J. Plasma Fusion Res. SERIES, Vol.7 (2006) 138-141

Pulsed Laser Ablation of Hydrogen-Implanted Graphite Target
SAKAWA Youichi∗ , SHIBAHARA Takahiro, SATO Kouji and TANABE Tetsuo
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan（Received: 12 October 2004 / Accepted: 28 April 2005）

Abstract
Ablation of the H+ -implanted graphite target by pulsed laser irradiation with an ArF laser (193 nm, pulse
length: τ  8 ns, intensity: I L < 3 × 109 W/cm2 ) or the fourth harmonic of a Nd:YAG laser (266 nm, τ  20ps, IL < 9 × 1011 W/cm2 ) has been investigated. The intensities for ablation threshold are I L  4 × 108 and 5 × 109 W/cm2 for the 193 and 266 nm lasers, respectively. Above the ablation threshold for the 266 nm
laser irradiation, two ablation regions, a weakly-ablated (WA) region and a strongly-ablated (SA) region, areobserved; the threshold laser intensity of I L  7×1010 W/cm2 is determined by the strong-ionization thresholdof C+ ions. When the target is irradiated by the 193 nm laser, only the WA region is observed; the averaged
mass number of emitted ions is  200 for initial few shots, and the most of the implanted H atoms is released
within a few 10 shots. In visible emission spectra, only C2 Swan band is observed in the WA region both forthe 266 and 193 nm laser irradiation. For the 266 nm laser irradiation in the SA region, the emitted ions aremostly C+ and C2+ , and the optical emission of C I, C II, and C III lines are dominant. For the removal of theimplanted H, the WA region seem to be suited, since a large amount of H 2 is released.
Keywords:
laser ablation, graphite, tritium retention, tritium removal, ArF laser, YAG laser
1．Introduction
Carbon is a candidate as plasma facing materials in
ITER and future D-T fusion reactors. Because of large
tritium retention in carbon, a significant fraction of tritium is expected to retain in the vacuum vessel as codeposits with carbon, and it is necessary to remove tritium periodically [1]. One of the methods to remove
tritium is a laser induced desorption. Skinner et al. [2]have used a low-intensity (I L = 8 × 103 W/cm2 )continuous-wave Nd laser (1.06 µm) for the tritium removal from the co-deposits of the TFTR carbon tiles.
Shu et al. have used a pulsed ArF laser (193 nm,IL < 3 × 108 W/cm2 ) in the ablative conditions to remove hydrogen from the co-deposits of carbon tiles [3].
In this paper, we investigate pulsed laser ablation of
the graphite target, in which 10 keV H+ ion are implanted, and examine eﬀects of laser intensity on theablation with using an ArF excimer laser and the fourthharmonic of a Nd:YAG laser.

Pulsed laser
TOFMS

Emitted ions

Visibleemission

Desorbed gases
TMPRP

QMS

IG

GraphitetargetFig. 1 Schematic view of the experimental setup.

the fourth harmonic of a Nd:YAG laser (ContinuumCustom PY61C-10, 266 nm, E L < 3 mJ/pulse, τ  20
ps, IL < 9 × 1011 W/cm2 , repetition rate: 10 Hz) was focused on the target surface by a quartz lens of the focal
length of 300 mm through a quartz window either with0◦ or 45◦ direction to the normal plane of the target. Wevaried IL by changing both E L and the distance between
the lens and the sample. A graphite plate made of IG110U (Toyo Tanso Ltd.) was used as a target. H+ ion

2．Experimental
Figure 1 shows the experimental setup. Experiments
were conducted under the vacuum pressure < 3 × 10−8Torr. A pulsed ArF excimer laser (Lambda Physik OPTex, 193 nm, energy: E L < 13 mJ/pulse, pulse length:τ  8 ns, IL < 3 × 109 W/cm2 , repetition rate: 5 Hz) orC 2006 by The Japan Society of Plasma

