J. Plasma Fusion Res. SERIES, Vol.5 (2002) 189-192

Study of Electron Bernstein Wave Emission on TST-2
SHIRAIWA Syun'ichi, NAGASHIMA Yoshihikor, USHIGOME Masayasur, KASUYA Naohiror,
NOZI\TO Hideaki, KASAHARA Hiroshi, YAMADA Takumar, WADA Hirohumi, IIJIMA Daisuke,
KOBORI Yosuke, NISHI Tomoichiro, TANIGUCHI Tomokazur, EJIRI Akira and TAKASE Yuichi
Graduate School of Frontier Sciences, University of Tolqo'Graduate School of Science, University of Tokyo(Received: ll December 2001 / Accepted: 15 May 2002)

Abstract
Abstact: Electron Bernstein wave (EBW) emission, a possible alternative to the standard electron
cyclotron emission diagnostic for overdense (@p" )) o"") plasmas, was studied on the TST-2 sphericaltokamatr:. Simultaneous measurements of the radiation temperature and the density gradient at the upperhybrid resonance layer were carried out using a newly developed instrument consisting of a heterodyne
radiometer combined with an AM reflectometer. It was found that the emission was fully unpolarized andthe radiation temperature from the core region was about 150 to 200 eV. The mode-conversion efficiency
was approximately 40 Va which led to the estimated electron temperature of 250 to 370 eV.

Keywords:
EBW, s pherical tokamak, temperature diagnostic

1. lntroduction
Standard techniques for temperature diagnostic orside cutoff coincide (B-X-O). In the second scenario,
proposed more recently by Efthimion l2l, the X-mode

heating and current drive based on the electron

tunnels through the evanescent region between the UHR

cyclotron wave (ECW) are not effective in ST, because
ST plasmas are overdense and cannot sustain ECW. A
promising option is to use the electron Bernstein wave(EBW), which has no density cutoff. For EBW is a shortwavelength electrostatic wave, its optical thickness atand the low field side cutoff (B-X-FX). In ST plasmas,
the triplet composed of the high density cutoff, the UHR

and the low density cutoff is formed within a few
centimeters width (see Fig. 1), which makes the modeconversion efficiency high
The B-X-FX power mode-conversion efficiency isgiven by [3]the cyclotron resonance is high and can be considered tobe black body emission.

A comple;<ity associated with the use of EBW isthat electrostatic wave cannot leave the plasma and must

( = {s-nt (l -,(1)"-zr.)|cos2({p)
where cos2(g) represents the interference effect between
the high density cutoff and the UHR,

be mode-converted to an electromagnetic wave. TheEBW undergoes mode-conversion near the upper hybrid
resonance (UHR) to the X-mode to which access ispossible via tu,o scenarios. In the first scenario, firstinvestigated by Preinhaelter and Kopeck'y in the early
1970's [], the X-mode is mode-converted again to thery=L_t2i-is the Budden parameter when the magnetic field is constant, Z, is the density scale length, all is the upper hy-O-mode. This scenario requires the optimum (1 such thatthe locations of the O-mode cutoff and the hieh field@2O02 by The Japan Society of Plasma

Corresponding author' s e -mail : shiraiwa@ plasma.phys. s. u-tolcyo. ac.jp
r89

Science and Nuclear Fusion ResearchShiraiwa S. et al., Study of Electron Bernstein Wave Emission on TST-2
fir+o1)F=.
ouI10-.{-10E1-

11or'I
110

0.40

0.50

0.60

0.70

RlmlFig. 1 Locations of critical layers and the perpendicularwave number for 6 GHz on TST-2. Parabolic profiles are assumed for the density and the temperature for the core plasma (inside the limiter radius)
and an exponential decay for the scrape-off layer.

The magnetic field on axis is 0.2 T, the central
density is 1.0x101s m-3 and the density at thelimiter is 1.0x1018 m-3. The perpendicular wavenumbers drawn by dashed lines are evanescentwaves.

brid frequency and Ois the electron cyclotron frequency
at the UHR. eq. (l) shows that the efficiency can be
100 Vo for optimum density scale length and phase factor. If a strong turbulence averages the phase factor, theaverage efficiency is simplified to Cuu" = )s-n! - s-n)
and depends only on the density scale length.
So far, comparisons of the radiation temperature
with the electron temperature carried out on CDX-U [4]and NSTX measured the density profile by Langmuirprobes and Thomson scattering. However, Langmuirprobes cannot be used in hot plasmas and Thomson
scattering does not have a continuous temporal
coverage. We have developed a new instrument thatconsists of a heterodyne radiometer and a reflectometer(radio-reflectometer) that can measure the radiation

Refl ectometer Signals Radiometer Signa

t'e =6Ql75OMHzl2nFig. 2 Block diagram of the radio-reflectometer. The fre-quency coverage is from 5-12 GHz that corresponds the 2nd and 3rd harmonic emission fromTST-2.

the receiving antenna. The difficulty of the sharing isthat the probing wave of the reflectometer may be
detected by radiometer. Using the different wave
polarization for radiometer and reflectometer may offer
a solution. However, the reflected wave from plasma

temperature and the density gradient simultaneously.

