25D01
High Mode Purity Excitation of HE11 modein Circular Corrugated Waveguides
円形コルゲート導波管における高純度HE11モード励起	 Hiroshi Idei1), Masatsugu Sakaguchi2), Michael Shapiro3), Richard Temkin3) and Walter Kasparek4)
出射	  浩, 坂口政嗣 Michael Shapiro, Richard Temkin and Walter Kasparek
1)
2)

Research Institute for Applied Mechanics , Kyushu University, 6-1, Kasuga-koen, Kasuga, 816-8580, Japan
Interdisciplinary Grad. School of Eng. Sci., 6-1 Kasuga-koen, Kyushu University., Kasuga, 816-8580, Japan
3)
Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
4)
Institut für Plasmaforschung, Universitat Stuttgart, Pfaffenwaldring 31, D-70569 Stuttgart, Germany
1)
九州大学応用力学研究所 〒816-8580	
  春日市春日公園６−１2)
九州大学総合理工学府 〒816-8580	
  春日市春日公園６−１

A new HE11-mode exciter system using overlapped 2 Gaussian beams is designed and proposed,
instead of a traditional Gaussian system. The system is composed of a scalar horn antenna, a beam
splitter and combiner, and quasi-optical mirrors. The amplitude distribution of a HE11 mode has
smaller kurtosis, compared to a Gaussian beam, and the field can become zero at the waveguide wall.
The amplitude distribution is controlled by interference of the 2 Gaussian beams. The phase profile isflat at the waveguide aperture. The optimum HE11 mode purity excited in this system is 99.98%.

field should be zero at the waveguide wall. In orderto obtain the broad amplitude profile of the HE11
mode with zero amplitude at the waveguide wall, anamplitude control is needed. A new HE11-mode
exciter system is designed and proposed here. In
this system, 2 Gaussian beams are overlapped witha phase matching to control the amplitude
distribution.
2. Conceptual Design
The HE11 mode of the CC waveguide has abroad amplitude and flat phase profiles. Here, a
new QO HE11 mode exciter using overlapped 2
Gaussian beams is proposed. Figure 1 shows 2Gaussian parabolic phase profiles with phasecurvatures of +/- R, indicating one is expanding andthe other is focusing. The diameter of CC
waveguide is 63.5mm, and the frequency is 170
GHz. A flat phase profile is obtained, resulting in
the interference of 2 beams. The amplitude profile
stemming from the interference is a cosine with anargument of the parabolic phase profile. The cosine
3

Phase [radian]

1. Introduction
Electron Cyclotron Heating (ECH) using highpower millimeter waves is an attractive method forplasma production, auxiliary heating, and current
drive in a nuclear fusion research. In planned fusion
experiments at the ITER, the injected total power is
20 MW for ECH. An output beam from high power
gyrotron oscillators is coupled into the oversizedcircular corrugated (CC) waveguide transmission
lines as a main transmitted HE11 mode. The higher
order modes are excited in the oversized condition
due to miss-alignment of the coupled beam or due
to manufacturing error of the components. These
cause undesirable arcing events and/or overheatingin the transmission line. In the ITER-ECH
transmission line, the total loss including unwanted
mode excitation is evaluated as 2% [0.088dB] ateach miter bend section. A precise understanding of
the mode contents transmitted in the CC
waveguides is one of most important issues in the
ITER-ECH transmission. In order to study the
unwanted higher mode excitation experimentally, apure HE11 mode should be first excited in the CC
waveguides. In this paper, the high HE11 mode
purity excitation is described in quasi-optical (QO)
systems.
The Gaussian beam was widely used to excite theHE11 mode at the CC waveguide aperture in the
QO system. The Gaussian beam can be converted
into the HE11 mode with an efficiency of 98%. The
field distribution of the HE11 mode has a smaller
kurtosis, indicating a strong flatness, compared to a
Gaussian beam distribution, and the HE11 mode

2

1

0
-0.03

0.00

r [m]

0.03

Fig.1 : Gaussian parabolic phase profiles ofoverlapped 2 Gaussian beams.

distribution by the interference has a smallest
kurtosis of 2.2 without taking Gaussian amplitude
profiles into consideration. The kurtosis of theHE11 mode and Gaussian beams are 2.7 and 3,
respectively. The small kurtosis (2.7) of the HE11
mode can be obtained by the interfered cosine
dependence and Gaussian distributions. The field at
the waveguide can become zero, provided that [0,π]phasing is given there, as shown in Fig.1.

(a)

(b)

amplitude and phase profiles in the optimized beam
parameters are shown in the figures. The high HE11
mode purities of 99.96% and 99.98% can be excitedin the optimization of the beam parameters (ω1,
ω2) and (ω, R), respectively. In the optimization of(ω1, ω2), the phase curvature R is taken to obtain
zero amplitude at the waveguide wall with the [0,π]phasing. The inferred amplitude profile matched for
the HE11 mode profile and flat phase profile can be
excited using a 2 phase-matched Gaussian system.
3. System Schematic
Figure 1 shows a schematic of the phase-matched
Gaussian system. In the phase-matched Gaussian
system, expanding and focusing Gaussian beams
should be injected to the CC waveguide. The scalarhorn antenna and a QO mirror are used to excite a
Gaussian beam, and wire grid polarizers are used a
combiner and a splitter. Some QO mirrors are used
to obtain the 2 phased-matched Gaussian beams.

(c)

Fig.3: Schematic of phased-matched Gaussian system.

(d)Fig.2 (a): Contour plot of HE11 mode purity for given
beam sizes (ω1, ω2), and (b): amplitude and phaseprofiles in the optimized (ω1, ω2). (c): Contour plotof HE11 mode purity for given beam size ω and phasecurvature R, and (d): amplitude and phase profiles in
the optimized (ω, R).

Figures 2 show contour plots of the HE11 mode
purity for beam sizes (ω1, ω2) and phase curvature
R of the 2 overlapped Gaussian beams. The

4. Component Design
The scalar horn antenna is designed forbroadband operation of 110-140GHz using amoment method simulator. The radiated amplitude
and phase profiles are measured in low power test
facilities. The measured Gaussian-like amplitude
profile coincided within more than 15dB
dynamic-range to the designed profile, and themeasured phase is exactly the same to the designed
one. However, the beam propagation cannot be
explained with Gaussian optics. The Kirchhoff
integral code is developed to design the QO mirror
surface, instead of the moment method simulator, toobtain a Gaussian beam. The wire grid polarizer isalso designed using the moment method simulator.
The beam can be split without significant influenceby the wire grid, depending on the polarization. The
design of this 2 phase-matched Gaussian beamsystem is finished and is going to fabricate and be
tested soon.