23P099-P
Research and Development of High Power Wideband Diplexerfor ECCD System
電子サイクロトロン電流駆動用ミリ波帯広帯域高速スイッチの開発研究
Kohei Atsumi, Sugawara Shuhei, Yamaguchi Tomoki, Saigusa Mikio,Oda Yasuhisa1), Takahashi Koji1), Kajiwara Ken1), Sakamoto Keishi1)
渥美幸平, 菅原修平，山口智輝，三枝幹雄，小田靖久1)，高橋幸司1)，梶原健1)，坂本慶司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4-12-1
1) 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801-1A diplexer for high power millimeter wave has been developed as a fast switching device in ECCD system.
It is an analogy of an optical ring resonator switch. The switching operation of diplexer with a frequency
shift was confirmed with the FDTD code developed in our laboratory. The diplexer was made up of twocross circular corrugated waveguides, two miter bends and two half mirrors. The switching
operations of designed diplexer in low power tests are reported.

Half mirrors

Output2

Input

Number of Time step(a)

Number of Time step(c)NormalizedTransmission Power

Normalized
Transmission Power

Output1
Fig.1. Principle of fast switching using a diplexer withshifting a frequency.

Normalized
Transmission Power

2. Principle of fast switching device
The proposed diplexer consists of two crosscircular corrugated waveguides, two miter bends
and two half mirrors as shown in Fig. 1.
Input power is divided into the reflected power to
Output1 and the transmitted power to ring resonatorby a half mirror. The transmitted electromagnetic
waves are turned around the inside of a ring
resonator and it is reached Output1. If it is in phasewith the electromagnetic waves reflected in the firsthalf mirror, most rf power go to Output1. And if it
is out of phase, it got to Output2. So, power ofoutput can be switched by shifting a frequency.

Normalized
Transmission Power

1. Introduction
It is expected that the neo-classical tearing
modes (NTMs) deteriorate energy confinement intokamak fusion reactors [1]. Electron cyclotron
current drive (ECCD) using high power
millimeter wave could suppress NTMs [2]. It isuseful that local current drive into O-point of amagnetic island of NTM for stabilization. Up tonow, ECCD with pulse modulated gyrotron
operation at duty of 50% have been done todrive current into only O-point. Recently, forimproving the stabilizing efficiency of NTM, thediplexer like a fast directional switch has beendeveloped [3]. It makes the duty of ECCD system
to 100% by fast switching beam direction fortracking the rotating O-point of a magnetic islandof NTM. For improving performance of fastswitching devices, the new wide band diplexer asa fast switching device was proposed [4]. In thispaper, the results of low power test of new
diplexer are reported, firstly.

Number of Time step(b)

Number of Time step(d)Fig.2. Normalized rf power of a fast switching device
simulated by FDTD code. (a)Output1 using sapphire
mirrors, (b) Output2 using sapphire mirrors, (c) Output1
using slotted mirrors, (d) Output2 using slotted mirrors.

Output Power [arb. unit]

4. Low power test of a fast switching device
The diameter of a circular corrugated waveguideused in the low power test is 63.5 millimeters. The
length of resonant ring is 650 millimeters.
The diplexer with slotted metal mirrors (slot
period:2 mm, slot width:1 mm, disk thickness:1.7mm) was assembled as shown in Fig.1. The
radiation patterns from Output1 and Output2 andthe switching feature at a frequency band of 170
GHz are shown in Fig.3. The radiation patterns ofpeak power look like HE11 modes, while the higher
mode was observed at minimum Output2 as shownin Fig. 3(d) because the slot period is longer than
the wavelength.

(a) 169.975 GHz(b) 170.2 GHz(c) 170.1 GHz(d) 170.425 GHz

Output1(a)(c)
Output2(b)(d)
Frequency [GHz](e)Fig.3. The result of low power test at 170 GHz band. (a)Radiation pattern of Output2 max, (b) Radiation patternof Output2 min, (c) Radiation pattern of Output1 max,(d) Radiation pattern of Output1 min, (e) Frequency
dependence of Output1 and Output2.

When a frequency band is 137 GHz, which is oneof the planed frequencies in JT-60SA, thewavelength is shorter than a slot period. Therefore
switching operation is clearly recognized withouthigher mode as shown in Fig.4.

Output Power [arb. unit]

3. Simulation by FDTD method
The electromagnetic field in the fast switching
device has been simulated with our developed codeusing FDTD method.
The diameter of corrugated waveguides is 21.0
millimeters. The slotted metal half mirrors and thesapphire mirrors are used as the half mirrors in thering resonator. The incidence HE11 mode is afundamental mode of a circular corrugatedwaveguide. When sapphire disks are used as half
mirrors, the input port must be shifted to takeaccount of a shift of center axis due to refractive
index. The characteristic of a fast switching device
in the case of both half mirrors are shown in Fig.2.

(a) 136.95 GHz(b) 137.2 GHz(c) 137.1 GHz(d) 137.375 GHz(a) Output1

Output2

(c)

(b)

(d)

Frequency [GHz](e)Fig.4. The result of low power test at 137 GHz band. (a)Radiation pattern of Output2 max, (b) Radiation patternof Output2 min, (c) Radiation pattern of Output1 max,(d) Radiation pattern of Output1 min, (e) Frequency
dependence of Output1 and Output2.

4. Conclusion
The high power wide band diplexer was designed
and fabricated as a fast switching device for ECCDsystem. The switching operation was checked bynumerical simulations and by low power tests. The
switching operation of diplexer has been recognized
clearly in the both cases of slotted mirrors andsapphire ones.
Acknowledgments
This work has been supported by the Grants-inAid for Scientific Research (c) of 22560818.
References[1] H. Zohm, Phys. Plasmas 4 (1997) 3433.
[2] H. Zohm, et al., Nucl. Fusion 39 (5) (1999) 577.
[3] W. Kasparek et al., Fusion Eng. Des. 84 (2009)1002.
[4] M. Saigusa, et al., Proc. of 13th AMPERE Toulouse(2011), p. 285.