present work, as the first stage of theinvestigation of complex bi-directional hydrogentransport phenomena, plasma-driven permeation
through a reduced ferritic steel alloy has beenstudied, using a laboratory-scale, well-diagnosed
steady-state linear plasma device.

2. Experimental
Used in this work is a steady-state linearlymagnetized device: VEHICLE-1[3] with a 1kW
ECR plasma source. A schematic diagram of thisdevice is shown in Fig. 1(a). Typically, theplasma density is of the order of 10101/cm3 and theelectron temperature is around 3eV. The hydrogen
flux is of the order of 10161/cm2/s, typical of thefirst wall environment, and the ion bombarding
energy can be controlled by DC-biasing between
the target and the test chamber up to a few 100V.
VEHICLE-1 is equipped with a variety ofdiagnostics important for plasma-wall interactions
research, including a Langmuir probe, total pressuregauge, partial pressure gauge, visible spectrometer,
and an IR pyrometer, all set aiming at the target
region.
In the test chamber of VEHICLE-1, a
plasma-driven permeation setup is installed and it
is illustrated in Fig. 1(b). Permeation targets are
made of reduced activation ferritic steel: F82H
and also SUS316 stainless steel as the reference,both in the form of circular disk with the diameterof 70mm. The central circular portion of thesetargets with the diameter of 30mm is machined
down to 0.5-1mm to allow hydrogen permeation.
In all the permeation experiments, as will beshown, the plasma column diameter is set at around
70mm by a donut limiter and the axial magnetic
field. This means that the effect of 2D diffusion inparallel to the surface is negligible unlike other“pencil beam” experiments.

(a)(b)Steady state
plasmaValve

Electricalinsulator

VEHICLE-1
test chamber

A typical permeation break-through curve, takenfrom an F82H target, is shown in Fig. 3. In thisparticular case, the ECR power is set at 500W, theplasma bombarding flux is about 3 x 10161/cm2/s
and no DC bias is applied. However, because of theheat flux associated with plasma bombardment, thetarget temperature increases gradually, reaching a
steady state of about 200oC. Notice that the steadystate is attained after about 25min, indicative ofrather rapid diffusion of hydrogen in F82H, asexpected from the database[5]. The steady state
permeation flux evaluated from downstream partialpressure gauge measurements is about 6 x 1013
H-atoms/cm2/s, which has been found to agree with
the DIFFUSE code [3] calculation executed undersimilar conditions to those used for experiments.

Permeation
targetPartialpressuregaugeTurbo
molecularpump

Fig. 1 (a) The VEHICLE-1 device [3]; and (b) thecurrent plasma-driven permeation setup.

3. Results and discussion
Shown in Fig. 2 is a photo of an F82H targetbombarded with hydrogen plasma. Notice that thetarget is attached with a thermocouple in theplasma-facing surface. At the same time, thesurface temperature is monitored by an IRpyrometer. As can be seen, the plasma diameter is
set about the same as the target diameter whereaspermeation occurs only in the central portion shownby the dotted circle.

Fig. 3. A permeation breakthrough curve for F82H.

3. Summary and future plans
A new plasma-driven permeation setup has been
installed in VEHICLE-1. Preliminary data on a
reduced activation ferritic steel alloy have beentaken successfully.
Future plans up until the plasma 2011conference
include;1. A comparison between the permeation behavior
through SUS316 stainless steel and F82H willbe made.
2. The effect of DC-biasing will be investigated.
3. Permeation behavior at higher plasma
bombardment fluxes will also be investigated.
4. More detailed data analysis by the DIFFUSE
code will be done;
References[1] A. Sagara et al., Fusion Technol. 39(2001)753-757.

Fig. 2 Hydrogen permeation experiments inVEHICLE-1.
The dotted circle area with adiameter of 30mm is the thinned permeation area.

[2] G. A. Esteban et al., J. Nucl. Mater. 335(2004)353-358.
[3] Y. Hirooka et al., J. Nucl. Mater. 337-339(2005)585-589.
[4] M. I. Baskes, “DIFFUSE83” Sandia Rep. SAND83-8231.
[5] B. L. Doyle and D. K. Brice, J. Nucl. Mater.
122-123(1984)1523-1530.

(a