24P133-P
Excitation Experiments of Electron Bernstein Waves via O-X-B mode
conversion in the Internal Coil Device Mini-RT
内部導体装置Mini-RTにおけるO-X-B変換による電子バーンスタイン波励起実験
Kenichiro Uchijima, Wataru Makino, Yuichiro Hosaka, Junji Morikawa and Yuichi Ogawa
内島 健一朗，牧野航，保坂友一朗，森川惇二，小川雄一
Graduate school of frontier sciences, the University of Tokyo
5-1-5, Kasiwanoha, Kasiwa, Chiba 277-8568, Japan
東京大学新領域創成科学研究科 〒277-8568 千葉県柏市柏の葉5-1-5Electron Bernstein Waves, which have no cut-off density, are excited at the Upper Hybrid Resonance layer
by mode conversion from X-mode waves. The possibility of O-X-B mode conversion was investigated in
the internal coil device Mini-RT. The magnetic and electric field profiles in the plasmas were measured
directly with interferometry system by injection diagnostic microwaves different from that of ECH. As aresult, wave characteristics corresponding to EBWs, i.e. short wavelength, longitudinally polarized, andbackward wave mode, were observed only at some initial injection angles that were expected by numericalanalysis.

1. Introduction
The Mini-RT is an internal coil device that wasconstructed to confine high beta plasma by amagnetic field similar to that of a planet. In thisdevice, so-called overdense plasmas have been
observed with levitation of an internal
superconducting magnetic coil [1], and heatingwith Electron Bernstein Waves (EBWs) isexpected.
Since the cutoff density exists, it is difficult togenerate and heat the high-density plasma withelectron cyclotron heating (ECH). Therefore,
EBW heating is expected to be one of the mostpromising methods for generating and heating
high-density plasmas. Inserting antennas enables
the direct investigation of waves in the plasma. E.
Yatsuka et al. developed directly measurement
systems in Mini-RT [2], and revealed wave
characteristics of the Electron Cyclotron Range ofFrequencies (ECRF) corresponding to EBW inFX-SX-B mode conversion in Mini-RT [3].
We have an interest other excitation scenario ofEBW. Here, to examine O-X-B mode conversion
in Mini-RT experiments, we are attempting toinvestigate the propagation of waves in the ECRF
in overdense plasmas, sifting the initial injection
angle of diagnostic microwaves.
2. O-X-B Conversion Scheme
To excite EBWs in plasma, microwaves have toaccess to the Upper Hybrid Resonance (UHR) layer,and in the O-X-B conversion, there are two mode

conversion processes i.e., the first transition from
O-mode to slow X-mode and the second one fromslow X-mode to EBW mode. The efficiency of theO-SX transition process in the two dimensions isgiven by Mjolhus [4],where,
is the wave number vector, ωis the wave angular frequency, k0 is the wave
number of the incident wave in vacuum and L is thecharacteristics density scale length at the UHR
respectively. The maximum conversion efficiency
is obtained at N∥=N∥opt , which corresponds to theoptimal angle.
Directmeasurement

Excitation system

Interferometrysystem

Fig. 1 Measurement system

3. Internal Coil Device Mini-RTThe Mini-RT device has an internal coil with ahigh-temperature superconductor (HTS) that
produces a purely poloidal magnetic field in avacuum vessel.
A plasma is produced in Mini-RT by ECH with
continuous injection of microwaves at 2.45 GHz
and 2.5 kW. The plasma confinement region anddensity profiles can be changed easily by applying alevitation coil current.
4. Experimental Setup
In the Mini-RT device, waves at frequencies
lower than 2.45 GHz are injected to diagnose wave
propagation in overdense plasmas; the plasma
produced by 2.45 GHz microwaves acts as anoverdense plasma with respect to lower frequency
diagnostic microwaves. In this study, diagnostic
O-waves at 1GHz and 10W are injected from low
field side, and the angle between wave numbervector k and the external magnetic field can bealtered by changing position and angle of elementof excitation antenna. To examine the mode
conversion of waves in the internal coil device,electromagnetic and electrostatic components aremeasured with interferometry system by probingantennas
inserted
directly
intoplasmas.
Interferometry enables us to obtain a snapshot of
the electric or magnetic field. Probing antennas
detect the injected diagnostic microwaves and send
them to the mixer. They are modulated by IQ
demodulators and output as sine and cosine
components that contain information on theamplitude and phase of electromagnetic field. Fig.1
shows a schematic diagram of the diagnostics.
5. Initial Results
Figure 2 shows the density profile measured by
the triple probe. Cutoff density for 1 GHz
microwaves is 1.24×1016 m-3, so the region inside
the major radius R<285mm is the overdense region
for diagnostic microwaves, and this density profileand magnetic field configuration give transmission
coefficient profile depending on injection angel(Fig. 5) and the optimum injection angle θopt = 64.7°.
In the radial electric field measurement, shown inFig. 3, a short wavelength mode (λ = 20 mm) isobserved at only the initial injection angel θ = 66.5°,
while in other injection angle (ex. in Fig. 3 θ =71.0°) this short wavelength mode waves are notobserved. Figure 4 shows radial profiles of thephase in θ = 66.5° and 71.0°. The phase is functionof the spatial position and length of thetransmission lines; the gradient of the phase gives

Cutoff density for 1 GHz

Fig.2 Electron density profile

Fig.3 Longitudinal polarized wave amplitudes

Fig.4 Phase profile

Fig.5 Injection angel dependence of Transmission

the wave number vector. The figure confirms areversal of the phase gradient around the UHR in θ= 66.5°. This indicates a change in the direction ofthe phase velocity.
These results suggest that EBWs were mode
converted from electromagnetic waves at the UHR.
References[1] T. Goto et al.: Jpn. J. Appl. Phys. 45 (2006) 5917.
[2] E. Yatsuka et al.: Trans. Fusion Sci. Tech. 51 (2007)
310.
[3] K. Uchijima et al.: Plasma and Fusion Res. 6 (2011)
401122.
[4] E. Mjolhus: Plasma Phys. 31 (1984) 7.