25F01
Plasma loading on the slow-wave excitation antennaof the magnetospheric plasma device RT-1
磁気圏型プラズマ装置RT-1における遅波励起用アンテナのプラズマ負荷特性
Yoshihisa Yano1, Zensho Yoshida1, Haruhiko Saitoh1, Junji Morikawa1 and Atsushi Fukuyama2
矢野善久, 吉田善章，齊藤晴彦，森川惇二，福山 淳
1Grad. Sch. of Frontier Sci., Univ. of Tokyo, 2Dep. of Nuc. Eng., Kyoto Univ.
5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
東大新領域 〒277-8561 千葉県柏市柏の葉5-1-5

The first antenna loading experiment in the magnetospheric configuration was conducted by using a testantenna for the slow-wave excitation. In the experiment, a weak radio-frequency wave, the frequency was
1MHz – 3MHz and the power was lower than 0.1W, was transmitted into the plasma from the 3-turncircular antenna which was installed at the high field side of the ICRF resonances. Then the plasma loading
resistance was evaluated from a comparison between the antenna resistance with and without the plasma.
The results agree qualitatively with a wave calculation. The maximum plasma loading resistance was about
22Ω at 3MHz and 2.0Ω at 1MHz, which is considered to be sufficient.

1. Introduction
The Ring Trap-1 (RT-1) device can confine ahigh-beta plasma in a magnetospheric configuration
which is realized by a levitating superconducting
coil. So far, we have verified that very high beta
plasma, whose local beta value exceeds 70%, isstably confined in the RT-1 device with an electron
cyclotron heating (ECH) system. So the high beta
plasma in RT-1 is composed of hot electrons whosetemperatures are 10 - 50 keV and whose densities
are 1016 - 1017m-3[1] .
As a next step of the RT-1 project, we areplanning to heat ions by using an ion cyclotron
heating (ICH) system. Then two-fluid effects will
be experimentally examined [2] when the ion beta
value is sufficiently high.
Although the ICRH system has already become abasic plasma heating method in other types offusion device (Tokamak etc.), there have been noprevious ICRH systems in dipole configuration like
RT-1. In the magnetospheric configuration, themagnetic field strength (0.01-0.3T) and plasma
density (1016 - 1018 m-3) is much smaller than otherconfigurations. These features generally result in
low plasma impedance such as a resistance and areactance. If the plasma impedance is much lessthan the impedance of antenna itself, almost all of
the energy is wasted to heat the antenna and the RFmatching circuit become very difficult to bedesigned. Therefore, in this experimental research,
the loading resistances of the RT-1 plasma when aslow wave is excited are examined and a feasibilityof the ICRH system is considered.

Fig.1. Cross-sectional image of RT-1. The
magnetic field lines, the magnetic fieldstrength, the antenna location and theinterferometer sight lines are drawn.

2. Experimental Setup
2.1 Test Antenna
Three-turn circluar antenna, made from stainlesssteel wire, is installed around the center stack inorder to launch a slow wave from the high-fieldside of the ICRF resonances. There were no faradayshields (F. S.) at the beginning of the experiment.
At the latter half of the experiment, about 2/3 partof the antenna was faraday-shielded. A range of atested frequency was about f=1-3 MHz(|B|=0.0656–0.197 T for a hydrogen ion cyclotron
resonance).

2.2 Target Plasma
Hydrogen plasma heated by 8.2GHz ECH system
was mainly used as a target. The floating coil wasusually supported.
2.3 Resonance Method
Resonance method is used to evaluate the plasnaloading impedance. A change of the impedance
after the plasma is discharged is measured.

Fig.2. Photograph of the test antenna and thefloating coil. About 2/3 part of the antenna is
faraday-shielded.

Fig.3. The geometry of the test antenna. The ICRF
resonances for f=1, 3MHz are shown.

3. Experimental Result and Consideration
Experimental results are shown in Fig. 4. The
maximum loading impedance is higher than 20Ω at
f=3 MHz and 2.0Ω at f=1MHz. The plasma loading
resistance was decreased about 50% by the F.Seffect. Although the plasma density is dramatically
improved when the floating coil is levitated, theresistance is not increased.
Experimental results say that the plasma loading
resistance is related to the plasma density when thefilling gas pressure is lower than 20mPa. This iswell explained by the wave calculation code
(TASK/WF code). On the other hand, when thepressure is higher than 20mPa, the resistance isrelated to not the density but rather the filling gas
pressure. We guess that the plasma density near theantenna is much different from the density where
the interferometer is seeing when the filling gas
pressure is very high.

Fig.4. Experimental results of the plasma loading
resistance. Vertical axis means the loading
resistance and the horizontal axis means theline-averaged density (at r = 0.62 m). Marks ＊,○ and ● respectively denote those with the
faraday shield (F.S,), those without F.S. andthose when the floating coil is levitated without
F.S. The filling hydrogen pressure is alsoshown.

4. Summary
The first antenna loading experiment in themagnetospheric configuration was conducted. We
found that the maximum observed resistance ismore than 20Ω at f=3 MHz and 2.0Ω at f=1MHz.
Then the sufficient feasibility of ICRH system in
RT-1 is ensured. More experimental results anddetailed comparison between experiments andcalculations are shown in the presentation.
References[1] H. Saitoh et al., 23rd IAEA Fusion Energy
Conference (2010).
[2] Z. Yoshida et al., Physics of Plasmas 17, 112507 1-7(2010).
[3] D. Q. Hwang and R. W. Gould, Phys. Fluids 23, 614(1980);