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“Advanced Imaging and Plasma Diagnostics”

Advanced Laser Diagnostics for Electron Density Measurements

K. Kawahata, T. Akiyama, R. Pavlichenko, K. TanakaK. Nakayama, S. Okajima1), S. Tsuji-Iio

National Institute for Fusion Science, Chubu University1)

IntroductionIntroductionCottonCotton--Mouton Mouton polarimeterpolarimeter on CHSon CHSTwo color laser interferometerTwo color laser interferometer

16th International Toki ConferenceCeratopia Toki, Toki, JAPAN

December 5 - 8, 2006

2/14

Optimization of probe beam wavelength

LHD

Pneumatic

Windows

2.0

m

3.9 m 7.8 m

Waveguide

FIR LaserInterferometer

Isolator

Housing

Helical Coil

ReflectorsRetro-

Frame

6.0

m

CO2 LaserInterferometerHousing

10.6

m

Poloidal Coil

7.3 m

Proposed Polarimeter system for ITER

FIR Laser Interferometer installed on the LHD

On LHD, a 13-channel 119-μm CH3OH far infrared laser interferometer has been routinely operated for the precise measurements of the electron density profile .

The maximum densities of over 4 x 1020 m-3

were achieved by using pellet fueling and LID pumping.

Need of a short wavelength laser

λ：~ 50 μmA poloidal polarimeter system has been designed for

the measurements of current density profile in ITER.Proposed laser oscillation line is 119-μm CH3OH, but

the beam bending effect due to plasma density gradientis large.

Laser oscillation lines around 50μm are considered to be suitable from the consideration of the beam refraction and signal-to-noise ratio.

3/14

Choice of probe beam wavelength• The optimum wavelength suitable for

interferometry/palarimetry can be selected from the following considerationsPhase shift: Cutoff frequency: Beam bending effect:

(Example: Zo = 3.8 m, ne(0) = 1x1020/m3 leads to λ< 330 μm)

Mechanical vibration:

Faraday rotation:

Cotton-Mouton effect:

2161 1097.8)(sin λα oc

o

c

om n

nn

nn −− ×=≅=

2/12/1 )(2)(2πλ

απλ o

moo Z

ZZ

d ≤→=

mnZ oo3/1210 )(1016.1 −×≤λ

D. Veron

1

2

152.82 10 ( )z

zn z dzϕ λ−= × ∫

15 2 -3c 0 1.11 10 / (m )

n λ= ×

12 Lλφ π −ΔΔ =2

1

13 22.62 10z

znB dzλ−Ω = × ∫

11 3 22.45 10 eB n zε λ− Δ= ×

0

0.3

0.6

0.9

1.2

0

1

2

3

4

-1 -0.5 0 0.5 1

sh64359-1136-b120-a2020

n / no

Te(eV)

n /

no Te (keV)

ρ

n(0) = 4.2x1020m-3

T(0) = 0.87 keVβ(0) = 4.2 %

at B = 2.64 T

~50μm

k B

k B⊥

4/14

Selection of Wavelength for ITER DiagnosticsDependence of phase difference by

CM effect on wavelength of laser

∫ ⊥−×=

−=

]m[]T[]m[]mm[104.2

]rad.[23-320

XOCM

dlBneλ

φφφO

X

Linearly Polarized Beam

Elliptically Polarized Beam B

Plasma

101 10210-2

10-1

100

101

102

103

104

Wavelength (μm)

Cott

on-M

outo

nP

has

e S

hift

(deg.

)

10.6 μm57 μm

119 μm

337 μm

52

5.7

Ne=1x1020m-3, B0=5.3T, L=4m

5/14

101 10210-1

100

101

102

103

Wavelength (μm)

Far

aday

Rota

tion (deg.

)

14

60

101 10210-2

10-1

100

101

102

Wavelength (μm)

Dis

plac

em

ent

(mm

)

4

0.9

101 102100

101

102

Wavelength (μm)

Phas

e S

hift

(Fringe

)

4320

4 mm0.9 mmDisplacement

52 deg.5.7 deg.Cotton-Mouton

60 deg.14 deg.Rotation

43 fringes20 fringesPhase

Measurement119 μm57 μm

Selection of Wavelength (continue)

∫ ⊥ dlBne23λ

02nλ

∫ dlBne2λ

∫ dlneλ

Ne=1019+1x(1-ρ2)1020m-3, B0=5.3T, L=4m, Ip=15MA

Ne=1x1020m-3, B0=5.3T, L=4m Ne=1019+2x(1-ρ2)1020m-3,

B0=5.3T, L=4m

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HCN laserCHS

SRG

Optics for multi-channel

Frame Laser light

CHS plasma

Optics for detection Basement

Acrylic resin pipe for waveguide

5950

CottonCotton--Mouton Mouton PolarimeterPolarimeter for Electron Density for Electron Density Measurement with HCN laser on CHSMeasurement with HCN laser on CHS

• Path length from the laser to optical frame is about 15 m.• Dielectric waveguide (φ 47 ID) made of acrylic resin is used to transmit laser beams.

The transmission efficiency is totally about 80%. • Detectors are schottky barrier diodes.

