16th International Toki Conference

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16th International Toki Conference
P5-14
Advanced Imaging and Plasma Diagnostics
R.Pavlichenko, K.Kawahata, A.J.H.Donné (1)
Design of the 48, 57 mm Poloidal Polarimeter for
ITER
National Institute for Fusion Science, Toki,
Gifu 509-5292, Japan (1) FOM-Institute for Plasma
Physics Rijnhuizen, NL-3439, Nieuwegein,
Netherlands
Ceratopia Toki, Gifu, JAPAN December 5-8, 2006
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Concept of interferometry-polarimetry (I)
Faraday rotation
CottonMouton effect
circular shaped plasma (idealization)
The Faraday rotation is caused by the presence of
a magnetic field parallel to the direction of
propagation of probing beam.
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
the rotation angle (related to the Faraday effect)
the ellipticity (related to the CottonMouton
effect)
After DeMarco F., Segre S. E. Plasma Phys. 14
(1972) 245.
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)
Design of the 48, 57 mm Poloidal Polarimeter for
ITER
P5-14
3
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
  CottonMouton and the Faraday effects in a
  magnetized plasma.

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

Both ne, B must be measured along same chord
simultaneously.
Design of the 48, 57 mm Poloidal Polarimeter for
ITER
P5-14
4
Concept of interferometry-polarimetry (III)
Two general approaches exist to evaluate plasma
current profile
Polarimeter - polarimeter
Polarimeter - interferometer
B
B
polarimeter (Faraday)
polarimeter (Faraday)
ne
ne
polarimeter (CottonMouton)
interferometer
Advantages

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

• There is no fringe jumps in principal

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

• 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 CottonMouton polarimeter
  (W7-AS)

• Fringe jumps
• Mechanical vibrations

• Small Faraday rotation in the core plasma region

Design of the 48, 57 mm Poloidal Polarimeter for
ITER
P5-14
5
Test of two color FIR interferometer
SINGLE CHANNEL TEST
FIR Laser
1.06 mm YAG laser
Silicon B.S.

• 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

GeGa Detector
10 fringes/div.
C.C. Mirror
Optical path length was modulated by using an
electro-mechanical vibrator.
10 ms/div.
Design of the 48, 57 mm Poloidal Polarimeter for
ITER
P5-14
6
Transmission of the Gaussian beams
Design of the 48, 57 mm Poloidal Polarimeter for
ITER
P5-14
7
Laser beam deviation at the CRR
Beam aperture diameter at CRR must obey
Plasma cut-off frequency
Beam bending angle
Preferable choice of probe beam wavelength
Limited by vacant space inside the BSM (blanket
shield modules)
Distance from plasma center to CRR (changed with
the beam chord)
Central electron density
The optimum wavelength for the polarimeter
Design of the 48, 57 mm Poloidal Polarimeter for
ITER
P5-14
8
Circular waveguides diameter optimization
Gaussian beams in hollow circular dielectric
waveguides
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
Waveguide transmission
After Crenn J.P. Int.J.IRMMW. V14,No10
(1993) 1947.
p0.5588
0.5721
?1 48 µm
?2 57 µm
0.6313
Gaussian beam have to matched into the 40id
dielectric WG to avoid power loss and mode
conversion
?3 118 µm
Ø1 40 mm
Ø2 40 mm
Ø3 90 mm
Design of the 48, 57 mm Poloidal Polarimeter for
ITER
P5-14
9
Waveguides, windows, corner retroreflectors
Miter bends conversion losses
Up to 8 Miter bends per channel
VV Port vacuum interface
Dielectric WG
Corner retroreflector modules at the HFS BSM
Design of the 48, 57 mm Poloidal Polarimeter for
ITER
P5-14
10
Window and Beam splitter materials
Tested Silicon windows
Measured properties of Crystal Quartz,
CVD-Diamond and Si
Refractive index Refractive index Refractive index Absorption coefficient (cm-1) Absorption coefficient (cm-1) Absorption coefficient (cm-1)
48 µm 57 µm 119 µm 48 µm 57 µm 119 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
Design of the 48, 57 mm Poloidal Polarimeter for
ITER
P5-14
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SUMMARY

• Proposed poloidal polarimeter will operate at
  48,47 mm
• The output power of 57.2mm laser is estimated to
  be over 1.6 W and that of 47.6mm is 0.8 W.
• Two color beat signals are simultaneously
  detected by a GeGa 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 GeGa 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.

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