Nuclear Fusion Research and Development

4-2

A Laser System for Plasma Diagnostics Has Achieved the World s Best Performance=Development of YAG Laser for the Edge Thomson Scattering System in ITER=

Newly developped laser
7.66 J, 100 Hz, 766 W

WW

Japan(JT-60)
7.46J, 50Hz, 373W

ager
Av

2006
2003

er
wepo
=

1

10m

2001 Russia
Japan(LHD) 2000
Korea(KSTAR)
UK(MAST),USA(NSTX)
China(HT-7)
Czech(COMPASS)
USA(Alcator C-MOD)
Germany(ASDEX-U) USA(DIII-D)

2W

Performance ofcommercial laser

2002

4.

0EU(JET)

10Output energy (J)60

005W080 W
04020

10Target for ITER
5 J, 100 Hz, 500 W

1.5 m

100
Repetition rate (Hz)Fig.4-6 Comparison of plasma diagnostic lasers in theworld
Letters in parenthesis indicate the name of fusion experimentaldevice.

Fig.4-7 Laser system in operation
Bright light of pumping flash lamps in laser amplifiers is visible.
Because this YAG laser emits an infrared ray, the laser beam isinvisible.

We are responsible for developing the edge Thomson
scattering system that will measure electron temperatureand electron density profiles in an edge region of ITERplasmas. The Thomson scattering is a plasma diagnostic
technique that evaluates electron temperature and density byanalyzing laser light scattered by electrons when a highpowered pulsed laser light is injected into the plasma.
Photons scattered from electrons are extremely weak, and foreach hundred billion photons injected into the plasma, only
about one is scattered. To attain measurements with sufficientaccuracy, the pulsed laser light must be fired with highenergy. Moreover, to determine the plasma state, which varies
from moment to moment, the laser pulses must be fired at ahigh repetition rate. In this diagnostic system, the Nd: YAG
laser needs to provide 500 W of average output power; thatis, 5 J/pulse of energy needs to be fired at 100 Hz. Meetingthese design requirements for both high output energy andhigh repetition rate had been the biggest issues.
We have developed a laser system that provides 373 W(7.46 J, 50 Hz) of average output power; so far, this system
has been shown the best performance than any other systemin the world (Fig.4-6). However, to develop the laser system

required by ITER, the repetition rate must be doubled.
Moreover, the heat created in the laser amplifier must becontrolled, typically by controlling the input energy to thelaser amplifier. However, decreasing the input power also
decreases the gain in amplification; this has posed a serious
problem in that energy in an amplifier cannot be efficiently
extracted as laser light. However, by using samarium-doped
special glass tubes in the laser amplifier, the noise of thelight, which was interfering with the improvement in
amplification gain, is selectively absorbed. Therefore, even at
half-input energy, the amplifier gain has been approximately
doubled, compared to conventional performance. As a result,it becomes possible to efficiently extract energy from theamplifier, leading to an average output power of 766 W (7.66 J,100 Hz). This corresponds to twice the conventional average
output power (Fig.4-6, Fig.4-7). Thus, we have succeeded indeveloping a laser system that exceeds the target performancein ITER, and electron temperatures and densities can bemeasured with high accuracy.
A part of our research was accomplished by the support of
the Institute of Laser Engineering, Osaka University.

Reference
Hatae, T. et al., Development of a YAG Laser System for the Edge Thomson Scattering System in ITER, Review of Scientific Instruments, vol.83, issue 10,2012, p.10E344-1-10E344-3.
JAEA R&D Review 201255