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Tritium concentration in tungsten surface exposed
to low-energy, high-flux D plasma
K. Isobe , V. Kh Alimov , T. Yamanishi and Y. Torikai

1) Tritium Technology group, Japan Atomic Energy Agency,
Tokai-mura Naka- gun, Ibaraki, 319-1195, Japan
2) Hydrogen Isotope Research Center, University of Toyama
Gofuku 3190, Toyama 930-8555, Japan(Received December 13, 2011; accepted March 9, 2012)

Abstract

The limits on tritium inventory in the vacuum vessel and the need for prevention of impurity
ingress into plasma make plasma-surface interaction on tungsten an important issue. It is well known thatplasma exposure on tungsten makes some kinds of blisters on the surface and increases the hydrogeninventory. On the other hands, there is a possibility that plasma exposure would change the characteristicof surface and surface region in tungsten and cause the increase of tritium inventory. To understand thechange of tritium inventory in the surface region and surface in tungsten by plasma exposure, tritium
concentration in tungsten exposure by low-energy (38eV), high-flux D plasma with was examined with

BIXS after thermal exposure of tritium gas. D plasma exposure was carried out at specimen temperaturearound 495 and 550K. After that, specimens were exposed to gaseous tritium diluted with deuterium at
473K for 3 hours. The tritium concentration was measured with BIXS. The tritium concentration in surface
and surface region was found to be increased by plasma exposure. And its concentration of tungsten
exposed at 495 K was estimated to be twice higher than that of as-received tungsten.

1. Introduction

Tungsten is one of the candidate materials for plasma-facing components in fusion
reactor. The limits on tritium inventory in the vacuum vessel and the need for prevention of
impurity ingress into plasma, make plasma-surface interaction on tungsten an important issue.
Surface morphology and hydrogen inventory of tungsten after plasma exposure have beenstudied with many kinds of plasma condition. Most of the results on hydrogen isotope
retention and recycling for W have been reviewed by Causey and Venhaus,1,2 and Skinner etal.3 It has been found that deuterium plasma exposure to tungsten makes some kinds of blisteron the surface and increases the hydrogen inventory.4-10 On the other hands, there is apossibility that plasma exposure would change the characteristic of surface and surface regionin tungsten and cause the increase of tritium inventory. The authors have studied thedeuterium depth distribution of tungsten with Nuclear reaction analysis and clarified
deuterium depth distribution after D plasma exposure.10 Especially, deuterium was increased
with approaching surface and its concentration was two orders of magnitude higher than thatof inner region at 330 K of the fluence of 1026 D/m2. This result indicated that tritium
concentration in the surface and the surface region that was exposed to plasma is importantwith respect to not only the plasma surface interaction, but also the inventory and thedecontamination of tritium in fusion reactor. In case that the hydrogen inventory of tungsten
surface and surface region would be increased by plasma exposure, tritium inventory in thevacuum vessel should be taken into account not only hydrogen implantation but also theincrease of hydrogen concentration in surface and surface region by plasma exposure.

Beta

ray-induced-X ray spectrometry (BIXS) can measure the tritium concentration of surface
region in nm order due to the short path of beta-rays released from tritium. On the other hand,
this method required tritium and tritium has to be introduced into specimen by using somemethods. We have tried to introduce tritium to specimens with thermal exposure of tritium
gas and to apply BIXS method to measure the tritium concentration of recrystallized tungsten
exposed to D plasma. In this study, tritium concentration of tungsten exposed with
low-energy (38eV), high-flux D plasma was examined with BIXS. Tritium concentration
measurements were carried out not only in the recrystallized tungsten but also in the ITER
grade tungsten that was not exposed to D plasma.

2. Experimental
The tungsten specimens exposed to plasma were the recrystallized tungsten(A.L.M.T. Corp., Japan) with 99.99 wt% purity and major impurities (in weight ppm) of Mo
and Fe around 10, C and O less than 30. The specimens were finally recrystallized at 2073 K
after cut into 10 × 10 × 2 mm and polishing. Fig. 1 shows the schematic diagram of liner
plasma generator used in this study. The plasma was ignited by an electron-emitting filament
made of LaB6 with a shape of a hollow dual-spiral and maintained by an arc discharge power
supply applied between the filament (cathode) and grounded chamber wall (anode), being
assisted by the confinement coil. The
water cooled specimen holder is isolated
from the grounded chamber wall so thatthe specimen can be negatively biased toadjust the energy of ions impinging onto
the specimen. The specimen is passively

