investigate laser induced desorption (lid) of hydrogen retained in co-deposited layers on jt-60...
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Investigate Laser induced desorption (LID) of
hydrogen retained in co-deposited layers on JT-60 open-divertor tile
20 ps-pulsed Nd:YAG laser for
wide laser-intensity IL regions (3 orders of magnitude) with
= 1064 nm () and 266 nm (4) emission
study properties of desorbed gases & emitted particles
to find optimum laser condition (IL & ) for LID
Purpose
7th ITPA meeting on SOL/divertor physics, November 6-9, 2006
Removal of redeposited carbon layers by laser ablation
Y. Sakawa, ILL. Osaka Univ. D. Watanabe Nagoya Univ.
K. Sugiyama, T. Tanabe, Kyushu Univ.
Ablation Sublimation of C (3800K)
■ Thermal desorption of H2
Ionization (C+ : 11.26 eV)
Multi-photon ionization (3-photon process of 266nm) enhancing release of ions, surpressing : neutrals
Inverse bremsstrahlung absorption: ↑ increase⇒
Laser Intensity (=266nm )
1010
W/cm2
1011
W/cm2
Non-ablation
Weak-ablation
Strong-ablation
Three regimes are distinguished
■ Bond breaking by laser photon : ↑ decrease⇒ Bonding energy : C-H (4.5 eV), C=C (7.24 eV) Photon energy : 266 nm (4.66 eV) 1064 nm (1.17 eV)
Y. Hirohata, et al., J. Nucl. Mater. 337-339, 609-613 (2005)
Tile
Co-deposits
Plasma
Co-deposits on JT-60 open-divertor tile
■ Sample JT-60 open-divertor tile Exposed to 1800 H2 discharge (June ~ October 1988)
H/C ~ 0.03 (1.4 x 1021 atoms m-2 mm-1) Thickness of co-deposits : 35 ~ 40 mm
■ Laser Nd:YAG laser (Continum Custom PY61C-10) Pulse duration : 20 ps Repetition rate : 10 Hz Wave length : 266 nm (4), 1064 nm (), Energy : < 3 mJ/pulse (4), 35 mJ/pulse () Intensity : < 9 x 1011 W/cm2 (4), 6 x 1012 W/cm2 (w)
■ Measurements Emitted ions : Time-of-flight mass spectrometer (TOFMS) Desorbed gases : Quadrupole mass spectrometer (QMS) Vissible light emission: optical spectroscopy Ablation spot size : Scanning electron microscope (SEM) Ablation depth : Optical microscope
Experimental setup
Vacuum pressure < 3 x 10-8 Torr
Beer’s low :
IL : Laser intensity
Iablation : Ablation threshold
: Absorption coefficient
d = ln1 IL
Iablation
Ablation threshold ~10GW both for 266nm and 1064 nm.Ablation depth is larger for 1064 nm with the identical IL.
0
50
100
150
200
250
300
109 1010 1011 1012
= 266 nm= 1064 nm
Abl
atio
n de
pth
per
lase
rsh
ot
d (n
m)
Laser intensity (W/cm2)
266 nm: =35m-1
1064nm:=15m-1
Non-ablation region
Weak-Ablation Region (WAR266)
Iablation < IL < Iionization
Iionization = 9 x 1010 W/cm2
Strong-Ablated Region (SAR266)
Iionization < IL
Laser Intensity IL (W/cm2)
0
40
80
120
Abl
atio
n D
epth
d (n
m/s
hot)
Ablation Depth
102
103
104
C2
CICIICIII
Em
issi
on I
nten
sity
(a
rb. u
nits
)
Visible Light Emission
101
104
107
109 1010 1011 1012
I[C
+]
/ Spo
t Siz
e(a
rb. u
nits
)
TOFMS(C+)
Three regimes for 266 nm laser
Iablation = 9 x 109 W/cm2
Non-Ablation Region (NAR266)
IL < Iablation
Iablation Iionization
Ablationthreshold
Ionizationthreshold
CII
0
2 104
4 104
Inte
nsit
y (
arb.
uni
ts )
C2
C
Nd:YAG532nm
C+
C2+
C
&C2+
C2+
&C2
C2
CC
C+
IL = 3.0x1011 W/cm2C2+
C+C+
C+
0
2 103
4 103
300 400 500 600 700 800 900Inte
nsit
y (
arb.
uni
ts )
Wavelength (nm)
C2 Nd:YAG 532nm
C2 C2 C2
IL = 3.0x1010 W/cm2
SAR266 : Emission from C2, C, C+, and C2+
WAR266 : Emission from C2
Visible emission
SAR266
WAR266
0
1
2
3
0 400 800
C+Cn
+
IL = 2.3x1010 (W/cm2)
t2 (s2)
TO
FMS
Inte
nsit
y(a
rb. u
nits
)
0
0.1
0.2
0.3
0.4
TO
FMS
Inte
nsit
y(a
rb. u
nits
)
C+ IL = 3.0x1011 (W/cm2)
SAR266 : C+ , C2+ ions are emitted WAR266 : Carbon clusters (Cn
+) are emitted
TOFMS
SAR266
WAR266
020406080
100120
Abl
atio
n de
pth
per
lase
r sh
ot
d (n
m/s
hot)
266 nm
0
1000
2000
3000
4000
109 1010 1011 1012 1013Abl
atio
n de
pth
per
lase
r sh
ot
d (n
m/s
hot)
Laser Intensity (W/cm2)
1064 nm
Weak-ablation region
Strong-ablation region
Non-ablation region
Weak-ablation region
Strong-ablation region
Non-ablation region
Ionization Energy of C
= 11.26 eV
266 nm
Photon Energy
= 4.66 eV
⇩3-Photon Ionization
1064 nm
Photon Energy
= 1.17 eV
⇩Need 11 photons
Iionization is lower for 266 nm owing to 3-photon ionization.
