J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Upgrade of 2-D Antenna Array for Microwave Imaging
Reﬂectometry and ECE Imaging
Daisuke KUWAHARA1) , Shunji TSUJI-IIO1) , Yoshio NAGAYAMA2) ,Tomokazu YOSHINAGA2) , Zhongbing SHI3) , Soichiro YAMAGUCHI4) , Masaharu SUGITO2) ,Yuichiro KOGI5) , Atsushi MASE6)
1)

Department of Eneargy Sceience, Tokyo Institute of Technology, Tokyo 152-8550, Japan
2)
National Institute for Fusion Science, Toki 509-5292, Japan
3)
SOKENDAI, Toki 509-5292, Japan
4)
Kansai University, Suita 564-8680, Japan
5)
Fukuoka Institute of Technology, Fukuoka 811-0295, Japan
6)
KASTEC, Kyushu University, Kasuga 816-8580, Japan(Received: 20 November 2009 / Accepted: 7 June 2010)

Two types of 2-D Microwave Imaging, Microwave Imaging Reﬂectometry (MIR) and Electron Cyclotron Emission Imaging (ECEI) have been developed for the Large Helical Device (LHD). These aremethods of 2-D / 3-D imaging diagnostics on electron density ﬂuctuations and electron temperature forthe investigation of micro-turbulence and magneto-hydrodynamic instabilities in magnetically conﬁned
plasmas. 1-D horn antenna array was developed for a 2-D receiver antenna array of the MIR (freq.
range: 50 - 75 GHz). This antenna is also able to be used for a receiver of the ECEI (freq. range: 95 110 GHz). To apply the ECEI receiver, and to extend the measurement range of these diagnostics, the1-D horn antenna array was upgraded.
Keywords: Microwave Reﬂectometry, Electron Cyclotron Emission, 2-D Antenna arrayas the black body radiation. Therefore, the plasma
electron temperature and its ﬂuctuations can be determined by measuring the intensity of ECE. ECEI isdeveloped for tokamaks, mirror and helical devices [3].

1. Introduction
Imaging is one of the most powerful tools for understanding of complex systems. In magnetically conﬁned plasmas, there are much complex phenomena to
study, such as transport and turbulence. In order tounderstand above-mentioned phenomena, lots of diagnostic methods have been developed. As one of them,microwave diagnostics have been studied and applied.
Microwave reﬂectometry is a radar technique forthe measurement of electron density proﬁles and density ﬂuctuations by probing the electron densitydependent cutoﬀ layer in a plasma. This measurement
has a good spatial resolution and sensitivity by usinga phase detection method [1, 2]. Microwave Imaging
Reﬂectometry (MIR), a multi-channel reﬂectometry
system equipped with the imaging optics, is one of
the most powerful tools to study turbulence and instabilities, since it enables us to observe locally.
Electron Cyclotron Emission Imaging is an advanced ECE diagnostic method. By the use of ECEI,
we can obtain 2-D/3-D structure of temperature ﬂuctuations. Thereby, the observing of temperaturerelating phenomena, such as anormalous transport,
instabilities and turbulence becomes possible. Whenthe plasma density and temperature are suﬃciently
high, the plasma becomes optically thick to some harmonics of the ECE. In LHD, since the electron temperature and the density are suﬃciently high, ECE is in
the Rayleigh-jeans regime and ECE can be regarded

2. MIR system
Recently MIR has been intensively developedat National Institute for Fusion Science (NIFS) [4,5]. The ﬁrst generation MIR system was developedin LHD, which is a world largest superconducting
heliotron-type fusion device. Probe beam of the ﬁrst
system has frequencies of 53, 66 and 69 GHz in Xmode generated by three IMPATT oscillators. Threewaves are mixed with waveguide directional couplers
and are launched with a horn antenna, the probe beam
is focused on the plasma by the use of the imaging optics. Reﬂected waves from three diﬀerent cutoﬀ layers
are received with three v-band pyramidal horn antennas. One antenna is arranged at center, the others arearranged in toroidal and poloidal directions, respectively. This arrangement is to observe both toroidal
and poloidal wave numbers. Signal is received with asuper heterodyne method by mixing reﬂected waves
and the local wave that is generated from a Gunn oscillator with a frequency of 63 GHz. Reﬂected waves
enter a mixer through a waveguide, then the reﬂected
waves are down converted to three intermediate frequency (IF) signals ( 3, 6, 10 GHz). The IF signal
is separated with band pass ﬁlters, then the power of
each frequency is detected with diode detectors. In

author’s e-mail: kuwahara.daisuke@LHD.nifs.ac.jp

125©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

D. Kuwahara et al., Upgrade of 2-D Antenna Array for Microwave Imaging Reflectometry and ECE Imaging

Torus Plasma

Illumination waves
f = 60.41 GHz
61.81 GHz
63.01 GHz
64.61 GHz

4.5 GHz (60.41 GHz)
Power Det.

Receiving Antenna Array
Toroidal 5ch x Poloidal 7ch

Phase Det.