Corresponding author’s e-mail: sakawa-y@ile.osaka-u.ac.jp∗ Present address: Institute of Laser Engineering, Osaka University, Suita 565-0871, Japan138

Science and Nuclear Fusion ResearchPulsed Laser Ablation of Hydrogen-Implanted Graphite Target[3] and 15 ns [4]. Figure 2 (b) shows the intensitiesof the H+ signal of TOFMS for the first 266 nm laser
shot. The threshold intensity is around 5 × 109 W/cm2 .
The H+ signal intensity decreases by increasing I L at
IL > 2.5 × 1010 W/cm2 .
Visible emission spectrum from the H+ implanted
target for the 193 nm laser irradiation with I L = 2.3×109
W/cm2 , which is above the ablation threshold, is displayed in Fig. 3 (a). We observed only C2 Swan band(d3 Πg − a3 Πu ) emission, for example at 469.7, 516.5,
and 589.9 nm; but optical emissions from C (C I), C+Intensity ( arb. units ) Intensity ( arb. units ) Intensity ( arb. units )beams were produced by an ion gun with the energy of
10 keV, and implanted to the target normal to the surface plane with the beam size of 8 mm in diameter. By
using Transport of Ions in Matter (TRIM) code, for theimplanted 10 keV H+ ions with the fluence of 1 × 1022
ions/m2 , it is estimated that H atoms exist 250 nm range
from the surface and that the H/C ratio on the implantedarea is  0.45. This value is similar to the H/C  0.4of hydrogen co-deposited carbon film observed in carbon wall tokamaks. Emitted ions and desorbed gases
were measured by a time-of-flight mass spectrometer(TOFMS) and a quadrupole mass spectrometer (QMS),
respectively. Visible light emissions were also monitored by a spectrometer (Hamamatsu C7473), in which
relative sensitivity on wavelength is calibrated.

1.21(a) 193 nm

Intensity ( arb. units )

0.8
0.6
0.4
0.208109108
2Laser Intensity IL ( W/cm )(b) 266 nm

64
20 9101010
Laser Intensity IL ( W/cm2 )C28000C2025000

532nm(2ω emission)(b) 266nm
IL=2.5x1010 W/cm2C220000
15000
C210000
5000

C2(a) 193nm
IL=2.3x109 W/cm2C2C24000

C2C3 C
2CI050000
40000

532nm
(2ω emission)

20000
10000
02000015000

CII

CII C2CII
CIII

1CICII532nm(2ω emission)

CICI(d) 266nm
IL=4.2x1011 W/cm2
CII

C2CII

CI
CII

5000

6C2CII
CIII

10000

0300(c) 266nm
IL=1.1x1011 W/cm2

C230000

Intensity ( arb. units )H+ Signal Intensity (arb. units)

H+ Signal Intensity (arb. units)

3．Results and discussions
Integrated intensities of the H+ signal of TOFMS
over the first laser shot to the 50th one with the 193 nm
laser irradiation for the H+ implanted target are shownin Fig. 2 (a). By extrapolation, one can note a thresholdfor the H+ emission at around 4 × 108 W/cm2 , whichcorresponds to the laser fluence of F = 3 J/cm2 . Thisfluence is nearly the same as the measured carbon ablation threshold of 1 J/cm2 for the ArF lasers with τ = 25

12000

C2400

500
600
700
Wavelength ( nm )

800

900

C2CI
CII
CIII(e) 266nm
0.1
10101011
Laser Intensity IL ( W/cm2 )1011
Fig. 3 Visible emission spectra from the H+ implanted
target for (a) the 193 nm laser irradiation with
IL = 2.3 × 109 W/cm2 ; and for the 266 nm laser irradiation with (b) IL = 2.5 × 1010 , (c) 1.1 × 1011 , and(d) 4.2 × 1011 W/cm2 , respectively. (e) Intensitiesof the optical emission, C III (407.0 nm), C II
(426.7 nm), C I (909.4 nm), and C2 (516.5 nm) asa function of IL .

