J. Plasma Fusion Res. SERIES, Vol.5 (2002) 184-188

Electron Temperature Measurement Using Electron
Gyclotron Emission on LHD
NAGAYAMA Yoshio, INAGAKI Shigeru, ITO Yasuhiko, KAWAHATA Kazuo,MORISAKI Tomohiro, KOMORI Akio, NARIHARA Kazumichi, OHYABU Nobuyoshi,
SAKAKIBARA Satoru, TANAKA Kenji and LHD Experimental Group
National Institute for Fusion Science, Toki, 509-5292, Japan(Received: 21 December 2001 / Accepted: 2 August 2002)

Abstract
The ECE diagnostics have been developed in LHD. Two antenna sets are installed in the outboard
side of LHD. The ECE detected by the perpendicular antenna has a linear polarization, which isdetermined at the plasma boundary, but the ECE emitted in the tangential direction is not linearlypolarized. The ECE system is calibrated with a hot (800 K) radiation source and a high gain amplifier,which are calibrated with a cold (17 K) radiation source. The ECE temperature agrees with the Thomson
scattering measurement and the diamagnetism in LHD. The ECE is employed to observe the magnetic
island, which is generated with a local island divertor field. The island width approaches to the vacuum
width as the plasma beta increases, unexpectedly.

Keywords:
electron cyclotron emission (ECE), large helical device (LHD), local island divertor (LID), polarization,
electron temperature, magnetic island

1. Introduction

temperature in the plasma center may be measured using

ECE. In LHD an ECE antenna of the tangential sightline is newly installed in the outboard side. This paper
reports the recent progress of the ECE measurement in
LHD, and also reports the first experimental result

The electron cyclotron emission (ECE) diagnostics

is a well established technique for the continuous
measurement of the electron temperature profile intokamaks. In helical systems, however, very few
experiments have been reported on ECE measurements.
Fundamental differences between helical systems andtokamaks in terms of ECE measurement are theobtained by using the tangential antenna.
The intensity calibration is an important issue in thepolarization and the magnetic field profile. In helicalsystems, the magnetic field profile is peaked and theangle of field line varies from -30 to +30 degrees, while
the magnetic field is monotonic and the angle of field
line varies very small in tokamaks. The emissivity ofECE at the plasma center, where the magnetic field ispeak, is smaller than the black body radiation. Along thesight line tangential to the plasma, however, the peak of
the masnetic field shifts to inboard side. So, the electron

temperature, the ECE intensity has to be calibrated by a

ECE diagnostics. In order to obtain the electron

black body radiation source with the known
temperature. This paper reports the calibration using a
hot radiation source in LHD. In order to reduce noise,
the signal has to be integrated in long period. Since thesignal from our hot (800 K) source is three times
stronger than that from the usual cold (77 K) source, thesignal to noise ratio increases by factor three with the@20O2 by The Japan Society of Plasma

C o r r e sp o ndin g aut ho r' s e - mail : na g ay ama @ ntfs. a c.i p

Science and Nuclear Fusion Research184

Nagayama Y. et al., Electron Temperature Measurement Using Electron Cyclotron Emission on LHD

same integration time. In order to examine thereliability, the ECE temperature is compared with theelectron temperature obtained by the other apparatus,

magnetic field is still increased at the plasma center, it isexpected that the ECE from the plasma center can bedetected on the tangential sight line. The perpendicularsuch as Thomson scattering and the diamagnetism, inthis paper.

antenna and the tangential antenna can be switched shotby shot using a switch in the corrugated waveguide

The variation of temperature profile can besystem.

distinguished easily using ECE diagnostics because ofThe ECE system with the fast scanning Michelsoninterferometer (Michelson) is calibrated using a hot (800low noise. This feature is adequate to investigate
magnetic island structure. Recently, the physics ofmagnetic island attracts attention [5]. This paper
presents the first ECE measurement of the magneticK) radiation source in order to obtain the electrontemperature. In order to increase the calibration signal, apolarizer, which is to be set at 45 degrees in the plasma

island structure, which is produced by using the local
island divertor GID) field.

experiment, is set at 0 degrees, and a high-gain activelow-pass filter is additionally used in order to reduce
digitally sampled error (so called bit-noise). The hotsource and the other additionally used components inthe calibration process are also calibrated with a liquid

2. ECE System and Electron Temperature
Measurement
Schematic diagram of ECE measurement system inthe outboard side is shown in Fig. 1. The detail of theconventional ECE system in LHD is presented in refs.
[1-3]. We also present experimental results using thenitrogen cooled (77 K) radiation source. The calibration
signal of one million scans is integrated. The radiometerdata is calibrated to the Michelson temperature.
The electron temperature obtained by ECE iscompared with other diagnostics, such as the Thomson
scattering and the diamagnetism. In Fig. 3, the electron
temperature profile measured using ECE and Thomson
scattering in cases of low density and high density.
Electron temperature measured with ECE and Thomson
scattering agrees well in wide range of density. In Fig.

