23P093-P
Improvement of the Central-cell ECRH Antenna in GAMMA 10GAMMA 10セントラル部ECRHアンテナの改良研究
Hitomi AOKI1), Tsuyoshi IMAI1), Tsuyoshi KARIYA1),
Ryutaro MINAMI1), Yoshinori TATEMATSU2), et al.
青木瞳1)、今井剛1)、假家強1)、南龍太郎1)、立松芳典2)、沼倉友晴1)、飯泉英昭1)、
中林英隆1)、中澤和寛1)、江口濯1)、 河原崎遼1)、市村真1)、中嶋洋輔1)、吉川正志1)
1) Plasma Research Center, University of Tsukuba 1-1-1 Tennohdai, Tsukuba, 305-8577, Japan
2) Research Center for Development of Far-Infrared Region, University of Fukui
3-9-1 Bunkyo, Fukui, 910-8507, Japan
1)筑波大学プラズマ研究センター 〒305-8577 つくば市天王台1-1-12)福井大学遠赤外領域開発研究センター 〒910-8507 福井市文京3-9-1
The ECRH antenna system of the central cell in GAMMA 10 is newly developed for efficient electron
heating. A new mirror for this system was designed for injecting waves directly to the resonance surface so
as to improve microwave mode purity and the transmission efficiency is about 79%. The performance test
was carried out outside the vacuum vessel and we got good agreement with it. This system was installed toGAMMA10 and obtained improved heating performance.

1. Introduction
In the tandem mirror plasma confinement device,GAMMA 10, the main plasma generated andheated by Ion Cyclotron Resonance Heating(ICRH) is confined in the central cell. Under thisplasma condition, the low electron temperature in
the central cell inhibits the ion temperature rise.
Thus the electron heating is required and the central
cell Electron Cyclotron Resonance Heating has
been developed. This time a new C-ECRH antenna
system is designed for better heating with hightransmission efficiency and mode purity.
The central ECRH system consists of a gyrotron,a transmission line, polarizers and an antenna. The
microwave from the 28GHz high power gyrotron
propagates through corrugated waveguides as HE11mode. The polarization control of the injected wave
is done by the polarizers, which are two miter bends
with twister polarizer and circular polarizer. Thenthe microwave is injected on the EC resonancelayer via the mirror. In order to heat electrons by
the ECRH, the microwave mode injected at theresonance is right-handed elliptically polarized. The
extraordinary(X) mode couples this wave. 100 % X
mode injection is thought to be the best mode forthe ECRH.
The central ECRH antenna system has anellipsoidal mirror for converging of the wave on theresonance near the axis. Figure 1(a) shows theprevious ECRH antenna system of the central cell[1]. Though previous antenna system had the goodtransmission efficiency of 90 %, it couldn’t achieve
accurate X mode injection due to the first passcrossing the plasma, where the phase of the X andordinary(O) mode shifted differently [2]. So newantenna system with accurate X mode control isdesigned in this report.

Fig.1. Schematic views of the ECRH antenna systemof the central cell, (a) the previous system and (b)the new system. (c) The new mirror of the (b)system.

2. New antenna design
2.1 New ECRH antenna system
The directly injected antenna system is designed(Fig. 1(b)). The waveguide and the miter bend are
installed in the vacuum vessel and the microwave isobliquely injected from the mirror (Fig. 3(c)) of theupper side of the vacuum vessel. Thus, themicrowave doesn’t pass in plasma before the final
pass and the wave would have high mode purity.
2.2 Design
We use of elliptical mirror character that themicrowave irradiated from the elliptical focal point
converge to the other focal point. In order to obtain
the axisymmetric converging radiated pattern at theresonancesurface,wehaveusedtheelectromagnetic code developed in Plasma
Research Center, which directly solves the electricand magnetic field, E and H, at the resonancesurface from the field at the mirror [3].
The best calculation result with high transmission
efficiency is obtained when the mirror installed
position is put close to the resonance as much as
possible within the geometrical restriction. The
projected size of the new mirror is 145 × 60 mm2.
Figure 2(a) shows spatial profile of power density. The
1/e folding radius along X and Y axis is 29.8 mm
and 29.7 mm, respectively. The transmission
efficiency to the resonance surface within theplasma radius of 200 mm is about 79 % (Fig. 2(b)),
which a bit lower than that of the previous system.
But the beam doesn’t pass in plasma before thefinal pass for the accurate polarization this time, as
seen in Fig. 1(b).

results within the error.
4. Plasma experiment
The new developed antenna system has beeninstalled to the central cell of GAMMA 10, and theplasma experiment was performed. Figure 4 shows
diamagnetism (DM) increment in the preliminary
heating experiment using the new antenna system.
In 150 – 152 ms, the DM decreases at first and thenincreases with the ECRH microwave injection of100 kW. In 150 – 152 ms, vibrating SX radial
profile is observed. However the cause is not
obvious. This phenomenon was also observed in theprevious antenna system. The DM increase is about
7 % compared to without the ECRH. It is notclear whether new antenna has better heating
performance than before. We need more detailexperiments in GAMMA 10.
-4diamagnetism[10 Wb]

7Fig.2. The calculation results. (a) Spatial profile of power
density. Closed circles are X axis distribution and open
circles are Y axis one. (b) Transmission efficiency.

Fig.3. Spatial profile of temperature increment (dT)measured in (a) cold test and (b) hot test. Closed
circles are X axis distribution and open circles are Y
axis one.

3. Performance evaluation test
We performed tests for verifying performance ofthe designed mirror. The same configuration of thetransmission system as that used in GAMMA 10 ismocked up outside. Radiated power distribution of
the microwave at the absorber corresponding to theresonance surface is investigated by an infraredcamera.
Figure 3(a) and (b) shows the results of the cold
test with the power of 1 W and the hot test with 100
kW, respectively. Both test results are narrow along
X axis and wide along Y axis. This cause is thought
to be the measurement error of ±0.5~1.0 degree oftemperature increment due to infrared camera. Thus
these results give close agreement with calculation

65432219449

1C-ECH
0130 140 150 160 170 180 190
Time[ms]

Fig.4. Time evolution of diamagnetism of the centralcell. The C-ECH injection time is during 150 - 170
ms, with 100 kW and 100 % X mode.

5. Conclusions
The new antenna system of the central cell in
GAMMA 10 is developed. In this system, themicrowave with the accurate X mode purity by the
direct injection to the resonance can be achieved
with the transmission efficiency of 79 %.
The cold and hot test results give closeagreement with calculation results. In preliminary
plasma experiment, we obtained a slight incrementof the DM with the C-ECH injection. More detailed
studies will be done with the new antenna.
Acknowledgments
The author thanks the members of the GAMMA
10 group of the University of Tsukuba for theircollaboration and discussion. This work is partially
supported by National Institute for Fusion Science
Collaborative program (NIFS11KUGM050 andNIFS11KUGM053).
References[1] H.AOKI et al. 27th JSPF Annual Meeting, 02P84.
[2] H.KOUDOU et al. 27th JSPF Annual Meeting,
02P85[3] Y.TATEMATSU et al. Jpn. Appl. phys. 44(2005)
6791-6795.