Fig. 2 (a) Integrated intensities of the H+ signal ofTOFMS over the ﬁrst laser shot to the 50th one
with the 193 nm laser irradiation, and (b) intensities of the H+ signal of TOFMS for the ﬁrst 266 nm
laser shot, for the H+ implanted target. Lines areguide to the eyes.

139Pulsed Laser Ablation of Hydrogen-Implanted Graphite Target

IL = 3.8 × 107 W/cm2 (below the ablation threshold),
and the averaged mass number of the released ions wassmaller. This indicates that when I L is larger, the size ofthe emitted ion clusters is smaller due to the fragmentation in the ablated plume. When I L = 1.5 × 109 W/cm2 ,the signal intensities of H+ became the noise level after
the initial 30 shots for the H+ implanted target; whilefor the un-implanted target the H+ intensity decreasedmuch faster than that for the H+ implanted target. H+
signal intensity was remained the noise level for the unimplanted target, even when I L = 1.5 × 109 W/cm2 .
Figures 5 (a) and 5 (b) display the mass distributionsof the emitted ions at the first and 900th laser shots, respectively, when the H+ implanted target was irradiatedby the 266 nm laser with IL = 5.5 × 109 W/cm2 . At
this laser intensity, which is in the WA region, the ion
mass distributions and the visible emission spectrum aresomewhat similar to those for the 193 nm laser irradiation with IL  2 × 109 W/cm2 , as shown in Figs. 4 (b)and 3 (a), respectively. Figures 5 (c) and 5 (d) represent
the ion mass distributions at the first and the 900th laser
shots, respectively, with I L = 2.9 × 1011 W/cm2 (in theSA region). Only C+ and C2+ ions were emitted. Notethat the absolute intensity of C+ signal in the SA region[Figs. 5 (c) and 5 (d)] is more than two orders of magnitude larger than that in the WA region [Figs. 5 (a) and5 (b)]. The visible emission spectrum at this laser intensity, is similar to that shown in Fig. 3 (d), where C II
and C III emissions are dominant.
Before the laser irradiation, QMS signals were
dominated by residual gases, mostly m/e = 18 (H2 O),

0.2

2nd
10th1000th(a) 3.8x107 W/cm2

TOFMS Intensity (arb. units) TOFMS Intensity (arb. units)

TOFMS Intensity (arb. units) TOFMS Intensity (arb. units)(C II), or C2+ (C III) were not observed. We call thisablation region as a weakly-ablated (WA) region. Figures 3 (b) − 3 (d) show visible emission spectra for the266 nm laser irradiation with IL = 2.5 × 1010 , 1.1 × 1011 ,and 4.2 × 1011 W/cm2 , respectively, which are all above
the ablation threshold. When I L < 2.5 × 1010 W/cm2 ,only C2 Swan band and C3 emission were observed; it
is in the WA region. By increasing I L , C I, C II, andC III emission appeared in this order. Intensities of theoptical emission, C III(407.0 nm), C II(426.7 nm), CI(909.4 nm), and C2 (516.5 nm) as a function of IL , aredisplayed in Fig. 3 (e). The threshold intensities for theappearance of C I, C II, and C III are  2.5 × 1010 , 7 × 1010 , and  1.6 × 1011 W/cm2 , respectively. AtIL  7 × 1010 W/cm2 , not only emission of C II started
to appear but also C+ signal intensity of TOFMS drastically increased, i.e., strong ionization of carbon occurred. We call the ablation region at I L > 7 × 1010
W/cm2 for the 266 nm laser irradiation as a stronglyablated (SA) region.
Figures 4 (a) and 4 (b) show the mass distributions of
the emitted ions when the H+ implanted target was irradiated by the 193 nm laser with I L = 3.8 × 107 and1.5 × 109 W/cm2 , respectively, at the second, 10th, and1000th laser shots. For IL = 1.5 × 109 W/cm2 , which isin the WA region, the signal intensities of TOFMS weremore than an order of magnitude larger than that for0.10(b)8 1.5x109 W/cm22nd10th
1000th64200100m/q200300

