16th international toki conferenc e

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1 16th International Toki Conference Advanced Imaging and Plasma Diagnostics P5-14 Ceratopia Toki, Gifu, JAPAN December 5-8, 2006 Design of the 48, 57 m Poloidal Polarimeter for ITER R.Pavlichenko, K.Kawahata, A.J.H.Donné (1) National Institute for Fusion Science, Toki, Gifu 509-5292, Japan (1) FOM-Institute for Plasma Physics Rijnhuizen, NL-3439, Nieuwegein, Netherlands

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16th International Toki Conferenc e. P5-14. Advanced Imaging and Plasma Diagnostics. R.Pavlichenko, K.Kawahata, A.J.H.Donné (1 ). Design of the 48, 57 m m Poloidal Polarimeter for ITER. National Institute for Fusion Science, Toki, Gifu 509-5292, Japan - PowerPoint PPT Presentation

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Page 1: 16th International Toki Conferenc e

1

16th International Toki Conference

Advanced Imaging and Plasma Diagnostics P5-14

Ceratopia Toki, Gifu, JAPAN December 5-8, 2006

Design of the 48, 57 m Poloidal Polarimeter for ITER

R.Pavlichenko, K.Kawahata, A.J.H.Donné (1)

National Institute for Fusion Science, Toki, Gifu 509-5292, Japan(1) FOM-Institute for Plasma Physics Rijnhuizen, NL-3439, Nieuwegein, Netherlands

Page 2: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

Concept of interferometry-polarimetry (I)

dzzxBzxnxZ

Z e ),(),(1062.2)( ||213 2

1

The Faraday rotation is caused by the presence of a magnetic field parallel to the direction of propagation of probing beam.

circular shaped plasma

(idealization)

Calculated Faraday rotation angles (double pass through the plasma) for a horizontal fan of chords (right top) and the corresponding ellipticity values (right center) ; with q-profile, pressure profile and electron density profile on the left.

Very small ellipticity (Cotton-Mouton effect)

μm 75or μm 48

Faraday rotation

z

yx

yxp

Bme

BBme

BBme

c

2

22

22

22222

3

2

1

3

21,

the rotation angle (related to the Faraday effect)

the ellipticity (related to the Cotton–Mouton effect)

The state of polarization can be described by the Stokes vector s(z). The evolution along the line of sight (z-direction) is given by:

)()()( zszdzzsd

z

e dzBn 2311CM 1045.2

After: DeMarco F., Segre S. E. : Plasma Phys. 14 (1972) 245.

Cotton–Mouton effect

Page 3: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

Concept of interferometry-polarimetry (II)

• Control of the current density profile becomes a paramount issue for the modern tokamak experiments. • Polarimetry can provide information on the density and magnetic field distribution inside plasma (current profile), utilizing the Cotton–Mouton and the Faraday effects in a magnetized plasma.

z

e dzBn 106312.2 ||p213

F

z

e dzBn 2t

311CM 104568.2

• In order to evaluate B|| from the rotation angle, the electron density is necessary.

Both ne, B|| must be measured along same chord simultaneously.

Page 4: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

Concept of interferometry-polarimetry (III)

Two general approaches exist to evaluate plasma current profile

Polarimeter - polarimeter Polarimeter - interferometer

polarimeter (Faraday)

polarimeter (Cotton–Mouton)

B||

ne interferometer

B||

ne

Advantages• There is no fringe jumps in principal

Drawbacks

• Fringe jumps

• Mechanical vibrations

• Lots of application on major tokamaks (JT-60U, JET, TotaSupra, RTP,NSTX,MST…)

• Complicated channeling and calibration due to coupling of Faraday rotation and Cotton-Mouton effect.

• Despite promising theoretical and numerical results there is very limited experimental support. 1ch pure Cotton–Mouton polarimeter (W7-AS)

Shorter wavelength laser, with smaller refraction and two color interferometer resolve the problems

polarimeter (Faraday)

• Small Faraday rotation in the core plasma region

Page 5: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

Test of two color FIR interferometer

• For each channel same detector simultaneously detects the beat signals of the 57- and 48-mm laser lines;

•Each interference signal can be separated electrically – the 57.2 µm at 0.6 MHz and the 47.6 µm at 1.6 MHz.

•Mechanical vibration can be compensated by two color interferometer

Ge : Ga Detector

1.06 mm YAG laser

FIR Laser

Silicon B.S.

C.C. Mirror

Optical path length was modulated by using an electro-mechanical vibrator.