5.9 GHz (61.81 GHz)
Power Det.

B.S. 2Phase Det.

7.1 GHz (63.01 GHz)
Power Det.

BPF Array

Beam Splitter 1
Imaging Optic
Cutoff Layer@ 64.61 GHz @ 61.81 GHz@ 63.01 GHz @ 60.41 GHz

Phase Det.

8.7 GHz (64.61 GHz)
Power Det.

LO wavesf = 55.8 GHz

Phase Det.

Fig. 1 Schematic view of the MIR system.

this system, we observed the edge harmonic oscillation (EHO). The EHO is also observed with magnetic
probes and heavy ion beam probe diagnostics. However, wave numbers could not be estimated due to lackof receiver channels. In this system, phase detection
was not available.
The second generation MIR system was planned
for improving the above-mentioned problems. We developed a 2-D antenna array [6]. and a frequencystabilized microwave source. To observe the phasesignals, we developed a v-band (50 - 75 GHz) superheterodyne detection system by the use of two Voltage
controlled oscillators (VCO) and frequency multiplier.
One VCO is at the frequency range of 8 to 13.5 GHz.
It makes base frequency of the illumination waves.
The other VCO is at frequency of 110 MHz. it isused as IF and is carrier of reﬂected signals. Two signals from two VCOs are mixed with a single side-band
upconverter. The frequency of this upconverted signals is multiplied by six with the frequency multiplier.
Through these processes, v-band illumination wave ismade. And the LO wave uses base frequency. The
illumination and reﬂected wave section of the imaging
optics were the same as the former imaging optics, but
LO wave injection system was added. In this system,
waveguide coupling method could not be used because
the number of antenna channels is too large. Waveguide components, such as waveguides and waveguidemixers are large and expensive. Consequently, the LO
signal is injected to antenna elements by the use of optics. Each antenna element must have a compact andlow-cost mixer. The channel numbers of toroidal andpoloidal directions were 5 × 8, because it is expectedthat plasmas are more perturbed in the poloidal direction than toroidal direction.
At present, the third generation of the MIR system has been developed. Figure 1 shows schematic
view of the MIR system. In order to probe the plural cutoﬀ layers, this system employed a double superheterodyne detection method. The ﬁrst IF signal at
the frequency of 18.33 MHz is divided into four witha four way power divider. The ﬁrst IF signals are upconverted with four signals, the second IF signals at
frequencies of 750, 983, 1183, 1450 MHz. Four upconverted signals are upconverted further with the basesignal at the frequency of 9.3 GHz, and the frequencyof four signals are increased six fold. Finally, four illumination waves at frequencies of 60.41, 61.81, 63.01,
64.61 GHz are generated. Reﬂected waves are modulated with ﬂuctuations signals of each cutoﬀ layers.
Reﬂected waves are downconverted with LO waves at
the mixer of the antenna element. The signals fromeach antenna output are 4.5, 5.9, 7.1, 8.7 GHz + 110
MHz + ϕ1 to ϕ4 . The frequencies of 4.5, 5.9, 7.1, 8.7
GHz and 110 MHz are corresponds 6 times the 2nd IF
signals and the 1st IF signal, respectively. These four
signals are separated into 4 ways at a frequency separator, and are downconverted with the 2nd IF signals
increased frequencies by six. By the above-mentioned
processing, four ﬂuctuation signals, 110 MHz + ϕ1 to
ϕ4 are obtained. The frequency range of these signals
can be easily observed for the phase and amplitude
information by the use of commercial ICs for telecommunication.

3. MIR antenna array
Requirements of the MIR antenna array are as
follows:• Channels : 5 (toroidal), 7 (poloidal)• frequency range : 55 - 65 GHz (V-band)• mixer circuit (each channel)• IF frequency range : 4 - 9 GHz
We have developed a fabricating method of 2-D
antenna array [6]. It based on microstrip-line technique by the use of a high frequency printed circuitboard (PCB). Thereby the microwave antenna can beeasily made. The type of the antenna element is end-126

D. Kuwahara et al., Upgrade of 2-D Antenna Array for Microwave Imaging Reflectometry and ECE Imaging

ﬁre planer antenna. In the case of end-ﬁre type antenna, microwave is received on one side and IF signal
is outputted on the other side, thus antenna, mixer,
and IF circuits are aligned in line. By setting this unit
in parallel on a printed circuit board, a 1-D antenna
array can be assembled. And a 2-D antenna array canbe formed by stacking 1-D antenna array.