20.3(a)C+(c) IL=2.9x1011 W/cm2#1C+

IL=5.5x109 W/cm2

1.5

#10.2

10.1
0.5010.8(b) IL=5.5x109 W/cm2
#900

C+
C4+

00.6

0.4

C2+(d) IL=2.9x1011 W/cm2
#900
C+

0.6
0.4

C6+
C8+

0.2

0.2
00100

200m/q300

400001020m/q

304050Fig. 5 Mass distributions of the emitted ions at (a)the ﬁrst and (b) 900th laser shots measured byTOFMS when the H+ implanted target was irradiated by the 266 nm laser with IL = 5.5 × 109 W/cm2(in the WA region). Mass distributions of the emitted ions at (c) the ﬁrst and (d) 900th laser shotswith IL = 2.9 × 1011 W/cm2 (in the SA region).

Fig. 4 Mass distributions of the emitted ions at the second, 10th, and 1000th laser shots measured by
TOFMS when the H+ implanted target was irradiated by the 193 nm laser with (a) IL = 3.8 × 107 (below the ablation threshold) and (b) 1.5 × 109 W/cm2(in the WA region).

140

QMS Signal Intensity (10-7 A)

Pulsed Laser Ablation of Hydrogen-Implanted Graphite Target

426(C2H2)

2 (H2)
3intensity, since a large amount of H2 is released.
When the H+ implanted target was irradiated by the193 nm laser with IL = 1.5 × 109 W/cm2 , the measured
mass distribution of desorbed gases was similar to thatshown in Fig. 6, i.e., the ratio of the H2 and C2 H2 signal
intensities was similar to each other in the WA and SA
regions. The signal intensities of both H2 and C2 H2 decreased with the laser shot number and became steadystate within 30 shots. In the WA region, no clear differences in the amount of H2 -desorption were observed
between the 193 and 266 nm laser irradiation.

28 (CO + C2H6)

25(C2H2)

16(CH4)215(CH4)144 (CO2)

00102030
Mass Number

4050Fig. 6 Mass distribution of desorbed gases for the H+ implanted target after several shots of the 266 nm
laser irradiation with IL = 1.8 × 1011 W/cm2 (in theSA region). Signal intensities before the laser shot
were subtracted from those after the laser shot.

4．Conclusions
We have studied ablation of the H+ -implanted
graphite target by irradiation of pulsed 193 and 266 nm
lasers. Two ablation regions, the WA and SA regions,are distinguished for the 266 nm laser irradiation, while
only the WA region is observed in the 193 nm case. For
the removal of the implanted H, the WA region seem tobe suited, since a large amount of H2 is released.

17 (OH), 2 (H2 ), 28 (CO), and 44 (CO2 ). Mass distribution of desorbed gases for the H+ implanted target
after several shots of the 266 nm laser irradiation with
IL = 1.8×1011 W/cm2 (in the SA region), by subtractingthat before the laser shot, is shown in Fig. 6. The dominant mass numbers of the desorbed gases are m/e = 2(H2 ) and 26 (C2 H2 ), 28 (CO+C2 H6 ), and 44 (CO2 ).
The QMS signal intensities of H2 and C2 H2 increased
drastically at IL  4 × 109 W/cm2 , which is close to
the ablation threshold intensity derived from Fig. 2 (b).
The signal intensities of both H2 and C2 H2 peaked atIL  1.3 × 1010 W/cm2 , which is in the WA region, anddecreased by increasing I L ; similar trend as the H+
signal intensity shown in Fig. 2 (b). I L = 1 − 2 × 1010
W/cm2 in the WA region seem to be an optimum laser

[1]
[2]
[3]
[4]

141

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