23P098-P
Research and Development of High Power Wide-band Polarizer for ECCD
System電子サイクロトロン電流駆動用ミリ波帯広帯域偏波器の開発研究
Naoya Sugiyama, Suguru Ikehara, Mikio Saigusa, Shinichi Moriyama1), Takayuki Kobayashi1)
杉山直弥，池原 優，三枝幹雄，森山伸一1)，小林貴之1)
College of Engineering, Ibaraki University
4-12-1, Nakanarusawa-cho, Hitachi 316-8511, Japan
茨城大学工学部 〒316-8511 日立市中成沢町4-12-11) Japan Atomic Energy Agency
801-1 Mukoyama, Naka 311-0193, Japan
日本原子力研究開発機構 〒311-0193 那珂市向山801-1
A high power wide-band polarizer has been developed for ECCD system using step tunable gyrotrons, Thepolarizer has been designed at frequency bands of 110,130 and 140 GHz has been analyzed with the finite
differential time domain method. The wide-band polarizer was designed for low-power experiments.

1. Introduction
Electron Cyclotron Current Drive (ECCD)
system is one of the most promising method forheating and stabilization of fusion plasmas.
Incident wave from gyrotron was needed toconvert linearly polarized wave into specificelliptically polarized wave in order to excite thesuitable wave at high mode purity. The polarizer
is used for generating the specific polarization.
Until now, the polarizer corresponding to single
frequency has been used. However, it may be able
to let the electromagnetic waves of different
frequency on the same transmission line pass if
wide-band polarizer which can be installed in a
waveguide transmission line can be developed.
Therefore, transmission line composition can besimplified sharply.
In this paper, the possibility of development ofwide-band polarizer for ECCD system isreported.
2. The principle of polarizer
The linearly polarized wave, which is injected atright angles to a grooved mirror, can be divided intoa electric field and a magnetic field component
parallel to a groove direction. The former isreflected on the top of the grooved mirror, becauseof cutoff in groove, the latter is reflected at abottom of it as long as the cycle a is less than half
wavelength, as shown in Fig.1. Phase difference
between fast polarization: FP and slow polarization:
SP can be controlled by the groove depth of h.
In order to suppress high order diffraction waves,the cycle a must be given bya   /(1+sin cos  )(1)where  is a incident angle,  is a rotation
angle, and  is a wavelength [1].

Fig. 1. A schematic view of grooved mirror.

A universal polarizer generally consists of a pairof grooved mirrors. The groove depth of the one
mirror is about quarter wavelength, so that thephase difference between two reflected polarization
waves is about 180 deg. The groove depth of theother one is about 1/8 of the wavelength, and thephase difference between two reflected polarization
waves is about 90 deg. Arbitrary elliptical polarizedwave can be generated by rotating these twogrooved mirrors.
3. Analysis by the FDTD method
The polarizer has been analyzed by finite
differential time domain (FDTD) method. Themodels of grooved mirror for analysis were defined
as shown in Fig. 2. The dependence of phasedifference between FP and SP on the depth andwidth of groove have been analyzed. The groove
period: a was assumed to be 1.1664 mm from Eq.
(1).

Phase differenceof FP and SP [rad]4b/a=0.5 y1 = 6.6594x1 - 0.9123
b/a=0.67b/a=0.78 y2 = 5.2154x2 - 0.5787

3.5
3
2.5
2
1.5
1
0.5
0(a)
(b)
Fig.2. Modeling of the grooved mirror used for analysis:(a) rectangle type and (b) trapezoid type.

Phase differenceof FP and SP [rad]The analysis results in  =45 deg. and S=0 deg.
of grooved mirror rotation angles at 110,130 and140 GHz are shown in Fig. 3.
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0

0.2

0.4

0.6

0.8

0.6

0.8

(a)
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0

0.40.60.8Fig. 4. Phase difference between FP and SP on thegroove depth in various groove shapes at 110 GHz.
Table I. Maximum electric fields and the reduction ofphase differences in various groove shapes at 110 GHz.

Rectangle TypeRoundish Type 1
Roundish Type 2
Roundish Type 3

Groove depth [mm]0.2Groove depth [mm]140 [GHz]
130 [GHz]
110 [GHz]

0

Phase differenceof FP and SP [rad]

0Efp

Esp

2.78
2.40
2.31
2.25

2.60
2.19
2.08
2.10

Phase difference
(rad)
2.92
2.86
2.79
2.71

The grooved mirror and the flange attached to amitre-bend was designed for a low power test as
shown in Fig. 5.

140 [GHz]
130 [GHz]
110 [GHz]

00.2

0.4

Groove depth [mm](b)Fig. 3. Phase difference between FP and SP on thegroove depth; (a) rectangle groove, (b) trapezoid groove,at the both grooved mirror form ratios: b/a=0.78.

4. The design of the wide-band polarizer
On the assumption the groove period is 0.467
and the ratio of the width of a slot to ridge is 1 to 1,
the depth with optimal diffraction grating is 0.25
and 0.149 [2]. At 110 GHz, these are 0.681 mm
and 0.406 mm, respectively.
As shown in Fig.4, if they are substituted for x1,
the phase difference y1 will be set to 3.618 rad and1.791 rad. Further, they are substituted for y2, thedepth of the slot x2 will be set to 0.804 mm and
0.454 mm. As shown Table I, in order to use electric
power-proof, by deleting the angle at the tip of adiffraction grating, phase difference decreased by
0.21 rad at the maximum.
The depth of the slot is set to 0.85 mm and 0.49mm as a result of taking these all into consideration.

Fig.5. The erection diagram of the polarizer

5.Conclusion
The wide-band polarizer has been investigatedin numerical calculations. It appears that the large
ditch ratio rectangular grooved mirror is suitable forwide-band. The rectangular wide ditch grooved
mirror installed in a miter bend was designed for awide-band polarizer.
Acknowledgments
This work was performed by Japan Atomic Energy Agency under contract with Ibaraki University(Contract Number: 22I124).
References[1] J. Maloneyand M. Kesler: Computational Electrodynamics The FDTD Method. 13 (2004) 553.
[2] M. Harigae, et al., Fusion Sci. and Technol. 55(2009) 136.