ECE detection system in the inboard side of LHD in thisconference [4]. In order to reduce the transmission loss,
the ECE system fully uses the corrugated waveguidesystem in LHD. A tangential antenna is newly installedin LHD. The calculated magnetic field along the sightline is shown in Fig. 2. The corresponding normalized
plasma radius p is also plotted in Fig. 2. In this case, thetangential sight line is shifted vertically, this line does
not pass the right center of plasma, unfortunately. Since
the magnetic field is peaked at the plasma center, theECE from the plasma center cannot be detected on the4, the ECE temperature is compared with thediamagnetic temperature in various plasmas. Here, thediamagnetic temperature, Totu is defined as:^i

Wo = 3 ln"(p)T"(p)dV.

Here, the T. and n"pr#t", are assumed typical profiles

perpendicular sight line. On the other hand, since the1.2

LHF plas*a

B/Bo 1

Perpendicular Antenna
(a)

1.2
B/B^

I

0.8

Tangential Antena

0.8

u.o0.6
l0.4

o.4

p11

PerpendicularAntenna

TangentialAntenna

0.5Michelson$pec{rorneterkx'

Notch Eg
Filters ffiil(82.7GH2)

trl

0.8

0.6

-0.5

o.4
o.2

z.J

Polarization

33.5 4R (m)Rotator4.5q .522.533.5s (m)Fig. 2 (a) Magnetic field on the sight line of the perpen-(84GHz)$witch(,c)

0dicular antenna. (b) Magnetic field on the sightline of the tangential antenna. (c) Normalized

Pofariaers

plasma radius on the sight line of the perpendicular antenna. (d) Normalized plasma radius on thesight line of the tangential antenna.

Fig. 1 Schematic diagram of ECE measurement systemin the outboard side in LHD.
185

Nagayama Y. et al., Electron Temperature Measurement Using Electron Cyclotron Emission on LHD

These results indicate that the obtained electron temperature by these diagnostics is reliable in moderatein LHD, as follows:
T;(p) = T"(p) = Tai" (l - f), n" (p) = n"o (l - p8;. Figure4 shows that the ECE temperature is consistent with thediamagnetic temperature, basically. In low-density plasmas when the ECE is optically thick, however, the ECEtemperature is saturated while the diamagnetic temperature increases. This is very interesting phenomena.

.adensity LHD plasmas.

3. Polarization
Figure 5 shows polarization of ECE detected by theperpendicular and the tangential antennas. The magnetic
field is 2.8T and the line averaged electron density isl.4xl0rem-3. ECE signals of 148 GHz and 127 GHz
have a peak at 25 degrees and bottom is at 70 degrees
on the perpendicular sight line, as shown in Fig. 5(a).
Note the ECE signal detected by the tangential antenna
is not calibrated. ECE signal of 75 GHz has a peak at 70degrees and bottom is at 25 degrees. The difference issi
douddea45 degrees. The rotation angle of the ECE polarization
is double of the polarization rotator angle. So, thedifference of the polarization angle is 90 degrees. Whenthe microwave source (Gunn-oscillator) is set at theangle perpendicular to the field line of the last closed
flux surface, the peak is at 25 degrees. Therefore, the148 GHz and 127 GHz have the same polarization angle

I€

Thomson

3(c)

3.5 4

4.5

R (m)

#337d4 (t=1Radiomet€r _ Michelson(P{%)^.affi}. (H%)

as the X-mode on the last closed flux surface, and the 70

GHz has the same polarization angle as the O-mode.

PerDendicular AntennaTangential Antennao 20 40 60 80 100

o 20 40 60 80 100

Rotator Angle (degree)(c)

Rotator Angle (degree)

R (m)Fig. 3 Electron temperature profile measured with ECE
and Thomson scattering in (a) high density case,(b) low density case, (c) high beta case (F=0.69%),(d) low beta case lF=0.25%1.
#26282-29't74

4o-!a e

2.5

uJ

1.5

lll AFz

11el9;7p'fl nlsle=2/'6

ll'l,{'\ifi i in=iE8I

0r50(keV);V100 150 200 250
Frequency (GHz)dramag21.5'I0.5nl50

100 150 200 250
Frequency (GHz)Fig. 4 Electron temperature obtained by ECE and theFig. 5 (a) Polarization of ECE detected by the perpen-diamagnetic measurements. Different marks indicate shots in different days. Closed marks indicate
F",=3.5 m, open marks indicate 4,=3.6 m, andcross marks indicate &"=3.75 m.

dicular antenna. (b) Polarization of ECE detectedby the tangential antenna. {c) Spectrum of ECE
detected by the perpendicular antenna. (d) Spectrum of ECE detected by the tangential antenna.
186

Nagayama Y. et al., Electron Temperature Measurement Using Electron Cyclotron Emission on LHD

The different frequency coffesponds to the different

1(a)radiation place, where the field angle is different. So, thepolarization of ECE is determined at the plasma

1.5

Tr",

iiz (m)0.5(kev)

tr- i- i-boundary. The spectrum has peaks at74 GHz and at 148

l+l

GHz, which corresponds to the fundamental O-mode

and the second harmonic X-mode in case ofi- --'-0.5perpendicular antenna.
Figure 5(c) shows the spectrum of ECE detected by
the perpendicular antenna, when the polarization is at X--1

mode (27 degrees) and at the O-mode (70 degrees). The

0.12

2. J

3 3.5 4(b)Ni#[;i,Ni\i"=r..i.rri
R-=q6om i

OL
4.5

3.6 3.8R (m)i4 4.2 4.4
R (m)

