29pB02
高密度直線型プラズマ発生装置PISCESにおいてD-He-Be混合プラズマ照射したWの微細組織と重水素リテンション特性
Microstructure and Deuterium Retention Property of W Exposed to D-He-Be
Mixture Plasma in the linear divertor plasma simulators PISCES
宮本光貴1, 西島大輔2，M.J. Baldwin2，R.P. Doerner2，上田良夫3，栗下裕明4，吉田直亮5，相良明男6
Mitsutaka Miyamoto1, Nishijima Daisuke2, M.J. Baldwin2, R.P. Doerner2, Ueda Yoshio3, Kurishita Hiroaki4,Yoshida Naoaki5, Sagara Akio6
1

島大院総理工，2UCSD，3阪大院工，4東北大金研，5九大応力研，6核融合研
1
Shimane Univ., 2UCSD, 3Osaka Univ., 4Tohoku Univ., 5Kyushu Univ., 6NIFS

Microscopic damage and D retention in tungsten have been investigated for W exposed toITER-relevant mixed species (D, He, Be) plasmas in the linear divertor plasma simulators PISCES-A
and -B. It was observed that seeding of He into pure D plasma resulted in a significant reduction of D
retention and suppression of surface blistering. TEM observations and ellipsometric measurements
revealed that nano-sized high-density He bubbles were formed and percolated in the near surface
region. A similar reduction of deuterium retention was also observed by ion irradiation experimentswith an increasing fluence of He ions. Injected D atoms are thought to diffuse back to the surface
through the percolating bubbles during exposure. The seeding of Be into D+He mixture plasma
eliminates this He effect on the reduction in D retention.
1. Introduction
In the ITER DT phase, the burning plasma will
expose tungsten (W) simultaneously to helium (He)and other trace impurities such as C and Be besideshydrogen isotopes. Thus, the effect of these
impurities on hydrogen isotope retention in W needs
to be explored under mixed species plasma
exposure for a more reliable prediction of T
retention.
In this study, systematic mixed species (D, He,Be) plasma exposure experiments on W andcomplementary ion irradiation experiments havebeen conducted to investigate microscopic damage
and hydrogen isotope retention for W in the burning
plasma conditions.
2. Experimental
Mirror-polished W samples were exposed to pure D
plasmas, He-mixed D (D+He) plasmas, Be-mixed D(D+Be) plasmas, or He and Be mixed D (D+He+Be)
plasmas in PISCES-A and –B at the University ofCalifornia, San Diego [1,2]. The He+ (cHe+) and Be+(cBe+) ion concentrations were spectroscopicallydetermined [3], and were set to be ~1-20% and ~0.1%,
respectively. The incident ion energy, Ei, was ~60 or~120 eV, and the sample temperature, Ts, during theplasma exposures was kept fixed at ~573 or ~773 K
by adjusting the rate of flow of forced-air cooling.
Subsequent to the plasma exposure, surface
morphology and internal microstructures wereexamined by means of a scanning electron microscopy(SEM) and a transmission electron microscopy (TEM),
respectively. To investigate D retention properties ofthe bulk samples, gas-desorption profiles weremeasured using a thermal desorption spectroscopy(TDS) system, capable of distinguishing between
mass four He and D2 signals.
In addition to the plasma exposure experiments,
sequential ion irradiation experiments were also
performed with 3 keV-He+ and then 1.5 keV-D+.
3. Results and discussion
Figure 1 shows SEM images of the W samples
exposed to pure D plasma and D+He mixture plasmas
at Ei ~ 60 eV, D ~ 5x1025 m-2 and Ts ~ 573 K. On thesurfaces of stress-relieved (SR-W) and re-crystallized(RC-W) pure W exposed to pure D plasma, highdensity micron-sized blisters are clearly seen, while
the samples exposed to D+He mixture are ascomparably smooth as the surfaces of the unexposedsamples. In contrast, toughness enhanced, fine-grained
W-1.1wt.%TiC (TFG W-TiC) [4] shows no significant
changes in the surface morphology at all for exposure
to both pure D and D+He mixture plasmas. D
retention in TFG W-TiC was much smaller than that inFig.1. SEM images of several W samples exposed topure D and D+He (cHe+ ~ 20%) mixture plasmas
at Ts ~ 573 K, Ei ~ 60 eV, and D ~ 5x1025 m-2.

