Plasma and Fusion Research: Regular Articles

Volume 4, 004 (2009)

Visualized Blow-oﬀ from Helium Irradiated Tungsten in Response
to ELM-like Heat Load∗)
Shin KAJITA, Noriyasu OHNO1) , Wataru SAKAGUCHI1) and Makoto TAKAGI1)
EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan1)
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan(Received 28 November 2008 / Accepted 25 December 2008)
Laser-induced blow-oﬀ from a tungsten surface that was exposed to helium plasma is investigated experimentally in the divertor simulator NAGDIS-II. The pulse width of the laser is submillisecond and is similar to theduration of type-I edge localized modes in ITER. The temporal evolution of blow-oﬀ particles, which are visualized by the electron impact excitation in the surrounding plasma, is investigated by using filter spectroscopy. We
demonstrate the eﬀect of helium irradiation damages on the tungsten ejection behavior in response to a transientheat load.
c 2009 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: NAGDIS-II, helium irradiation, tungsten, transient heat load
DOI: 10.1585/pfr.4.004

1. Introduction

surface roughness [4]. Moreover, in the case of a fiberform
nanostructure, it is interesting to note that the structure easily melts and is destroyed even when irradiated with a weaklaser pulse [11]. This is because the eﬀective thermal conductivity is decreased significantly by the fine structure [5].
Although the modification of the surface nature has been
discussed on the basis of the experiments, the impurity release from the material in response to a transient heat load
like ELMs has not been investigated as yet. Since the massloss or quantity of the injected impurity in response to transient heat load aﬀects the lifetime of the material and theperformance of the core plasma, it is important to investigate the characteristics of impurity ejection from a heliumirradiated surface.
In this study, we investigate the eﬀects of a transientheat load on a helium-irradiated surface by using submillisecond laser pulses in a linear plasma device. Since thelaser pulses are injected while the target material is exposed to the plasma, a laser-induced blow-oﬀ is visualized
by the surrounding plasma, that is by the electron impact
excitation followed by the emission, which can be detectedby using a fast camera. From a comparison of the emission of tungsten neutral WI in response to the transientheat load, we will discuss how the damages caused by thehelium irradiation aﬀect the impurity release from the material.

In ITER, tungsten is a candidate material for the divertor plate and the first wall. This is because tungsten
has good thermophysical properties, a high melting point,
a low sputtering rate, and a low tritium inventory. Moreover, tungsten is assumed to be an in-vessel mirror material
because it has a rather good optical reflectivity in a wide
wavelength range [1]. It is known that helium bombardment leads to helium-induced damages accompanying microstructural evolution, such as dislocation loops, helium
holes/bubbles [2], and fiberform nanostructures [3]. Since
helium is produced by a nuclear fusion reaction, it is necessary to take into account the eﬀect of helium bombardmentin future fusion plants. The helium irradiation damages
could result in decreases in the optical reflectivity [4] andin the thermal conductivity near the surface [5], and additionally, an increase in the hydrogen retention [6]. An important issue concerning the helium irradiation damages isthe change in the eﬀects of the transient heat load accompanied by ELMs (edge localized modes) and disruptions
on the material.
From the laser irradiation experiments, it has been revealed that, when using a short nanosecond laser pulse,
the surface roughness is significantly enhanced [7] and thethreshold laser pulse energies for melting and ablation decrease because of submicrometer-sized bubbles [8, 9]. In
contrast, in the case of a long submillisecond laser pulse,
the pulse width of which corresponds to the duration of
typical type-I ELMs in ITER, it has been found that thesurface roughness decreases [10], and the optical reflectivity considerably recovers because of the decrease in the

2. Experimental Setup and Calculation
Experiments were performed in the linear divertor
plasma simulator NAGDIS (NAGoya DIvertor Simulator)II [12, 13]. Figure 1 shows the schematic representationof the cross section of the experimental setup. Helium

author’s e-mail: kajita@ees.nagoya-u.ac.jp∗) This article is based on the invited talk at the 25th JSPF Annual Meeting(2008, Utsunomiya).

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response to the laser pulse was calculated numerically bysolving a heat conduction equation. The details of the numerical method can be found in Refs. [5, 14]. As shownin Fig. 2, temperature dependences of the thermophysicalproperties were taken into account. As for the thermal conductivity and specific heat, the data in Ref. [15] were used,
and for density, we extrapolated the data in Ref. [16] to themelting point. Moreover, it is known that optical reflectivity decreases with temperature; therefore, this eﬀect wasalso taken into account on the basis of experimentally obtained value [17].