(c)
0.12 J9)lslandfundamental O-mode, of which frequency is between 50
GHz and 75 GHz, is stronger than the fundamental X-

width0..l0.1(m)mode. The second-harmonic X-mode, of which

0.08

0.08

than the seconcl-harmonic O-mode. ECE between 75 and0.06

0.06

100 GHz and between 150 GHz and 200 GHz is notpolarized. These ECE are generated near the coil where

0.04

0.04

frequency is between 100 GHz and 150 GHz, is stronger

o.15 0.2 0.25 0.3 0.35 0.4
beta (%)

the magnetic field is higher than the plasma center. They

have to be scattered at the wall, and loose thene(R-_4.27m) 11 olem-3;

Fig. 6 (a) Calculated magnetic island structure. (b) Elec-polarization when enter the antenna.

tron temperature profile measured using ECE(Michelson spectrometer) with and without LID

Figure 5(b) show the polarization of ECE detectedby the tangential antenna. The tangentially emitted ECEfield. (c) Trajectory of shot 19903 on the plane ofisland width vs. beta. (d) Trajectory on the planeof island width vs. electron density.

is not linearly polarized in this experiment. This isconsistent with the theoretical prediction that thetangentially ernitted ECE has a circular polarization.
Figure 5(d) show the spectrum of the tangentiallyemitted ECE. As we expect in the introduction of thisthe region between R = 4.27 m and R = 4.43 m is lessthan that in the other places. This region is close to thepaper, the spectrum of the second harmonics isvacuum magnetic island structure in Fig. 6(a). The
island region is not flat in the temperature profile, but
the electron temperature profile in the inner region
surrounded by the island is reduced due to the island.
The temperature profile is shifted as much as thevacuum island width. Now, we define the experimental
island width as the radial displacement in the electron
temperature profile. Figure 6(c) shows trajectory of theisland width on the beta during a single discharge. The
magnetic island grows as the beta increases during thegrowing phase, and it shrinks as the beta decreases in
the after-grow phase. This trajectory indicates that theisland width depends on the beta. Figure 6(d) shows
trajectory of the island width on the electron density. In
the growing phase the island grows as the density
increases. In the after-grow phase, however, the density
is constant while the island width decreases. The island
width does not look to depend on the electron density
but it depends on the beta. Most interesting point is thatsymmetric with the center frequency of 155 GHz. The
higher frequency than 155 GHz may come from thehigh field region, which is on the sight line of thetangential antenna. Interestingly, the spectrum less than
100 GHz detected by the tangential antenna are quite

different from that detected by the perpendicularantenna. The fundamental ECE spectrum have twopeaks, one at 62 GHz another at 78 GHz. The peak at 78

GHz may correspond to the ECE from the center ofplasma. However, the peak at 62 GHz is rather
mysterious. Tangentially emitted ECE has not been wellunderstood because theoretical and experimental works
are very limited.

4. Magnetic lsland
Observation of magnetic island structure is one ofuseful applications of ECE measurement, since therelative error is small in ECE measurements. Figure 6(a)shows the calcr,rlated magnetic island structure generated

as the beta increases the magnetic island approaches tothe width in vacuum, in which the beta should be zero.

by the LID field in vacuum. Figure 6(b) shows theelectron temperature profiles of LHD plasmas with andwithout LID field. The electron temperature gradient in187

Nagayama Y. et al., Electron Temperature Measurement Using Electron Cyclotron Emission on LHD

The ECE diagnostics have been developed in LHD.

plasma beta increases, unexpectedly. It requires further
investigation to understand these phenomena.

Two antenna sets are installed in the outboard side ofLHD. The ECE detected by the perpendicular antenna
has a linear polarization, which is determined at theRefereneces
Y. Nagayama, K. Kawahata, A. England et al.,

5. Conclusion

tll

Rev. Sci. Instrum 70,1034 (1999).
l2l Y. Nagayama, K. Kawahata, S. Inagaki, H. Sasao
et al.,Fusion Eng. Des. 53,2O1(2001).
[3] P.C. de Vries, K. Kawahata, Y. Nagayama et al.,Phys. Plasmas 7, 37O7 (2000).
[4] S. Inagaki, Y. Nagayama et al., presented in thisplasma boundary, but the ECE emitted in the tangential

direction is has not linear polarization. The electron
temperature is calibrated with a hot (800 K) radiation
source and a high gain amplifier, which are calibrated
with a cold (77 K) radiation source. The ECE
temperature agrees with the Thomson scattering
measurement and the diamagnetism in moderate density
plasmas in LHD. The ECE is used to observe themagnetic island, which is generated with a LID field.
The island width approaches to the vacuum width as theconference.

t5l K. Narihara, K.Y. Watanabe, I. Yamada et al.,Phys. Rev. Lett.87,24 (2001).

188