In the present paper, details of the newly built

was found to contain nearly l0 70 cross-polarizedradio-reflectometer are described in Sec.2. Section 3 isdevoted to the description of initial results of the radioreflectometer on TST-2. The results and perspectives aresummarized in Sec. 4.

component. A better way is to use two frequencies andto make the frequency difference greater than theradiometer bandwidth. Hence, we employ the amplitude
modulated reflectometry [5].
A block diagram of the radio-reflectometer is

2. Radio-Reflectometer
In order to measure the density gradient of theshown in Fig. 2. It has a single microwave YIG
oscillator that is fed to two mixers. The first one is used
to generate probing waves of slightly different
frequencies. Representing the YIG oscillator frequency

region where the wave received by radiometer is modeconverted, the radiometer and reflectometer should share190

Shiraiwa S. et al.. Study of Electron Bernstein Wave Emission on TST-2

by F, the launched wave frequencies are F+75 MHz andF-75 MHz. The probing waves are amplified to power
levels of about 15 dBm and fed to the ridged hornantenna whose frequency range is 5 to 18 GHz. The
mixture of the reflected waves and the EBW emission
are received by the second ridged horn antenna,
amplified by 33 dB and fed to the second mixer and thediode detector.

The second mixer is used to detect the EBWemission. Since the bandwidth of mixer's IF port is 300
MHz, the output of the second mixer contains theemission in the frequency range from F-300 MHz toF+300 MHz and the intense reflected wave components
at DC, 75 MHz and its harmonics. The bandpass filter
composed by a 50 MHz low pass filter and a 3 MHzhigh pass filter at the input stage of the logarithmic(c)i| lllll

X-mode0-modet)10F

amplifier is used to eliminate the reflected wave

515components and reduce the radiometer bandwidth to 100t6

17187920time [ms]MHz (Double Side Band detection). Finally, thelogarithmic amplifier, Analog Devices AD8307, detects

Fig. 3 Comparisons of EBW emission for different re-the filtered EBW signals. The radial resolution
determined by the bandwidth is about 5 mm. The

ceiving antenna orientation and polarizations. (a)the antenna arrangement (b) X-mode emission in
the direction perpendicular to flux surface (black)is smaller than emission in the oblique direction(gray). {c) X-mode (black} and O-mode (gray) havethe same intensity.

radiometer is calibrated using liquid nitrogen.
A diode detector is used to detect the beat signal
between two probing waves at 150 MHz. Contribution
produced by nixing other harmonics generated at thefirst mixer such as F+150 MHz and F are less than -30
dB in power and are negligible. A diode detector

greater the solid angle that the antennas view the plasma

produces the liequency components such as 75 MHz

and the interference of waves reflected at various parts

and 225 MHz. A filter is used to eliminate them. Withof the plasma surface becomes more serious. The second
reason is to restrict the range of the parallel wave
number received by the radiometer to as close to zero as
possible. This is because higher X-mode emission weredetected at an oblique angle (t11*0) than in the normal
direction (ftrr=O) (Fig. 3(b)), when the antenna was
located 50cm away from the window.
The plasma current, the measured 3rd harmonic
EBW emission from the core region and the phase delayof the reflectometer of typical discharge are shown in
Fig. 4. Several sudden drops of emission coincide withreconnection events, while the reflectometer signal

the local signal generated by the frequency doubler, thephase sensitive detector performs quadrature phasedetection and the phase delay of the beat wave isobtained.