Fig. 1 Schematic representation of the experimental setup in
the divertor simulator NAGDIS-II. The emission of theejected particles from the specimen in response to thelaser pulse is observed by high-speed camera.

3. Results and Discussion
3.1 Temporal evolution of emission
Irradiation experiments were performed under three
diﬀerent experimental conditions, cases (i)-(iii). Deuterium plasma was used in case (i), while helium plasma
was used in cases (ii) and (iii). The incident ion energy forcases (i)-(iii) were 40 eV, 15 eV, and 50 eV, respectively,
and the surface temperatures for cases (i), (ii), and (iii)
were 1200 K, 1730 K, and 1780 K, respectively. In the deuterium plasma, because the surface temperature was considerably high, blistering did not occur. Thus, there was nochange in the surface morphology in case (i), while damages due to helium irradiation were observed in cases (ii)and (iii).
In Fig. 3, images show the tungsten emission profiles
in response to the laser pulse. Figures 3 (a)-(a”), (b)-(b”),
and (c)-(c”) correspond to cases (i), (ii), and (iii), respectively. The temporal evolutions of the emission intensity
in the three cases are shown in Figs. 3 (d), (e), and (f), respectively. The emission intensity was obtained by integrating the CCD count in the emission area, approximately
15 mm × 15 mm in front of the laser-irradiated side of theplate. Because of the exposure to the helium plasma, helium holes/bubbles are formed in case (ii), and fiberform
nanostructure is formed in case (iii), as shown in Figs. 3 (e)and (f). It is suggested that the main emission originatesfrom the electron impact excitation of the ejected tungsten neutrals by the surrounding plasma. The laser ablation process that produces high-density plasma did nottake place because the laser power in the experiments was∼109 Wm−2 , which was suﬃciently lower than the typicalrequired laser power for ablation of >1013 Wm−2 [18]. In
case (i), the emission region is smaller than those in cases(ii) and (iii). This could be attributed to the facts that theplasma density in the deuterium plasma was an order ofmagnitude lower than that in the helium plasma and thatthe tungsten emission could be enhanced by the helium irradiation in cases (ii) and (iii). Further discussion about
the diﬀerences among the three cases is provided later.
It is interesting to note that temporal evolution ofemission intensity has a peak around 0.1-0.2 ms in case(iii), whereas the peak in case (i) is at approximately
0.3 ms. Figure 4 shows the calculated temporal evolutions

Fig. 2 Temperature dependences of the thermal conductivity,
specific heat, and density used in the calculation.

or deuterium plasma was produced in the source region
by dc arc discharge, and a specimen was exposed to theplasma in the downstream test region, as shown in Fig. 1.
The electron density and the electron temperature in thehelium plasma were 2 × 1019 m−3 and 6 eV, and those in
the deuterium plasma were 2 × 1018 m−3 and 6 eV. Powder metallurgy tungsten with a thickness of 0.2 mm wasused as the specimen. Ruby laser pulses were injectedto the vacuum vessel through a viewing port. The pulse
width was approximately 0.6 ms, and the pulse energy was< 5 J. By focusing the beam with a lens, the pulse energy per unit area was controlled up to several MJm−2
at the specimen. The emission from the ejected tungsten neutral particles (λ = 400.9 nm) was detected with
an image-intensified CCD (charge coupled device) camera (Photoron: FASTCAM-MAX 120K I2) through a narrow band-pass optical filter. The frame rate was 30 000 fps(frames per second) in the experiments. The surface temperature was measured by using a radiation pyrometer.
The temporal evolution of the material temperature in
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Volume 4, 004 (2009)of the surface temperature. For case (i), the reflection of
the laser beam, which was typically 50%, was taken into
account, but, for case (iii), it was assumed that all the energy was deposited on the surface because the surface reflectivity became almost zero. The fiberform nanostructure had so complicated structure that the photons thatentered the surface were not reflected back. The calculated surface temperature had a peak at approximately 0.30.4 ms, which was consistent with the observed temporal
evolution of the emission intensity in deuterium plasma of
Fig. 3 (d). In case (iii), on the other hand, the emission intensity is not consistent with the temporal evolution of thecalculated temperature. Since the fiberform nanostructure
would drastically decrease the thermal conductivity near
the surface, an anomalous temperature increase could takeplace [5] in case (iii). However, even if such eﬀects weretaken into account in the calculation, it was diﬃcult to obtain a result that had a temperature peak at approximately
0.1-0.2 ms. We suspected that the fiberform structure wasanomalously heated even in the earlier phase. The anomalous heating would destruct the fine structure, and a part of
the heat might be used for the sublimation process, which
consumed considerable heat; consequently, the emission
decreased from at approximately 0.3 ms even if the laser
power was almost maximum at that time. In case (ii), theemission duration was longer than that in cases (i) and (iii),
as shown in Fig. 3 (e). It was speculated that the bursting of helium bubbles would take place and result in sometungsten droplets [9] even after the laser pulse irradiation.
Because the helium bubbles could burst even if the temperature was lower than the melting point [8], the burstingof bubbles could take place while the laser heat was conducted toward radially in an outward direction from thelaser-irradiated area.

