24P064-P
Response of optical materials to high heat load given by laser pulses
光学材料の高熱負荷時の光応答
Kenichiro Furumoto, Kentaro Hirata, Tetsuo Tanabe ,古本健一郎, 平田健太郎，田辺哲朗
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University
6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
九州大学大学院 総合理工学府 〒812-8581 福岡県福岡市東区箱崎6-10-1

Ceramics or g lasses such as alumina (Al2 O3 ) o r silica (SiO2 ) are candidate materials for optical o r electricaldevices for fusion reactor. However, it is concerned degradation of optical or electrical properties causedby irrad iation. Since these materials are forecasted to be subjected high heat load, it needs to find out the
optical response fro m materials under such a condition.

1. Introduction
Optical, electrical and thermal properties ofceramics or glasses such as alumina (Al2 O3 )and, silica (SiO 2 ) which will be used opticalor insulating materials in fusion reactors aredegraded by irradiation of energetic photons
and/or particles appearing as radiation- induced
electricaldegradation(RIED)
andradiation- induced
conductivity(RIC).
Degradation of optical properties, such ascoloring and loss of transparency caused by theirradiation to optical materials are also
concerns for their use as fiber optics andwindows. Because electric excitation plays
important role in some of the degradation of the
optical properties, in-situ observation under theirradiation is required to understand the opticaldegradation.
In a fusion reactor, glasses or ceramics areexposed to high thermal load (energetic
particles and photons) escaping from edge orcore plasma, however, the optical response ofmaterials under such conditions have not beenstudied well.
In this work, we have observed lightemission from alumina induced by high power(approximately MW/cm2 - GW/cm2 ) Nd:YAG
laser exposure. Because laser irradiation

damage in materials occurs in connection withphotochemical reaction, it needs to examine
effects of surrounding atmosphere andirradiation temperature.
2. Experime ntal
Fig. 1 shows a schematic diagram ofexperimental arrangement. A specimen discmade of a single crystal sapphire (Al2 O3 ) wasused. A specimen disc was 10 mm in diameterand 1 mm thickness. A pulsed laser light wasirradiated normal to the specimen and lightemissions to both backward and forward
directions were observed. The laser light usedhere was the Fourth harmonic of Nd:YAG laser(wavelength 266 nm) with a pulse width of 20
psec and frequency was 10 Hz, and the energy
density was up to 1.5 J/cm2 /pulse.

Fig. 1 schematic d iagram of experimental arrange ment

The laser light was focused on the sample
surface by a quartz lens of the focal length of
300 mm. Light Emission was detected by aspectrometer through optical fibers and a notch
filter to cut the incident light.
3. Result and discussion
Fig. 2 shows observed light emission
spectrum of alumina induced by 266 nm
Nd:YAG
laser
irradiation
with500
2mJ/cm /pulse laser energy density. Observed
emission spectrum shows three broad band
centered at 320 nm (3.8 eV), 420 nm (3.0 eV),attributed to F+, F center. And 720 nm broad
emission band was likely caused by impurities
such as Fe3+ and Ti3+. In addition, black bodyradiation due to the temperature increase of thesample surface could have some contribution.
The F+, F center emission bands observed
here were very similar to those observed incathodoluminescence (CL) [1] or in the otherenergetic particle induced luminescence [2]. In
addition, several line emissions were observed.
The 693 nm (1.7 eV) emission is attributed tochromium impurity in alumina (Cr3+ center),and the 396 nm (3.1 eV) emission is from Al I
caused by laser ablation.
The forward emission intensities of both F+
and F center stayed nearly constant with thenumber of laser pulse shots. While, thereflection of primary laser beam monotonicallyincreased. These observations suggest that thelaser irradiation is not likely to create oxygen
deficiencies. However, the increase of thereflection suggests some modification ofsurface and subsurface region owing toablation.

Fig.2 lu minescence spectrum induced by 266 n m
Nd:YA G laser irrad iation

4. ConclusionWe
haveconductedlaser
inducedluminescence studies. The observed emission
spectra were constructed from the emissions
due to oxygen deficiency defect centers (F andF+ centers), and various line emissions causedby Cr3+ impurity, ablated Al atoms and ions.
The emission intensities were different for
backward and forward emissions. Therefore
detailed studies of the laser induced
luminescence would give useful information on
laser damaging of ceramics or glasses.
References[1]A. Morono, E.R. Hodgson J. Nucl. Mater 224 (1995)
216-221[2]A. Al Ghambi, P.D. Townsent, Nucl. Instr. Meth. B
46 (1990) 133-136