3. Experimental setup and results
The radio-reflectometer is installed at the low fieldside mid-plain of the TST-2 [6]. TST-2 is a small
spherical tokarnak with major radius R=0.36 m, minor
radius c = 0.23 m, aspect ratio 4-1.6 and elongation
r-1.3-1.8. We measured the ohmic plasmas withplasma current of 80 kA, discharge duration of 20 ms,
ion temperature of 50-100 eV and toroidal field of 0.15
to O.2 T . As shown in Fig. 1, the plasmas are calculated

shows no obvious change. It is inferred that the emissiondrop represents the core temperature decrease. By
sweeping the YIG oscillator frequency by shot by shotmanner, the spectrum of EBW emission and density
profile around the UHR were measured. Figure 5 shows
the spatial profile of EBW emission obtained by

to be overdense except in the very peripheral region andmode-conversion takes place near the limiter surface.
We locate the antennas 90 cm away from thevacuum window for two reasons. The first reason is tosuppress the interference effect of the reflectometer. The
closer the antennas are located to the window, themapping the frequency to the radial location. It is shownthat the 3rd harmonic emission from the core plasma is
100 to 150 eV and monotonically decreases as minor
191

Shiraiwa S. et aI.. Studv of Electron Bernstein Wave Emission on TST-2

vacuum vessel have randomized the wave number

reconnecuon events,/\',_._.<f..**+./

0t200
F-CH.o.
0W- Group delay

s8:3

0.4
Brn)

\ ::-::=lv *p & Iptrr.":

10spectrum and polarization.

Presently, a full wave treatment is underpreparation for precise reconstruction of the density
profile, because the wavelength of the probing wave iscomparable to the measured density scale length. As an--iT-aindependent absolute electron temperature measurement,:1,1*.ritl*l tltrlltr.,.tt'
nrt,u![,1'lrt'4

ll''*Thomson scattering is also under preparation.
404. Conclusion
In this paper, we report the newly constructed

time [ms]Fig. 4 Temporal evolution of radiometer signal and thephase of reflectometer. The frequency of the ra-EBW emission diagnostic that consists of a radiometerand an AM reflectometer. The instrument is installed on

dio-reflectometer is 11 GHz.

TST-2 and the radiation temperatures of the 2nd and 3rd
harmonic emission and the density scale length are, forthe first time, measured simultaneously. Initial results
show that the radiation temperature is about 100 to 150
eV in the plasma core. The density scale length of about
3 cm was obtained from reflectometer using the WKB

t=18.5ms

zoo

a 'Znd harmonlc+ 3rd harmonic150.F-50

approximation. The calculated mode-conversion
efficiency is about 40 Ea, rcsulting in the estimated+? rooF-ioo+-{-electron temperature of 250 to 370 eV.

++-Acknowledgments
The one of authors (S.S) is grateful to Dr. Gary00.35 0.40 0.45 0.50 0.55 0.60 0.65

Taylor, Dr. Phil Efthimion and Dr. Brent Jones for pilesof discussions and advices which have signinicant
importance for conducting the EBW research on TST-2.

Rlml
Fig. 5 Spatial profile of EBW emission at f=18.5 ms. In

order to map the frequency to the radial location,
the 1/B dependence of the toroidal field and a linear spatial profile of the poloidal field are as-References[1] J. Preinhaelter and V. Kopeck'y, J. Plasma Phys.
10, 1, (1973)
t2l P.C. Efthimion, J.C. Hosea, R. Kaita, R. Majeskiand G. Taylor, Rev. Sci. Instrum. 70, 1018 (1999).

sumed.

radius increases. Using WKB approximation for thesake of simplicity, the density scale length of about 3

t3l A.K. Ram and S.D. Schultz, Phys. Plasmas 7, 4084

cm is obtained. The mode-conversion efficiency
calculated from eq. (l) is about 40 Vo and leads to thet4l G.Taylor, P.C. Efthimion, B. Jones, T. Munsat,(2000).
J.C. Hosea, R. Kaita, R. Majeski, R.W. Harvey andS. Shiraiwa, Bull. Am. Phys. Soc. 46,3O7, (2001).
[5] J. Srinchez, B. Braflas, E. de la Luna, T. Estradaand V. Zhvavlev, Rev. Sci. Instrum. 63, 4654

central electron temperature of 250 to 370 eV.
In the B-X-FX scenario, the evanescent region isnarrowest for ft',=0 and mode-conversion becomes mostefficient. Hence, the result shown in Fig. 3(b) seems tocontradict the one dimensional mode-conversion theory.
Another contradictory result is shown in Fig. 3(c), in(r99D.

t6l Y.Takase, A. Ejiri, N. Kasuya, T. Mashiko, S.
Shiraiwa, L.M. Tozawa, T. Akizuki, H. Kasahara,
Y. Nagashima, H. Nozato, H. Wada, H. Yamada,T. Yamada and K. Yamagishi, Nucl. Fusion 41,1543 (200n.

which O-mode emission has almost the same intensity
as X-mode emission. The phenomena shown in Fig. 3(b)and (c) are not thoroughly understood. A possibility isthat multiple reflection between the plasma and thet92