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