S8-4

Plasma Diagnostics by Laser with Phase Conjugation MirrorMasahiro Nakatsuka, Takaki Hatae* and Hidetsugu Yoshida* !"Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan
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Naka Fusion Research Establishment, Japan Atomic Energy Research Institute,801-1 Mukouyama, Naka-machi, Naka-gun, Ibaraki 311-0193, Japan
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The phase conjugation effect is described by Stimulated Brillouin Scattering process in a transparent
medium. This nonlinear effect was used to compensate a thermal effects occurred in the strongly pumped
solid-state YAG laser at Thomson scattering system of JT-60U. Also a new scheme of scattering
technology will be presented to improve an energy efficiency of laser system by SBS-PCM, achieving arepetitive use of the laser beam in a scattering optics.

1. Introduction
In the field of Nonlinear Optics[1], there is a term,
Stimulated Scattering in which category an acoustic
wave in material works as a grating to scatter a lightbeam, called as Stimulated Brillouin Scattering(SBS). The scattering rate of a light, wherein it isreflectivity because of a back-scattering process,depends on the incident light energy, reaching near
100% for an intense laser beam. The reflected light
has a property of the phase-conjugated wavefront
against an incident light. Then this process isusually used for phase compensation during passing
through a transparent aberrator.
In this presentation, the principle of SBS process
is introduced in detail, and its application will be
discussed to compensate a wavefront aberration inthe solid-state laser used in the Thomson scatteringsystem at JT-60U, JAERI, and also to present a newscheme of scattering technology for improving anenergy-efficiency of the laser utility.
2. Principle of Phase Conjugation Mirror
The phase conjugation effect was discovered in
the early 1970s by Zel’dovich et al.[2]. SBS process
is the three-wave interaction between an intense
incident pulse, a reflected wave that builds up froma noise level and an acoustic wave that is pumped
by interferometry effect of previous two opticalwaves. As this acoustic grating has a same shape ofwavefront as the incident pulse, the reflected wave
has a phase-conjugated wavefront of the incident
one. This is one of the most common nonlinear
optical processes for achieving a quasi-phaseconjugate wave with a small Brillouin frequency
shift of 1 GHz. The part of the nonlinear opticalmedium that causes phase conjugation is called a“phase-conjugate mirror (PCM)” to distinguish itfrom a standard reflective mirror.

Fig.1 Difference of reflected-beam wave fronts from a) conventional mirror and b) SBS-PCM.

In recent years, several liquid or solid materials,
having a very high laser-damage threshold and highreflectivity, were found applicable to the highperformance SBS-PCM[3-5]. Heavy fluorocarbon
liquids after treatment of severe purification werefound to be an ideal medium for the SBS-PCM. For
example, fluorinert FC-75 made at 3M, Ltd. shows
very high reflectivity ~98% at 14 J of an incident
laser energy. For the average peak power laser, arepetitive operation of PCM was also developed up
to 20 kW depending on absorbance of liquid.

Fig. 2 Reflectivity of SBS cell for 1064-nm, 20-ns YAG pulse.

3. Compensation of Thermal Effect by PCM
The SBS-PCM is a good tool to compensate thethermal lens effect that causes in a heavily pumped
YAG laser rod. The thermal load to a laser rod
makes strong temperature gradient in a rod that
affects a beam transmission by the distribution of anindex coefficient as a lens. Figure 3 shows theamplified beam patterns with and without using
SBS cell as a reflector in the double pass amplifier.
The depolarization effect in the crystal rod gives
also a strong loss through a polarized loss in an
amplifier system that including many polarizing
optics. The phase conjugation mirror reflects a laser
beam with very accurate returning path at YAG rod,and then depolarization can be compensated bypolarization rotation with a Faraday rotator set infront of a SBS cell.
The far-field patterns are also shown in the figurethat makes evidence of phase conjugation effectkeeping a diffraction limited beam property.
NFP

FFP

14 mm

D.L. x 2(a) single-pass

NFP

FFP

14 mm

D.L. x 2

Thomson scattering diagnostics at JT-60U, JAERI.

5. New Scheme of Scattering Diagnostics
Another proposal at the Thomson scatteringsystem is a double-pass configuration with using aSBS-PCM set at an initial dumper position after
through JT-60U plasma (see Fig. 5). The laser beampassing through a plasma is reflected back again tothe plasma chamber for a second pass. In theexperimental test at JT-60U, a scattering light wasincreased about 60%.
Figure 5 shows a new scheme of scattering optics
for multi-pass system with two SBS cells set bothside of a plasma chamber[7]. The laser pulse will be
confined in a scattering pass by an large Pockelscell optical switch and a thin-film polarizer. A
simple calculation for designing this optics shows adrastic increase of the scattering light level to 20
times higher than a case of single pass if a systemloss is only 5% showed in the insert of Fig. 5. The
small frequency shift in SBS cells does not affect aspectral response of Thomson scattering.

(b) double-pass with SBS-PCM

Fig. 3 Near-field and far-field pattern a)without and b) withusing SBS-PCM at YAG rod amplifier.

4. Improvement of Thomson-Scattering Laser
System.The phase conjugation mirrors were installed on
the YAG laser system in the Thomson scattering
diagnostics at JT-60U, JAERI. Without majorchanges of the components of laser system, a laser
staging was changed from a MOPA (masteroscillator and power amplifier) configuration to adouble-pass amplifier with SBS-PCM as a reflector.
After changing a system design to a two-beamstaging, the output power level was increased
finally 8 times higher than the original design asshown in Fig. 4. The beam brightness is also
improved about 10 times by improving thedepolarizing property of the beams[6], that was very
effective in focusing beams in to the plasma vessel.

Fig. 5 New scheme of laser transmission into Tokamak plasma
increases scattering light by multi-pass configuration.

6. Summary
The phase conjugation method by the stimulated
Brillouin scattering is very useful tool to compensate thethermal effects in the YAG rod amplifier, and also toincrease the Thomson scattering light from plasma by
multi-pass configuration.
The purification of liquid material for SBS process
gives a very high laser damage threshold to use a highpeak power laser, and a flowing method of liquid in a
SBS cell is verified to achieve an availability of over 20kW average laser power into a cell.

References

Fig. 4 Improvement of output power of YAG laser system of[1] R. Menzel; Photonics, (2001, Springer-Verlag)[2] B.Ya.Zel’dovich, et al.; Sov.Phys. JETP 15, 109(1972).
[3] H. Yoshida, et al.; Applied Optics 36, 3739 (1997).
[4] H. Yoshida, et al.; Optics Eng. 36, 2557 (1997).
[5] H. Yoshida, et al.; JJAP 38, L521 (1999).
[6] H. Yoshida, et al.; JJAP 42, 439 (2003).
[7] T. Hatae, et al.; J. Plasma and Fusion Res. (2005 , tobe published )