29E06PGAMMA 10におけるECH二枚ミラーアンテナ系の改良
Development of ECH antenna system with two mirrors in GAMMA 10

中澤和寛, 今井剛, 南龍太郎, 假家強, 沼倉友晴,中林英隆, 江口濯, 河原崎遼, 南齋宏駿, 浅野徳馬
K. Nakazawa, T. Imai, T. Kariya, R. Minami, T. Numakura,
H. Nakabayashi, T. Eguchi, R. Kawarasaki, H. Nanzai, T. Asano
筑波大学プラズマ研究センター
Plasma Research Center, University of Tsukuba
In the GAMMA 10, Electron Cyclotron
Heating (ECH) is utilized for the formation ofthe plasma-confining potentials in plug/barriercell. The central ECH (c-ECH) is another key toimprove plasma performance of the GAMMA 10,
since low electron temperature causes thestrong electron drug of hot ions in the centralcell. To increase the electron temperature of theplasma, it is necessary to inject the strong
electron cyclotron waves into the core plasma inthe central cell. The c-ECH antenna system has
been improved in terms of the transmission
efficiency, the control of wave polarization, andradial profile. The antenna system with one
mirror (M4) used in the last time enabled toinject accurate X-mode wave, since it directly
see the EC resonance layer to remove wave
polarization deterioration due to plasma. Thetransmission efficiency of the antenna system is
80% in calculation. In the plasma experiment,
when we increase the injection power of thegyrotron of the c-ECH from 50kW to 150kW, thediamagnetisms tend to decrease. This results
would be caused by the increase of the stray RF
field generated by rest of the microwave powerthat couldn’t be covered on the antenna.
Therefore, it is necessary to design the newc-ECH antenna system in order to reduce thestray RF radiation.
The c-ECH antenna system was newly
designed to improve the previous antennasystem with two mirrors (M1&M2) in 2009. The
new antenna system is composed of two mirrors“M1a” and “M2”. The M1a is set on the end of
the waveguide, and the M2 is the same asprevious. The previous M1&M2 antenna system
had only about 70% transmission efficiency
because the size of the M1mirror had been
limited by the port size. Clearing this limit from

the M1 by being separable, it becomes possible
to enlarge the size of the M1a about two times,
from 40.88×50mm2 to 52.5×100mm2 and to
expand the diameter of the waveguide at theopen end from 31.75mm to 45mm. The
transmission efficiency of the new antenna
system is improved up to about 95% in
calculation and the radial profile is ideallyround. This calculation values are obtained byuse of the computing code which calculates thesurface currents on the mirror derived from theradiation at the waveguide mouth, and thencalculates the electromagnetic field excited bythem. [1,2]In order to check mirror performance, weperformed the cold radiation tests by using 1 Woscillator. The heat map at the resonance layer
measured by infrared camera is compared withthe calculation. The experimental data are ingood agreement with the calculation. The
transmission efficiency of the new antenna M1aachieves about 83%, it is about 20% increase ascompared with the previous M1 system.
The author thanks the members of the GAMMA 10group of the University of Tsukuba for theircollaboration and valuable discussion during this study.
This work is partially supported by NIFS collaborative
program (NIFS11KUGM050).

[1]Y.Tatematsu et al.,Jpn.Appl.phys.Tech45(2006) 7911-7913[2]Y.Tatematsu et al., Jpn.Appl.phys.Tech44(2005)6791-6795