Fig. 1. Schematic view of a linear plasma generator
that can produce low-energy and high flux plasmas.㩷

heated by the plasma itself and the temperature rise was adjusted by inserting several micaplates of different thickness between the specimen and the holder. The temperature ismonitored using a type K thermocouple tightly pressed to the backside of specimens. The
ions species can be controlled by adjusting the operational parameters of plasma generator. In
this study, plasma beams highly enriched with a single species of D2+ with a ratio of over
80% were obtained, and the main impurity in the plasma was oxygen with a concentration
less than 1 ppm. The area of the specimen exposed to the plasma was 9 mm in a diameter, andincident flux and energy were fixed at 1022 D+/m2/s and 38 eV/D, respectively.

D plasma

exposure to recrystallized tungsten was carried out at around 495 and 550 K of specimen to
the same fluence of 1026 D/m2. The temperature dependence of deuterium inventory oftungsten exposed such conditions have been reported.10 After plasma exposure, the surface oftungsten specimen was observed with scanning electron microscope (KEYENCE VE-9800)
at a tile angle of 45 o.
Tritium was introduced into the specimens thermally by exposure to hydrogen gas
containing tritium. Fig. 2 shows the schematic view of tritium exposure apparatus. The
tritium exposure apparatus consisted of a vacuum pumping system, a quartz tube for
specimen heating and a tritium source. The tungsten specimens, the tungsten after plasma
exposure, as received recrystallized tungsten (non-exposure) and ITER grade tungsten, wereplaced in quartz tube of specimen
QMS

heating. ITER grade tungsten (A.L.M.T.

CM

IG

Chamber

Corp., Japan) used in this study werewith over 99.96 wt% purity and major
impurities (in weight ppm) of Mo and Fe

TMP

SIP

RP

around 3, C and O less than 10. The

Tritium source Specimen

specimens were finally annealed forFig. 2. Schematic view of tritium exposure apparatus.㩷

stress relief after cut into 10 × 10 × 2 mm and polishing. Glass wool were inserted between
each specimen to avoid the contact of each specimen. After the installation of specimen in theheating tube to the apparatus, specimen tube and the apparatus except the tritium source were
evacuated by the vacuum pumping system to less than 10-4 Pa. Before the tritium exposure,specimens were heated at 473 K for degassing the quartz tube, the glass wool and specimens.
At this temperature, we have already confirmed that deuterium in tungsten was not released
to the vacuum by thermal desorption spectroscopy. After the high vacuum was obtained, avalve above the specimen heating tube was closed and the tritium source was also heated up
after opening the valve above tritium source. Tritium in the source was diluted with
deuterium and the tritium concentration is 7.8 %. During the heating of tritium source, thetritium was released into the chamber equipped with CM. The pressure of hydrogencontaining tritium reached around 1 kPa, the valve above tritium source was closed. And
tritium released into the chamber was introduced into the specimen heating tube by opening
the valve above specimen. The exposure of tritium was kept 5 hours. After tritium was
re-stored into the tritium source sufficiently, specimens were removed from the tritium
exposure apparatus.
BIXS measurement was carried out in the chamber in which Ar gas flowed
continuously. Details of the BIXS measurement have been reported.11 Thin Be plate was usedas the window of X-ray detector (CANBERA SSI 50150 Model), because of the energy of
X-ray induced by tritium is very low. BIXS measurement of each specimen was continued
until the peaks appeared clearly.

3. Results
Fig. 3 shows the results of surface observations with SEM after D plasma exposure.
On the surface of tungsten exposed at 495 K, two kinds of blister appeared. One type hadlarger diameter over 10 µm, and the other had small

495K

diameter under 1µm. On the other hand, there wasonly one kind of blisters, its diameter was under 1 µm,
on the surface of tungsten exposed at 550 K. This

20µm

temperature dependence of blister morphology has
been already reported in [10] and these observations

550K

suggested the deuterium inventory of tungsten in thisstudy was the same as the previous study. According
to previous results, a maximum deuterium inventory

10µm

at the 1026 D/m2 fluence is 7×1021 D/m2 around 500 K.

