22F14
Excitation and Propagation of Electron Bernstein Waveby OXB injection on LATELATE装置におけるOXB入射法による電子バーンスタイン波の励起と伝播
NOGUCHI Yuto ,FUKUNAGA Tadahiko,KURODA Kengoh,OMI Shota,KATSUMA Jun, KANEMITSU Toshiyuki, HAYASHI Ryota, MIZOGAMI Hikaru,
WATANABE Fumitake, UCHIDA Masaki, TANAKA Hitoshi, MAEKAWA Takashi
野口悠人, 福永忠彦, 黒田賢剛, 逢見翔太, 勝間淳, 金光俊幸,林良太, 溝上晃, 渡辺文武, 打田正樹, 田中仁, 前川孝
Graduate School of Energy Science, Kyoto University
Kitashirakawaoiwakechou, Sakyouku, Kyoto-city 606-8502, Japan
京都大学 〒606-8502 京都市左京区北白川追分町
A linearly polarized microwave(1.5GHz,10W) is obliquely injected into overdense ECR plasmas(typicalpeak density is 3×1016 m-3 , Te ~10eV), which are produced and maintained by 2.45GHz, 1kW microwave
injection. Optimum injection angle is determined by using cold plasma model. Wave electric fields aredetected with movable monopole probes and downconverted by mixer circuits. The wave amplitude andphase change of 1.5GHz microwave are obtained. A steep spike in electric field amplitude has been
detected near the UHR layer. Comparison between the experimental results and the numerical analysis will
be presented.

1. Introduction
Electron cyclotron resonance (ECR) heating isa useful and often indispensable method to heat aplasma. However, if the plasma frequency ωpeexceeds the injected microwave frequency, themicrowave is reflected and can no longer beabsorbed at the plasma core. One possibility toovercome this problem is to use electron
Bernstein waves (EBWs), which have nohigh-density cutoff and are very well absorbed at
ECR and its harmonics. EBWs are electrostatic

Figure1. Electric field measurement systemwaves that need to be mode-converted from theinjected electromagnetic waves, and theordinary-extraordinary-Bernstein(OXB)
andextraordinary-Bernstein(XB) mode conversion
process are known for efficient mode-conversion.
The both method have restrictions in the range ofdensity gradient and injection angle.
This study aims to examine the whole process
of excitation of EBW at the UHR layer,
propagation toward the ECR layer, and strong
resonance absorption near the ECR layer, and tolook for the condition to obtain the optimum
coupling efficiency from electromagnetic waves
to EBW.
The experiments are carried out in the Low
Aspect ratio Torus Experiment (LATE) device.
The advantage of low aspect ratio tokamak
plasmas to study EBW is that the ratio of electron
cyclotron frequency to frequency of the injected
microwave varies over a wide range in the plasma,
therefore entire process of excitation, propagation
and absorption of EBW can be separately studied.
2. Numerical determination of optimal injectionangle
To estimate the optimal injection angle, we
conducted numerical calculations based on the coldplasma resonance absorption model. Calculation
results show that the optimal injection angle for
EBW excitation
is N//~0.6 oblique injectionwhile the mode conversion efficiency is very low
at N// =0 in the present plasma condition. Therefore
we employed the OXB mode conversion method. In
the OXB mode conversion process, first an
externary launched O mode is coupled to a X modein the region O mode cutoff. Then the X mode
propagates back to the UHR layer and is converted
into EBW. The EBW propagates into the overdense
plasma and is absorbed near the ECR layer.
Raytracing calculations were performed in order
to find the optimal trajectory of movable probes.
Wavelength of EBW is estimated from thedispersion relation[１] as ~2mm at the vicinity ofthe UHR layer.
3. Experimental set-up
A wave field measurement system has beendeveloped in order to excite and detect themode-converted EBW. Figure.1 shows the overall
view of the system.
An ECR plasma is produced and maintained by
2.45GHz, 1kW microwave injection. A 1.5GHz,
10W microwave is injected obliquely into theECR-plasma using waveguide launcher to excite
EBW with horizontal polarization(Quasi O-mode).
The radial profiles of the wave electric fields forthis microwave injection are detected using
movable coaxially-fed monopole probes, which are~4mm in length and 0.5mm in diameter. Received
signals are downconverted to 200kHz and bandpass
filtered by a mixer circuit. The phase and amplitudeof wave fields are evaluated from the detected wave
pattern. As the reference input, signals detectedwith another movable probe in the plasma andsignals divided from oscilator for micro wave
injection can be utilized.
. Electron density profile and electron temperature
are measured by movable probes and calibratedusing 70GHz RF interferometry system. Typicalpeak density is 3×1016 m-3 and Te is ~10eV.
4. Experiment and results
Figure 2. shows the radial profile of theamplitude and the phase change of measured
electric fields at horizontal polarization. The
reference signal has been divided from the oscilator
for microwave injection.
Although there is no short wavelength wave
pattern in phase change, we can see a steep spike atmajor radius R~29cm ,near the UHR layer. This
spike could not be seen at vertical polarization
injection (Quasi X-mode). In the present condition,mode conversion rate of QO mode is higher than
QX mode, therefore this steep spike might beassociated with EBW excitation.
5. Summary
Wave patterns near the UHR layer have beenmeasured by injecting a linearly polarized
microwave (1.5GHz, 10W) into the plasma
obliquely to magnetic field. The comparison
between experimental results and calculation using
a cold plasma slab model will be discussed.
References[1] T. H. Stix, Waves in plasmas (American Institute ofPhysics, New York, 1992).

Figure 2. Amplitude and phase change of detected
electric field