065-P
24P
4P0
Durability to repeated pulse laser irradiation
for laser transmission mirrors in ITER
ITERにおけるレーザ伝送用ミラーへのパルスレーザ照射耐久性
Masaya Sato1, Shin Kajita2, Noriyasu Ohno1, Ryo Yasuhara3, Masayuki Tokitani3,
and Yuzuru Tawara2
佐藤雅也, 梶田 信，大野哲靖，安原 亮, 時谷政行, 田原 譲
1

Department of Engineering and Science, Nagoya University,
Fro-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan
名古屋大学工学研究科, 〒464-8603 名古屋市千種区不老町
2
EcoTopia Science Institute, Nagoya University,
Fro-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan
名古屋大学エコトピア研究機構, 〒464-8603 名古屋市千種区不老町
3
National Institute for Fusion Science322-6, Oroshi-cho, Toki, GIfu, 509-5292, Japan
核融合研究所 〒509-5292 岐阜県土岐市下石町322-6
In ITER, laser transmission mirrors will be used for Thomson scattering diagnostics. For the lasertransmission mirrors, which transmit laser pulses to plasma, high reflectivity at 1064 nm, high multi pulse
LIDT (Laser induced damage threshold) and high durability for sputtering by plasma are needed. If thesurface of mirror is damaged by laser irradiation, reflectivity of the mirror decreases seriously. In this study,
the effects of the multi pulse LIDT on OFHC (Oxgen - Free High thermal Conductivity) - copper andpolished pure copper are experimentally investigated up to 7.3×104 pulses and 5.0×103 pulses, respectively.

1. Introduction
In ITER (International Thermonuclear Experimental Reactor), plasma diagnostics requires metalmirror. For Thomson scattering diagnostics, lasertransmission mirrors and a neodymium - doped
yttrium aluminum garnet (Nd:YAG) laser at 1064
nm will be used. The required specification for thelaser include a pulse energy 5 J with a repetition
frequency of 100 Hz and pulse width of ~10 ns [1].
The mirrors, which transmit laser pulses toplasma, high reflectivity at 1064 nm, high multipulse LIDT (Laser induced damage threshold) andhigh durability for sputtering by plasma are needed.
A leading candidate material is Au, Ag, and Cu.
It is necessary to maintain its optical properties
for more than 107 laser pulses [2]. If the surface ofthe mirrors is damaged by laser irradiation,reflectivity of the mirror decreases seriously.
Therefore, to investigate multi pulse LIDT isimportant.
In this study, the effects of the multi pulse LIDTon OFHC - copper (KUGLER Co.) and polishedpure copper are experimentally investigated up to
7.3×104 pulses and 5.0×103 pulses, respectively.
Experimental results predicted the expected multipulse LIDT for 107 pulses.
2. Experiments
The mesurement of multi pulse LIDT for OFHC- copper mirror and polished pure copper mirror
were done with the set up shown in Fig. 1. Our
experiment is based on the principle that once thesurface of the mirrors are damaged by laser
irradiation, reflectivity of the mirrors significantlydecrease.
The experimental measurement system consistsof pulsed Nd: YAG laser, 1064 nm wavelength, 10Hz repetition rate, 5-7 ns pulse duration (FWHM),
ND (Neutral Density) filter, and focus lens whichhave a focal length of 307 mm at 1064 nm. Thelaser beam has a Gaussian intensity profile. Spotsize at metal mirror is 7.0×10-3 cm2, measured with
CCD camera and lase view, free software.

Fig. 1. Experimental measurement system

3. Results
Single shot LIDT and multi pulse LIDT ofcopper mirrors were investigated. The single shot
LIDT was 4.8 J/cm2 for OFHC - copper mirror and
1.1 J/cm2 for polished pure copper. In this study,
single shot LIDT was defined by visible damage on
the copper mirror surface. Value of the former isabout four times that of the latter.
Experimental result of multi pulse LIDT isshown in Fig. 2. The effects of the multi pulse
LIDT on OFHC - copper and polished pure copper
are experimentally investigated up to 7.3×104pulses and 5.0×103 pulses, respectively. OFHCcopper mirror, multi pulse LIDT was 1.5 J/cm2 for
7.3×104 pulses. Polished pure copper mirror, multipulse LIDT was 4.5×10-1 J/cm2 for 5.0×103 pulses.

4. Discussion
Value of single shot LIDT on OFHC - coppermirror was higher than the predicted value in ref [1].
However the value of polished pure copper mirror
was about the same value. This difference isthought to be related to the roughness of the surface.
Roughness of OFHC - copper mirror is less than 3nm.
Experimental data on single shot LIDT and multipulse LIDT and regression curve for more than 103
pulses are shown in Fig. 4. Multi pulse LIDT on
OFHC - copper mirror is averaged over the sameenergy density. It is seen that the slope of the graphchanges between 103 - 104 pulses for both mirrors.
This graph predicted that the expected multi pulse
LIDT for 107 pulses is ~ 6.5×10-1 J/cm2 on OFHC copper mirror and is ~ 2.9×10-2 J/cm2 on polishedcopper mirror.

Fig. 2. Multi pulse LIDT on OFHC - copper andpolished pure copper mirror.

Power of reflected light from the copper mirror
decreases seriously if the mirror surface is damaged
by laser irradiation. Temporal evolution of reflected
laser power the that was measured by power meterand oscilloscope is shown in Fig. 3. When theenergy density was low, the power of the reflected
light decrease very slowly. This trend is also seen in
OFHC - copper and polished pure copper.
Strange emission was observed on the surface ofthe copper mirror at the knee appeared in thetemporal evolution.

Fig. 3. For polished pure copper, energy density is
6.8×10-1 J/cm2 for (a), 4.5×10-1 J/cm2 for (b).

Fig. 4. Single shot LIDT, multi pulse LIDT andregression curve for more than 103 pulses on OFHC copper and polished pure copper mirror.

References[1] S. Kajita, T. Hatae, and V.S. Voitsenya: J. Plasma
Fusion Res. Volume 3, 032 (2008)[2] A. Gorshkov, I. Bel'bas, V. Sannikov, and K.
Vukolov: Fusion Eng. Des. 74 (2005) 859-863[3] J.F. Figueira and S.J.Thomas: IEEE J. Quantum
Electron VOL. QE-18, NO. 9, 1982