24P134-P
Excitation experiment of electron Bernstein wave with injection of X waves fromstrong magnetic field in the Internal Coil Device Mini-RT
内部導体装置Mini-RTにおける強磁場からのX波入射による電子バーンシュタイン波の励起実験
Makino Wataru, Uchijima Kenichiro, Hosaka Yuichiro, Morikawa Junji and Yuichi Ogawa
牧野 航, 内島健一朗，保坂友一朗，森川惇二，小川雄一，Graduate school of Frontier science, The University of Tokyo
5-1-5, Kashiwanoha, Kashiwa 277-8568, Japan
東大新領域 〒277-8568 千葉県柏市柏の葉5-1-5
The Mini-RT device is an internal coil device that confines high beta plasma with dipole magnetic field
made by magnetically levitated High Temperature Superconducting coil. In this device, plasma has been
produced by electron cyclotron heating (ECH) by using 2.45GHz microwaves. Usually, it is hard togenerate and heat high density plasma with ECH because of the existence of the cutoff density. However,in this device, overdense plasma has been observed. The application of electron Bernstein wave (EBW) on
high efficient plasma heating in the Mini-RT device is expected.

1. Introduction
The fusion experimental devices such as sphericaltokamak, helical, internal coil device aim togenerate and heat high beta plasma. In these devices,
ECH by EBW was experimentally investigated to
study techniques for high density plasma heatingover cutoff density. In the toroidal plasma, X or O
mode electromagnetic waves are injected and send
to the electron cyclotron resonance layer for thepurposes of ECH or electron cyclotron current drive(ECCD). But in the overdense plasma, thesemethods cannot be used because of the existence ofthe cutoff density. In center of high beta plasma,electron density is high and electron plasma
frequency is several times higher than electron
cyclotron frequency. Compared with X or O mode
waves propagation conditions, EBW has no limit on
the density. Therefore, ECH and ECCD by EBW
are expected. At various devices, these methods
have been verified. But a lot of the verifications
used indirect measurements. We have conducted
experiments aimed at elucidating the mechanism of
EBW by direct measurement in the internal coildevice Mini-RT.
2. Mini-RT
The Mini-RT device is an internal coil device thathas major radius of R=0.5m and height of about
0.7m. The Mini-RT device can generate a dipole
magnetic field of coordination by energizing thesuperconducting coil. This magnetic field confines
high beta plasma. The major radius of the internalcoil is 0.15m and its weight is 16.8kg. The
maximum coil current is 50kAturns, and the typicalmagnetic field strength at the internal coil is 0.1 T.
A Copper coil is above the superconducting coil. It
is possible to control the plasma parameters andconfinement region by applying a levitation coilcurrent. Plasma has been generated and heated with
ECH by 2.45GHz microwaves. It is possible toproduce plasma whether internal coil is levitated or
supported, and overdense plasma which has twice
density of cutoff density has been generated withHTS coil magnetically levitated [2].
3. Electron Bernstein wave (EBW )In this section, EBW characteristics and methods
to convert electromagnetic wave to EBW areshown.
First, EBW has next characteristics listed below:-No cutoff density-Resonance at the harmonics of ECH
These two features make it possible to generatehigh beta plasma. Furthermore, EBW has othercharacteristics listed below:-Electrostatic waves-Convert electromagnetic waves into EBWs atUHR-Short wavelength-Longitudinal waves-Negative group velocity
EBW is excited by feeding X-wave to the UHR. So,
in the plasma, there are electromagnetic waves fed
from outside. These features are clues about
identification of EBW in experiments.
Second, it is well known that there are threemethods to convert electromagnetic wave to EBW.
-①Perpendicular injection of X-wave from highfield side (SX-B conversion)-②Perpendicular injection of X-wave from low
field side (FX-SX-B conversion)-③Oblique injection of O-wave from low fieldside (O-X-B conversion)①②③Fig. 2 density profiles(a)Fig 1: three methods to convert EMW to EBWPrevious studies in the Mini-RT device, EBW
excitation experiments by Perpendicular injectionof X-wave from low field side have been performed[4]. We have started a new planning for SX-B
conversion.
In the case of SX-B conversion, we have to becareful of new issues. One of those is existence ofL-cutoff. Figure 2 shows the two types of density
profiles in the Mini-RT, and these give thereflective index profiles perpendicular to magnetic
field (Fig.3). With growing of density, SX wave
touches L-cutoff and X wave cannot propagate(Fig.3 a). Therefore, we need to pay attention to thedensity distribution.
4. Objective
We try to set up new antenna on the high field sidein order to measure electromagnetic field at ECRF .
An objective of this work is to investigate SX-B
mode conversion in Mini-RT device and tocompared with the measurement result of previous
researches.
5. Measurement system
Figure 4 shows Mini-RT plasma measurement
system. Measurement of electromagnetic field at
ECRF is focused. In the Mini-RT device, waves atfrequencies lower than 2.45 are injected to diagnose
wave propagation in overdense plasma. In thisstudy, diagnostic X-mode waves at 1 GHz and 10
W are injected from the high field side.
References[1] MAEKAWA Takashi J.Plasma Fusion Res.Vol.78
No.6(2002)508-520.
[2] E. Yatsuka, D. Sakata, K. Kinjo, J. Morikawa, Y.
Ogawa, “Direct Measurement of a Mode-Converted
Electron Bernstein Wave in the Internal Coil Device
Mini-RT”, Plasma Fusion Res. 3 013 (2008).
[3] 岡部圭悟，“内部導体装置 Mini-RT におけるパル(b)Fig.3 N⊥2 profiles

Fig.4 measurement system in the Mini-RTス電磁波入射による電子バーンシュタイン波の直接測定”，東京大学修士論文（2011）[4] K.Uchijima, K.Okabe, A.Honda, E.Yatsuka, K.Kinjo,
J.Morikawa, Y.OGAWA, ”Wave Characteristics inthe Electron Cyclotron Range of Frequencies inOverdense Plasma in the Internal Coil Device
Mini-RT”,PlasmaFusion
Res.
Volume6,2401122(2011) .