Fig. 3 Images of tungsten neutral emission profiles at diﬀerent
times (a)-(a”) in case (i), (b)-(b”) in case (ii), and (c)-(c”)
in case (iii). (d), (e), and (f) show the temporal evolutions of the emission intensity in cases (i), (ii), and (iii),
respectively.

3.2 Pulse energy dependence
Figures 5 (a), (b), and (c) show the laser pulse energy
dependences of the emission intensity in cases (i), (ii), and(iii), respectively. In cases (ii) and (iii), the specimen was
exposed to the helium plasma before the laser irradiationfor 1200 and 4900 s, respectively. The relative optical reflectivity against the initial value, which was measured byusing a He-Ne laser [4], decreased because of the plasma
irradiation to 34% in case (ii), and to almost 0% in case(iii). The laser pulse energy was gradually increased toapproximately 2.0 MJm−2 . In case (iii), we irradiated thelaser pulse after the relative reflectivity decreased to lessthan 0.1 because of the helium plasma exposure. The timeinterval between the laser pulses was several minutes. In
case (ii), the time interval between the laser pulses wasapproximately 5 min. Thus, since the time interval was
shorter than the time required to form the micrometer-sizedhelium bubbles in case (ii), it should be noted that the surface condition could be changed between laser pulses. In
case (i), intensity gradually increased from approximately

Fig. 4 Calculated surface temperature evolution in response to
the laser pulse irradiation under two diﬀerent conditions.

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Fig. 6 Laser pulse energy dependences of the calculated maximum surface temperature in three diﬀerent cases.

Fig. 5 (c), the intensity at 1.5 MJm−2 was extraordinarily
larger than the others. This was because arcing was initiated in response to the laser pulse and this increased theemission intensity anomalously. It was found that the fiberform nanostructure made it significantly easier to initiate
arcing [19].
The pulse energy dependence of the emission did notchange considerably in cases (ii) and (iii) except for during arcing, although we had assumed that the fiberform
structure could significantly decrease the threshold pulse
energy for the emission in these cases as compared to thecase with bubbles. Indeed, the modification of the surface
nature and the local temperature response were considerably diﬀerent in the case of the fiberform structure and thatof a surface with bubbles [5]. However, with respect to theemission, that is, the ejection of material, the bursting ofhole, which could take place even if the surface temperature was considerably lower than the melting point, could
increase the emission by forming microdroplets. Therefore, we should say that it was not easy to conclude thediﬀerences between the two cases regarding with the impurity ejection, although we could say that the damages
due to helium irradiation enhance the ejection in both thecases.
Figure 6 shows the calculated maximum temperature
as a function of the laser pulse energy in three diﬀerent cases. We used the surface temperatures of 1200 K,
1720 K, and 1780 K, which corresponded to the cases (i)(iii). The optical reflectivity for a clean surface was usedin case (i), whereas half of this reflectivity was used incase (ii), and no reflection was assumed in case (iii). We
should say that the optical reflectivity in the actual situation was an unknown factor, and it could change while thelaser was irradiated because of a change in the morphology, as shown in Ref. [4]. However, the calculation withthe above assumptions in the reflectivity can provide us

Fig. 5 Laser pulse energy dependences of the emission intensity in cases (i)-(iii). The pulse energy was gradually increased to approximately 2 MJm−2 .