SR-W and RC-W by around two orders of magnitude
because of no blisters [5]. Since TFG W-TiC contains
more grains than either SR-W or RC-W, it isspeculated that a larger fraction of implanted D atoms
can diffuse back to the surface along grain boundaries
and leave from the surface during plasma exposure.

Fig.4. TDS of D2 from RC-W samples irradiated with
3keV-D2+ ions to fluence of 1x1021 m-2 after thepre-irradiation of 3keV-He+ ions.

Fig.2. TDS of D2 from SR-W samples exposed topure-D and D+He (cHe+ ~ 20%) mixture plasmas
at Ts ~ 573 K, Ei ~ 60 eV, and D ~ 5x1025 m-2.

A similar reduction of D retention was also
observed by sequential ion irradiation experiments asshown in figure 4. Pre-irradiation of 3 keV-He+ above
the fluence of 1.0x1023 He+/m2 caused a large D
desorption peak to disappear, while the desorption
peak increased with increasing the He fluence at alower fluence up to 1.0x1022 He+/m2.

Fig.3. Cross-sectional microstructure of SR-W exposed
to D+He mixture plasma.

In figure 2, thermal desorption spectra of D2 fromthe SR-W samples exposed to pure-D and D+He
mixture plasmas are displayed. D retention in thesample exposed to D+He mixture plasma issignificantly lower than that in the pure D exposed
sample. To clarify the mechanisms of reducing theretained amount of D, near surface cross-sections of
the plasma-exposed samples thinned by ion beamprocessing were examined with TEM. As can be seenin figure 3, the high density He bubbles (circularwhite contrasts) were created in the near surface
region of the sample exposed to D+He mixture plasma.
The bubble density and the mean diameter were
estimated to be 7.6x1025 m-3 and 1.8 nm, respectively,
and then the volume fraction of bubbles in thedamaged layer is ~23%. A comparable value was also
obtainedfrom
opticalmeasurementswithspectroscopic ellipsometry [6]. When a filling fractionof percolating particles exceeds the percolationthreshold of ~16%, the particles mutually percolate
and form an infinitely large cluster regardless of theirsize and alignment [7]. Thus, the bubbles caninterconnect and link between the surface and thedeep region. In fact, the interconnected bubbles wereobserved with TEM [6]. These linked bubbles can act
as a diffusion path to the surface for implanted D
atoms during the plasma exposure, resulting in theobserved significant reduction in D retention.

Fig.5. Total D retention in W samples exposed tovarious plasmas at Ts ~ 573 K, Ei ~ 60 eV, and D~ 5x1025 m-2.

On the other hand, a sample exposed to D+He+Be
mixture plasma at Ei ~ 60 eV and Ts ~ 573 K showedno significant He effects. At this Ei, most of incoming
Be onto the W surface is re-sputtered. Figure 5 shows
the total D retention in samples exposed to the D+Be
and D+He+Be mixture plasmas, compared with those
exposed to the plasmas with no Be seeding. It is foundthat He seeding to D+Be plasma does not reduce Dretention, as opposed to He mixture to pure D plasma.
Thus, one can say that Be seeding has a more
dominant influence on the D retention property inplasma-exposed W than He seeding.
The temperature dependence of microstructures andretention properties will be discussed in thepresentation.
References[1]D.M. Goebel et al., J. Nucl. Mater. 121 (1984) 277.
[2]R.P. Doerner et al., Phys. Scripta T111 (2004) 75.
[3]D. Nishijima et al., Phys. Plasmas, 14 (2007) 103509.
[4]H. Kurishita et al., J. Nucl. Mater. 377 (2008) 34.
[5]M. Miyamoto et al., Nucl. Fusion 49 (2009) 065035.
[6]M. Miyamoto et al., J. Nucl. Mater. 415 (2011) S657.
[7]H. Scher et al., J. Chem. Phys. 53 (1970) 3759.