Upper part of optical frame

Optics box

15 m

HCN laser11 3 22.45 10 eB n zε λ− Δ= ×

7/14

ω

ω ωb+O-mode

X-modePlasma Polarizer

π/4Detector

sin{( )t+ }ω ω φb o+

sin( t+ )ω φx

B

Measurement Method of CM effect

Phase difference due to Cotton-Mouton effect can be measured as a phase difference between probe and reference signal.

Free from amplitude variations due to oscillation instabilities of laser and beam deviation.

{ }[ ]{ } Const.)(cos

)sin()(sin

XOb

2XObprobe

+−+∝

++++=

φφω

φωφωω

t

ttIProbe signal:

Const.)cos( bref +∝ tI ωReference signal:CMφ=

S.E. Segre, Phys. Plasma 2 2908 (1995)

8/14

Optical Setup of CM Polarimeter with Int.

HCN

SRG

WaveguideBS

Wire Grid BS

CHS Plasma

BS

BS

λ/2 plate

ω

Polarizer

ω+ωb (1 MHz)

Detector

Reference

Interferometer

CM polarimeter

preamplifierBandpass Filter

Phase Counter(Interferometer)

ADC

sinωbt

sin(ωbt+φint.)

sin(ωbt+φCM)

• Beat signal of 1 MHz• Simultaneous measurement system of an interferometer

and a CM polarimeter with the same plasma center chord inorder to check the absolute value of CM phase difference.

This is almost same as the system proposed on W7-X.

9/14

0 50 100 1500

10

Time (ms)

n e, n

eCM

(1019

m-3

)

Response Time = 1 ms CM Polarimeter Interferometer

-

#126371Bt=1.76 T

NBI ECH Gas Puff

-Measurement Results

Phase difference due to Cotton-Mouton effect can be measured successfully.

The result of the polarimeter is almost consistent with that of theinterferometer except initial phase of the discharge. Amplitude of phase variations is within ±0.5 deg. with time constant of 1.0 ms.Interferometer shows fringe jump errors twice (t=135,144ms)

After improvement of cross and back talks

Cf. JET CM Measurement

CM 1deg.with 10t msδφ = Δ =

10/14

Development of a new two color FIR laser diagnostics

The Highest Output Power

CO2 Laser

FIR LaserStark Cell

0

2

4

6

8

10

00:00 05:00 10:00 15:00 20:00 25:00

Time (min.)

Las

er

outp

ut

(arb

it. unit)

CO2 laserCavity length: 3 mDischarge length: 1.25 x2Output coupler:ZnSe, 20 m radius curvatureGrating: 150 g/mm, blazed at 10.6 μm

FIR LaserCavity length: 2.9 mLaser tube: 25 mm inner diameterInput coupler: gold-coated copper M.Output coupler: Silicon hybrid coupler

57 μm laser output

cw FIR laser system

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Plasma

Two color FIR laser Interferometer

2

1

2

1

15

( )

2.82 10 ( )

z

ezc

z

ez

n z dzn

n z dz

πφλ

λ−

Δ =

= ×

∫

∫

57.2 μm is optimum value to avoid refractive effects in high density operation of LHD and future fusion devices.Both laser beams pass the same optical path in the interferometer without any optical path difference. One detector simultaneously detects the beat signals of both laser oscillation lines.

1E⊥2E

1E⊥2E⊥

12/14

10 dBm, 500 kHz/divTwo color beat signals detected by Ge:Ga photoconductor.

Beat frequency;

0.6 MHz for 57 µm

1.2 MHz for 48 µm

The frequency of the beat signals can be changed with the cavity length and pressure of the FIR laser.

Detection of two-color beat signals

Ge:Ga DetectorYAG Laser

He-Ne Laser

FIR Laser

C.C.M.Monitor

48 μm 57 μm

13/14

Demonstration of Two color FIR laser Interferometer

One detector simultaneously detects the beat signals of both oscillation lines, and each interference signal can be separated electrically – the 57.2 μm at 0.6 MHz and the 47.6 μm at 1.6 MHz.Two color FIR laser interferometer work was successful in demonstration of mechanical vibration compensation.

57 μmbeat signal

48 μmbeat signal

57 μmbeat signal48 μmbeat signal

57 μmbeat signal48 μmbeat signal

Detector (I)

Detector (II)

57-μm LaserInterferometer

48-μm LaserInterferometer

Optical path length was modulated by using a piezo-electric transducer.

PhaseCounter

Pie

zo D

rive

Vol

tage

(a.u

.)

0 100 200

-100

0

100

48 μm 57 μm 48-57 μm

Vib

ratio

nal

Dis

plac

emen

t (μm

)

Time (ms)

±20 μm

14

SUMMARY

(1) A Cotton-Mouton polarimeter has been developed on CHS, combining with an interferometer.

(2) The measurement results of the (2) The measurement results of the polarimeterpolarimeter showshowa good agreement with the interferometer.a good agreement with the interferometer.

(3) A two color FIR laser interferometer is under development for high density plasmas on LHD. Simultaneous operation at 57.2μm and 47.7μm is confirmed.

(4) (4) Two color beat signals are simultaneously detected by using a Ge:Ga photoconductive detector.

(5)(5) Two color FIR laser work was successful in Two color FIR laser work was successful in demonstration of mechanical vibration compensation.demonstration of mechanical vibration compensation.