Iablation Iionization
Iablation Iionization
226nm
1064m
1020
1021
1022
1023
109 1010 1011 1012Num
ber o
f Des
orbe
d H
2(a
tom
s m
-2)
Laser Intensity IL (W/cm2)
1020
1021
1022
0.1 1 10
H2
Des
orpt
ion
Rat
e
Laser Irradiation Time (s)
(ato
ms
m-2
s-1
)
4.6x1011W/cm2
1.8x1010W/cm2
266 nm : Number of desorbed H2 / Spot sizeincreases by increasing IL.
10Hz laser irradiation
H2 desorption rate
Spot size
Integrate over 30 s (300 shot)
Weak-ablation region
Strong-ablation region
Number of desorbed H2
Spot size
0.1
1
10
109 1010 1011 1012 1013
= 266 nm= 1064 nmN
deso
rbed
/Nre
tain
ed
Laser intensity IL (W/cm2)
Nretained : Hydrogen atoms retained in the ablated volume using density of hydrogen in co-deposits ~ 1.4 x 1021 atom / m2 m [Hirohata et al., J. Nucl. Mater. 337-339 (2005) 60.]
Ndesorbed : Number of desorbed hydrogen
H2 desorption efficiency is larger for 266 nmbecause of bond-breaking & ionization by laser photons
Thermal desorption from surrounding region
0
0.2
0.4
0.6
0.8
1
109 1010 1011 1012
N[C
2H2]
/ N
[H2]
Laser Intensity IL (W/cm2)
C2H2 production rate for 266 nm: Increases by increasing IL in Weak-ablation region Decreases by increasing IL in Strong-ablation region
C2H2 production rate =Number of desorbed C2H2
Number of desorbed H2
ConclusionsLID of hydrogen retained co-deposits on JT-60 open-divertor tile
Ablation by 266 nm and 1064 nm of a 20 ps-Nd:YAG laser are compared
1) Ablation properties
◆ Three regions of ILD were distinguished (NAR, WAR, SAR)
◆ Iablation was nearly identical for 266 nm and 1064 nm .
◆ Co-deposits can be removed faster for 1064 nm.
◆ Iionization was lower for 266 nm because of effective 3-photon ionization.
2) Hydrogen desorption properties
◆ Hydrogen-removal efficiency was largest in SAR and larger for 266 nm
◆ C2H2 production rate decreased by increasing IL in SAR for 266 nm
For the ablative removal of hydrogen, a short-wavelength and high-power laser irradiation is desirable.
266 nm:Desorption of H2, CnHmby laser irradiation
Desorption of H2O is small.
0 20 40 60 80 100
IL = 1.63x1011 W/cm2 (SAR)
IL = 9.52x1010 W/cm2 (WAR)
Number of Laser Shot
0
1
2
3
4
5
0 0.2 0.4 0.6Time (s)
H2
Inte
nsity
(10
-7A
)
1
0 5 10 15 20
IL = 1.63x1011 W/cm2 (SAR)
IL = 9.52x1010 W/cm2 (WAR)
I (1
0-7
A)
Number of Laser Shot
0.4
8
266 nm : Desorption of H2 per shot I[H2] is expressed by fast- and slow-decay processes
I[H2] = Ifast exp(- t /τfast)
+ Islow exp(- t /τ
slow)I[H2]
QMS
0
1
2
3
4
109 1010 1011 1012
τ fas
t (s)
Laser Intensity (W/cm2)
101
102
103
104
I /
Abl
atio
n sp
ot s
ize
(10-7
A/c
m2 ) Ifast
Islow
τslow ~ 500 s
266 nmIfast decreases in SAR, τfast increases in SAR
Islow proportional to IL, τslow is independent on IL
SARWAR
τfast
QMS
1019
1020
1021
1022
1023
0.1 1 10
H2
deso
rptio
n ra
te
Laser irradiation time (s)
(ato
ms
m-2
s-1
)
4.6x1011W/cm2
for = 266 nm
3.7x1011W/cm2
for = 1064 nm
1064 nm : Desorbed H2 decreases faster than 266 nm
because of larger ablation depth⇨10Hz laser irradiation
0
1
2
3
4
5
0 0.2 0.4 0.6Time (s)
H2 I
nten
sity
(10
-7A)
I[H2]
1021
1022
1023
0 5 10 15 20 25 30
Des
orbe
d H
2 m
olec
ules
[H
2]
Number of laser shots
3.7x1011W/cm2
for = 1064 nm
4.6x1011W/cm2
for = 266 nm[mol
ecul
es/m
2 ]
Desorbed H2 per laser shot
QMS
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