1 MJm−2 . In cases (ii) and (iii), it seemed that the intensity increased from a slightly lower pulse energy, typically
0.5 MJm−2 . Moreover, emission was observed at a considerably lower pulse energy: 70 kJm−2 and 150 kJm−2 forcase (ii) and 150 kJm−2 for case (iii). We believe that thedamage due to helium irradiation decreases the onset pulse
energy of the emission. In case (ii), the surface contains
submicrometer-sized holes. Thus, it is possible that the
bursting of these bubbles decrease the threshold laser pulse
energy. In case (iii), the anomalous increase in the temperature of the fiberform structure could lead to the reductionin the threshold pulse energy. In the experiment, the laser
pulses irradiated to the same position of the specimen. The
helium irradiation damage could be recovered by the laser
irradiation; moreover, the damage could be formed in between the laser pulse irradiation because the sample was
exposed to the helium plasma. Thus, we should say that thesurface nature was diﬀerent for each laser pulse irradiation.
If the surface roughness decreased because of the laser irradiation, the emission intensity might decrease at the next
irradiation. Consequently, it was thought that pulse-bypulse diﬀerence in the emission intensity increased. In
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conduct the experiments in a higher pulse energy range inwhich laser-induced vaporization would take place. If theintensity in case (i) significantly increased in the higher
pulse energy range, the signal in Fig. 5 (a) would be a precursor signal before the real blow-oﬀ signal.
Figure 7 shows the SEM micrographs after the seriesof laser pulse irradiations in case (iii). Figure 7 (a) corresponds to the peripheral region, where the laser pulse energy was lower than that in the central region shown in
Fig. 7 (b). At the peripheral region, micrometer-sized balls
were formed probably in the process of the melting of thefiberform structure. It was reported that a 1.5 µm tungstenball was formed on a microcone array by laser pulse irradiation to tungsten in a helium atmosphere [20]. It is of interest to point out that the size of the ball in our case is similar
to that given in Ref. [20], although the formation mechanism may be diﬀerent. It is indicated that such a ball canbe produced by a pulsed heat load if the fiberform structure
is formed on the surface. It is important to note that the
micrometer-sized balls can be high-Z dust particles, which
are destructive for the core plasma, if they are released
from the surface. In the central region, the surface melted
completely, and many cracks could be observed. The formation of a rough surface might be attributed to the factthat the laser pulse energy was gradually increased fromthe lower energy in the experiment. Since the fiberform
structure could easily be destroyed and form balls even ata low pulse energy, as shown in Fig. 7 (a), the surface mightretain the irradiation history. That is, if micrometer-sizedballs were formed on the surface, it might be diﬃcult tocompletely modify the micrometer-sized structure, which
was considerably larger than the fiberform structure, even
if the laser pulse energy was greater; and, consequently,
a rough surface might be formed after the series of laser
pulse irradiation.

Fig. 7 SEM micrographs after the series of laser pulse irradiations in case (iii). (a) shows the peripheral region of thelaser beam, and (b) shows the central region of the laser.

with a benchmark for temperature change, which is usefulfor understanding the phenomena in response to laser pulse
irradiation. In cases (ii) and (iii), the onset laser pulse energy required for observing the emission was slightly lessthan the melting threshold energy but close to the value.
As previously discussed, it was understandable in cases(ii) and (iii) that the emission was observed from a lower
pulse energy than the melting threshold. In contrast, thecalculations predicted that the melting threshold in case(i) might exist at approximately 1.8 MJm−2 , which wasslightly larger than the observation threshold of approximately 1 MJm−2 . In general, in order to enhance the emission, increases of tungsten neutral density and/or electron
density are necessary. In case (i), as discussed in the previous section, the emission intensity agreed well with thetemperature response. When the surface temperature is increased, tungsten neutral release is enhanced; moreover,thermionic electron emission is initiated. We speculatethat the emission in case (i) is mainly due to the electron
excitation by the electrons produced by thermionic emission. Indeed, in case (i), the emission area was significantly smaller than the area in cases (ii) and (iii), as shownin Figs. 3 (a)-(c). As a future work, it will be interesting to

4. Summary
The eﬀects of ELM-like transient heat load to thetungsten exposed to helium plasma were investigated byusing a submillisecond pulsed laser. The ejected tungsten
particles in response to the laser pulses were detected by
fast CCD camera. When the laser pulse energy was greaterthan 0.5 MJm−2 , clear WI emission was observed from thehelium-irradiated surface. The emission could be observed
at a considerably lower pulse energy sometimes, althoughsuch an emission was not observed from the surface without damages due to helium irradiation when the laser pulse
energy was lower than 1 MJm−2 . Two processes were proposed for the mechanisms that could cause the tungsten
ejection from the low pulse energy. One was the burstingof bubbles, which could produce tungsten microdroplets,
and the other was the anomalous temperature increase in
the fiberform nanostructure. The results suggested that thedamages due to helium-irradiation, that is, bubbles/holes
and the fiberform structure, could enhance the tungsten im004-5

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purity release from the material in response to the transientheat load like ELMs in future fusion devices. The formation of a micrometer-sized ball was observed in a region
where weak laser pulses irradiated the fiberform nanostructure. Since the ball formation could enhance the high-Zdust particles in fusion devices, it is important to investigate the formation mechanism, which will be dealt with ina future work.

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Acknowledgments
This work was supported in part by a Grant-in-Aid
for Young Scientists (Start-up) (No. 19860096) from theJapan Society for the Promotion of Science (JSPS).
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