Proceedings of ITC/ISHW2007

Optimization of 48, 57μm poloidal interferometer /polarimeter for ITER
Rostyslav PAVLICHENKO1, Kazuo KAWAHATA1,Tsuyoshi AKIYAMA1 and Shigeki OKAJIMA2
1

National Institute for Fusion Science , Toki-shi, 509-5292, Japan
2
Chubu University, Kasugai-shi, Aichi 487-8501, JapanThe determination of the profile of the toroidal plasma current (safety factor) has been considered
as essential diagnostic for ITER. In absence of neutral beam, the only reliable diagnostics which
can provide the information on the q-profile is polarimetry. About ten viewing chords could coversignificant part of the poloidal cross section of the plasma in a fan-like arrangement at theequatorial port. The probing beams enter the vacuum vessel through a penetration in the blanket
modules at the low-field side. The four additional quasi-vertical viewing chords are inserted
through the upper port. Retroreflectors at the high field side mirror the laser beams back towards
the initial penetrations and allowed to measure Faraday rotation and phase shift. The present work
considered the combined interferometer / polarimeter configuration for laser wavelength of about50 µm. At this wavelength the ellipticity is small but has to be taken for account. Thus, thepromising Cotton-Mouton effect, which is considered as an alternative for plasma density
measurements, will be difficult to implement. The initial optimization of the viewing chords
arrangement for several ITER plasma scenarios (sensitivity to the toroidal current profiles) has
been done. The additional issue of delivering laser radiation to the plasma will be addressed. The
advantages of the waveguide approach for beam focusing in the long (more than 50 m) optical
path are shown.

1. Introduction
Control of the current density profile becomes aparamount issue for the future tokamak experiments.
Polarimetry can provide information on the density
and magnetic field from which current profile could
be reconstructed. Previous system [1] was design tooperate at λ =118.8 μm CH3OH oscillation line. It is
well known that there are two main approaches tobuild the polarimetry system. To obtained information
about magnetic field one have to measure the value of
Faraday rotation angle αF, which is proportional topoloidal magnetic field Bp|| and electron density ne.

α F = 2.62 ×10−13 λ 2 ∫ ne ( z )B p|| ( z ) dz
Z(1)From αF values profile of the poloidal componentof the magnetic field could be calculated. It became
obvious that the electron density along same beamline have to be known. For this purpose along beam
chord a phase measurements by interferometer ormeasurements of the ellipticity angle αCM(Cotton-Mouton effect (CM)) have to be performed.

α CM = 2.62 ×10−11 λ 3 ∫ ne ( z )Bt2 ( z ) dz
Z(2)2. System description
Maximum number of twelve probing beams comes
into the plasma through the diagnostic plug at theauthor’s e-mail: pavlichenko.rostyslav@LHD.nifs.ac.jp

low-field side (LFS). At the high field side (HFS) ofthe blanket shield module (BSM) small ( ∅ 37 mm)
corner retroreflectors (CRR) are placed to reflect
backwards the laser beams. Recently for ITER-scale
experiments and for the plasma experiments on Large
Helical Device (LHD) we have been developing shortwavelength FIR laser. These research activities arecarried out to overcome the common ‘fringe jumps’
phenomena which occurs because of electron density
increasing during pellet injection experiments. OnLHD 13-channels 119 μm laser interferometer has
routinely operated to provide information on theelectron density profile.
The wavelengths of the new two-color
interferometer are 57.2 μm (1.6 W) and 47.6 μm (0.8W) in a twin optically pumped CH3OD laser [2].
For ITER experiments we advocate the classicaldual interferometer / polarimeter approach for its
considerable simplicity to reconstruct experimental
data. We consider the propagation of polarimeterbeams through a thin layer of plasma (thickness z0) in
the presence of a magnetic field. This calculation
takes into account both the Faraday rotation and theCotton-Mouton effect.

Proceedings of ITC/ISHW2007

Figure 2: Calculated Gaussian beam radius (waist of
1/e intensity value) for wavelength 118 and 57 μm.

Fig. 1: Calculated Faraday rotation (top) andellipticity (center), line density (bottom) 57 μm.

3. Alternated transmission line and numericalsimulation of the expected values of therotation angle and ellipticity
The calculation of the expected values of Faradayrotation angle and ellipticity for chosen ITER ‘plasmaburn’ scenario #2 (plasma current Ip=15.0197 MA, q0= 0.99, ‘flat’ electron density profiles are shown at theFig. 1. One can see that for the chosen wavelength of
57.2 μm Faraday rotation angle is about 25−30°,
which is still large enough.
Recently for ITER-scale experiments and for theplasma experiments on Large Helical Device (LHD)
we have been developing short wavelength FIR laser.
These research activities are carried out to overcome
the common ‘fringe jumps’ phenomena because ofrapid raise of the plasma electron density, which
occurs during pellet injection experiments. On LHD
13-channel 118.8 μm CH3OH laser interferometer has
routinely operated to provide information on theelectron density profile. The wavelengths of the new
two-color interferometer are 57.2 μm (power 1.6 W)
and 47.6 μm (power 0.8 W) in a twin optically
pumped CH3OD.

Since 47.6 μm and 57.2 μm have different
polarization a Martin-Puplett diplexer is placed in
front of the laser output. One of the most important
issues is the developing of high quality heterodyne
detection system with fast and sensitive characteristics.
One of the main differences from 118.8 μm system isthat instead of using quasi-optical transmission line
(evacuated tubes of 90-120 mm in diameter) 40 mm
dielectric waveguides become an attractive candidate.
Those waveguides were made from Pyrex®
borosilicate glass (with relative dielectric constant εr =4.6−5.0) or acrylic resin (εr = 2.7−6.0)). From theother hand an oversized waveguides offer an attractive
practical solution to transport light through thecomplicated geometry surrounding the fusion reactor.