Fig. 3. Surface observations result after D

Over 500 K, the deuterium inventory decreased

plasma exposure

drastically. And deuterium inventory at around 550 K was one order of magnitude smaller
than that for 500 K. Therefore, the deuterium inventory at the exposure temperature of 495 K
was considered to be higher than that of 550 K.
BIXS measurement results of tungsten exposed to D plasma, as-received
recrystallized tungsten and the ITER grade tungsten were shown in Fig. 4. The horizontal
axis is the energy of X-rays and the vertical axis is X-ray count rate. Since the BIXS
measurement time was wdifferent from each other, X-ray count was normalized into counts
per second. Two peaks appeared at theenergies of 1.8 keV and 3.2 keV in X-ray
spectrum. The peaks at 1.8 keV and 3.2
keV are characteristic X-rays of W (Mα)
and Ar (Kα) respectively. Beta-ray
releasedfromtritium

inducedcharacteristic X-rays depending on its
Fig. 4. BIXS measurement results of each specimen

depth. Beta-ray released from tritium on the surface can induce Ar atom in the chamber.
Therefore, Ar(Kα) depends on tritium concentration from surface to beta-ray escape depth intungsten. On the other hand, beta-ray released from tritium in tungsten can excite W atom.
Therefore, peak intensity of W (Mα) x-rays depends on tritium concentration in tungsten
from surface to tungsten bulk. Here the maximum depth of tritium inducing Ar(Kα) andW(Mα) x-rays was estimated to be in 20 nm and 500 nm, respectively.11 So that two peaks asshown in Fig. 4 were obtained by BIXS measurement, tritium existed in tungsten surface to
20 nm depths and to 500 nm depths were obtained successfully by the method of this study.
Especially, tritium concentration of surface region of tritium existed in 20 nm depths and oftritium in surface could be clarified by this measurement.

According to the count rate ofeach peak, the count rate of the tungsten exposed at 495 K was the highest. And the count rateof the tungsten exposed at 550 K was the second one. The count rate of ITER grade tungsten
was slightly higher than that of as-received recrystallized tungsten. This result agrees with theinventory and NRA results of previous paper.10 Therefore, BIXS measurement was shown its
performance to obtain the tritium concentration on surface and surface region. From the resultof fig. 4, tritium concentration in surface region and surface was found to be increased byplasma exposure. In other words, plasma exposure increased the amount of tritium trappingsite in tungsten surface region and surface. Therefore, the result of fig. 4 suggested thatplasma exposure would increase the tritium inventory of tungsten by not only hydrogen
implantation but also the increase of the amount of hydrogen trapping site. The reason of theincrease of trapping site could not be clarified in this study. To understand reasons of theincrease, the surface condition after plasma exposure and tritium distribution should beclarified in detail.
Here, hydrogen concentration in surface region and surface can be estimated fromthe count rate of Ar(Kα) x-rays. The count rates from 2.746 keV to 3.404 keV were chosenfor the start of the Ar(Kα) peak and the end of the peak respectively. The count rate
subtracted background was summed up between start of the peak and end of the peak. After
that, tritium concentration was estimated by radioactivity of Ar(Kα) x-rays. Tritium
concentration of as-received tungsten is 3×1014 T/cm2, tungsten exposed at 495 K is 7×1014
T/cm2 and tungsten exposed at 550 K is 4×1014 T/cm2, respectively. From this estimation, it
was found that the tritium concentration of tungsten exposed at 495 K is about twice higher
than that of as-received tungsten and the tritium concentration of tungsten exposed at 550 K
is slightly higher than that of as-received tungsten. This estimation of tritium concentration
also indicated that plasma exposure increased hydrogen trapping site in surface and surface
region and suggested that the amount of trapping site by plasma exposure was reached twice
higher than that of as- received tungsten .

4. Conclusions
To understand the change of the surface region and surface in tungsten by plasma
exposure, we have introduced tritium in tungsten specimens by thermal exposure to tritium
gas and applied BIXS method to measure the tritium concentration of recrystallized tungsten
exposed to D plasma. Two peaks were observed in BIXS measurement and tritium
concentration in the surface region and surface in tungsten were successfully obtained.
Tritium concentration in the surface region and surface was found to be increased by plasma
exposure. And its concentration of tungsten exposed at 495 K was estimated to be twice
higher than that of as-received tungsten.

Acknowledgement
A part of this work is performed with the support and under the auspices of theHydrogen Isotope Research center, University of Toyama Collaborative research program.

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