SINGLE CHANNEL TEST

10 fr

inge

s/di

v.

10 ms/div.

Page 6: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

Transmission of the Gaussian beams

Gaussian Beam Transformation

Free space multi-lense mutching approach

Waveguide approach

ADVANTAGES

1. Full 3D spatial “freedom”;2. Only two main components (lens, mirror)*

DRAWEBACKS

1. Easy to match modes (defined by WG diameter)2. Easy alignment and much more robust to the mechanical vibrations

1. Mode matching / mode conversion problems;2. Misalignment in the the middle will deteriorate all system; 3. Component support (base plane) much more complicated and bulky

1. Limited length of the straight WG components, for long WG systems addition precaution must be considered for WG multiple connections

Page 7: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

Beam aperture diameter at CRR must obey:

edD 1CRR 2.2

mm 37CRR D Limited by vacant space inside

the BSM (blanket shield modules)

02 Zd

Plasma cut-off frequency:

dZ 25.00m

][m 10115.1 -3215coff n

Beam bending angle:

20

16coff0coff0m 10972.8)arcsin( nnnnn

m 5.35.20 Z Distance from plasma center to CRR

(changed with the beam chord)

Laser beam deviation at the CRR

][m 1002 -3200 .n Central electron density

Preferable choice of probe beam wavelength:

][m 10583.0

3 200

10

nZ

The optimum wavelength for the polarimeter:

μm 110opt

Page 8: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

Circular waveguides diameter optimizationGaussian beams in hollow circular dielectric waveguides

p=0.5588

0.5721

0.6313

The hollow circular oversized waveguides are commonly used in some infrared and millimeter wave devices such as waveguide lasers or transmission lines for infrared interferometers/polarimeters. The advantages of the HE11 mode and Gaussian beams for propagation in these waveguides are well known: low attenuation, linear polarization, azimuthal symmetry

λ3 = 118 µm

λ2 = 57 µm

λ1 = 48 µm

Ø3 = 90 mmØ2 = 40 mmØ1 = 40 mm

22

2

21

exp1

1

1exp1

pp

qp

T

Waveguide transmission

radius waveguideis radius,intensity 1 is where, WG0WG0 aewawp

3WG

2WG

ak

GLq

r

rG

2

material WG theof

constant dielectric relative1.2 r

After: Crenn J.P. : Int.J.IR&MMW. V14,No10 (1993) 1947.

Gaussian beam have to matched into the 40id dielectric WG to avoid power loss and mode conversion

WG598.0 dDopt

Page 9: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

Waveguides, windows, corner retroreflectors

Up to 8 Miter bends per channel

Corner retroreflector modules at the HFS BSM

Dielectric WG

VV Port vacuum interface

miterbendper / dB 103.83dB 4.2CL 5-2

3

D

Miter bends conversion losses

Page 10: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

Window and Beam splitter materials

Measured properties of Crystal Quartz, CVD-Diamond and Si

Tested Silicon windows

Refractive index Absorption coefficient (cm-1)

48 µm48 µm 57 µm57 µm 119 µm119 µm 48 µm48 µm 57 µm57 µm 119 µm119 µm

Crystal quartz 2.260 2.2306 2.1691 4.90 2.90 0.46

Silicon 3.416 3.4164 3.4163 0.33 0.36 0.14

CVD-diamond 2.383 2.3830 - 0.25 0.19 -

nontransparent

After: K. Nakayama, S.Okajima et al. 29th Conf. IRMMW, 2004

Page 11: 16th International Toki Conferenc e

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P5-14Design of the 48, 57 m Poloidal Polarimeter for ITER

SUMMARY

• Proposed poloidal polarimeter will operate at 48,47 m• The output power of 57.2m laser is estimated to be over 1.6 W and that of 47.6m is ~0.8 W.

• Two color beat signals are simultaneously detected by a Ge:Ga detector.

• Preferable polarimeter-interferometer configuration• Well established techniques, a lot of experience .

• Shorter wavelength laser will significantly improve refraction problems.

• Small Cotton-Mouton effect

• Waveguided transmission line / miter bends – better focusing and tuning as well as much siple further maintenance …

• High power two color beat signals are simultaneously detected by a Ge:Ga detector; it is possible to suppress fringe jumps and mechanical vibrations

• Problems• Under some plasma condition there is a possibility of coupling Faraday and Cotton-Mouton effects.

• Small Faraday rotation angle along some chords.