Figure 3: Poloidal polarimetry transmission lines andport plug to tokamak.

Proceedings of ITC/ISHW2007
However, they can suffer from serious
radiation-induced
opticalabsorption
andradioluminescence. Special fabrication and glass
hardening techniques must be developed before
suitable radiation -resistance dielectric waveguides
can be used in ITER. By switching from purequasi-optical (QO) beam free space propagation to‘waveguide ideology’ for the transmission line thatlies outside port plug (transmission line that
correspond to the straight line at the Fig. 2) we canresolve several obstacles such as: mode matching /mode conversion, misalignment in the ‘middle part’ ofthe polarimeter optical path, which will be very
difficult to maintain.

the waveguide entrance, εr-waveguide material(relative dielectric constant) was chosen such as:

ε r = 2.1 . The calculation (refer to Fig. 4) showsthat transmission coefficient values for 47.6 and 57.2
coupling into 40 mm diameter waveguide are about
99.6−98.42%, which is 7−8% higher than that for118.8 μm.
The beam propagation inside the oversizeddielectric waveguide have the same efficiency as forthe free space, thus, preserves its polarization (99.6%).
It was already confirmed by the long-term operationof FIR interferometer at LHD [5] (acrylic resin
waveguide, length about 40m) and from several
reports on JET polarimeter diagnostic (Pyrex® glass
waveguide, length about 30m) that polarization of thelaser beam in the waveguide remains almost constant.
It was already shown [1, 3] by other research groups
mechanical and optical properties of the corner
retroreflectors became ultimately ‘Achilles heel’ ofthe system. The positions of the retroreflectors arelimited by mechanical design of blanket shieldmodules on HFS inner wall (see Fig. 5). It was foundthat for wavelength 118.8 μm previously, which CRR
can cope with misalignment up to 15 - 20 mm inpoloidal plane without any serious effect on thereflectivity. For the shorter wavelength those values
become twice smaller: up to 7.5 mm (see Fig. 6).

Figure 4: Coupling of Gaussian beam at the entranceof the dielectric waveguide. The inside diameter of thewaveguide is 40 mm
From other hand waveguide system has precise
mode matching (Fig. 2) (defined by waveguidediameter), easy alignment and more robust tomechanical vibrations. To deliver radiation from / toplasma each laser beam line is equipped with up to 8
miter-bends, with small conversion losses (Fig. 3). To
avoid beam power dissipation Gaussian beam mustenter the waveguide hawing optimized diameter. The
calculation of waveguide transmission coefficient
have been done for λ=48, 57, 118.8 μm⎛⎛R2 ⎞ ⎞T = ⎜1 − exp ⎜ − F 2 ⎟ ⎟ F −1⎜r0 ⎠ ⎟⎠⎝⎝ε +1 L 1where F = 1 + r
2 2
ε r − 1 R k r0(3)andR,

L-waveguide radius and length, k = 2π λ - laser beam
wavelength, r0-radius of the 1/e beam intensity level at

Figure 5: Outline of the retroreflector blockincorporated into the blanket shield module. Without
the plasma the beam is aimed at the center of theretroreflector.
This gives us much more freedom to cope with beamdisplacement caused by mechanical vibrations anddue to refraction in plasma. Present level of CRR
manufacturing could deal with ‘ideal’ sharp corner tosustain desirable sharpness which is about 5% fromthe laser beam width.

Proceedings of ITC/ISHW2007
4. Final remarks
Recently proposed poloidal high power polarimeter
will operate at 47.6 μm, 57.2 μm infrared oscillation.
The output power of 57.2 μm laser is estimated to beover 1.6W and that of 47.6 μm is about 0.8W. Twocolor beat signals are simultaneously detected by aGe:Ga detector with success. It was shown that
preferable polarimeter-interferometer configuration
will unveil some extra advantages in respect of‘full-polarimetric’ system. Shorter wavelength laser
will significantly improve (diminish) refraction
problems. For present chord alignments and beam
wavelength caused considerably small Cotton-Mouton
effect. Alternated waveguide transmission line (withmiter bends included) showed better focusing andtuning as well as much simple further maintenance,
the Cotton-Mouton polarimeter becomes clear only inthe case when the viewing chords are orientated in theequatorial plane, where the poloidal component of themagnetic field Bp is zero (pure toroidal polarimetry).
Under some plasma condition there is a possibility ofcoupling Faraday and Cotton-Mouton effects.

Figure 6: Beam displacement characteristics for thevertical fan of the chords at the position of the centerof the correspondent corner retroreflectors.
Small Faraday rotation angle along some central
chords suggests that placing additional beam lines
must be done to improve spatial resolution of thesystem. Promotion of the dielectric waveguide
addresses the issue of the radiation effect on thosecomponents. The appropriate additional ‘shielding’ of
the waveguides studies now under the consideration.
Further research is needed to define the most adaptedmaterials for dielectric waveguides for FIR
polarimetry.

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[2] K. Kawahata, et.al, Rev. Sci. Instrum., 77, (2006)
10F132.
[3] V. S. Voitsenya, et.al, Rev. Sci. Instrum., 76,(2005) 083502.
[4] H. Soltwich, Plasma Phys. Control. Fusion, 35,(1993) 1777.
[5] K. Kawahata, et.al, Rev. Sci. Instrum., 71, (1999)
707.