the middle of the waveguide, and is connected to theground pattern and the micro-strip-line. The diode isseparated by 1.8 mm from the waveguide end.
Figure 3 shows schematic view of the MIR 1-D
antenna array. Modiﬁed points from the second generation MIR antenna are as follows: 1) channel number,2) shape of the mouth of antenna element, 3) employment of more wide-band IF ampliﬁer, 4) gold platingof PCB. The toroidal and poloidal channel numbers
are ﬁve and seven, respectively. From former analysis,
it was conﬁrmed that if the channel numbers are morethan four channels, the wave numbers will be evaluated with suﬃcient accuracy [7]. Since it becomes easy
to adjust an optic system by the existence of a centerchannel, the poloidal channel number was changed toan odd number, seven. Due to the change in the channel number, the shape of an antenna element is necessary to be modiﬁed to ﬁt to a focus plane of the optics.
The mouth of the horn antenna is 13 mm × 13 mm.
The size of waveguide is 1.9 mm × 3.8 mm, which isthe same as the size of commercially available V-band
waveguides. The horn antenna receives both RF andLO waves and the mixer generates an intermediate frequency (IF) signal. The mixer bias is supplied from a
DC power supply through an inductor, and the bias
current is optimized. The IF ampliﬁer was modiﬁed
more wide-band since the third generation antenna array is planned to be used for ECEI. The IF ampliﬁers
are low-cost GaAs microwave monolithic ICs with thefrequency range of DC to 12 GHz and about 30 dB.
To prevent rusting, the PCB was plated with gold.
Rusting of PCB was seen in the antenna of the second
generation. It was conﬁrmed that the characteristicsof the strip-line does not deteriorate by plating.

(a) Upper Frame (reversed)
Lead-Out Slot(b) PCB
Schottky Diode(c) Lower Frame(e) 1-D Antenna Array

Fig. 2 Schematic view of the 1-D horn antenna array assembly. (a) underside view of upper structure, (b)PCB, (c) lower structure, (d) completed form

7 Channels are arranged

MIR Antenna Array
Single Diode Mixer
to B.P. F.
Array

13mm

14mm

3.8 mm15mm
Horn Section

Wide-Band Amp.
DC - 12 GHz
30 dB
Waveguide Section

0

12mm

H-Plane
E-PlaneRelational Amplitude (dB)

13mm

Fig. 3 Schematic view of the 1-D horn antenna array.

Figure 2 sketches a schematic view of the V-band
1-D horn antenna array assembly. It has three partsof upper structure, PCB, and lower structure. Figure2(a) shows a underside view of the upper structure,and Fig 2(c) shows the lower structure. The upperand lower structures are made of aluminum alloy, andhorn shape and waveguide slots are made by electricaldischarge machining. By attaching these slots, a hornantenna is formed. In the upper structure, anotherslot is formed for passing the micro-strip-line. Figure2(b) shows PCB, where the mixer diode is mounted.
RF ampliﬁers is installed on the PCB. By stacking
1-D horn antenna arrays, a 2-D antenna array is fabricated. The mixer is a surface-mounted type Schottky diode working in the frequency range between 20
GHz and 100 GHz. A mixer diode tip is mounted in-5-3dB Line-10-15

20.5deg.

-20

17.5deg.

-25
-30

-40

-20

0

20

40

Angle (deg.)
Fig. 4 Radiation Pattern of the MIR antenna element.

Figure 4 indicates the radiation pattern of theantenna element of the MIR antenna array at a frequency of 63 GHz. The vertical axis is the gain, andthe horizontal axis indicates angle. 0 degree corre-

127

Relational Amplitude (dB)

D. Kuwahara et al., Upgrade of 2-D Antenna Array for Microwave Imaging Reflectometry and ECE Imaging

4. ECEI system-26

An ECEI system is also developed for LHD. Thissystem uses the same microwave optics. The ECE signals from plasma are focused on the 1-D antenna arraythrough optics. The optics of ECEI system are madeby two, receiving optics and LO optics. The receiving
optics is the same as MIR receiving optics, since thediﬀerence in the frequency range is not aﬀected, andthe same diagnostic position is selected.
Figure 7 shows the ECEI receiving system. In
our ECEI system, the 1-D antenna array is adopted
for obtaining the 2-D ECE proﬁle. 2-D ECEI diagnostics (observing radial and poloidal directions) arebeing developed on several fusion devices and theygave much results. These systems use surface emission type 1-D antenna arrays. But our antenna array,
end-ﬁre 1-D antenna array can easily compose a 2-D
antenna array. It is a simple fact that 3-D diagnostics can obtain more information than 2-D diagnostics. The frequency range of the ECEI antenna arrayis 97 - 108 GHz, and the LO wave is at a frequency of
95 GHz. Therefore, the frequency range of IF signals(downconverted ECE signal) is 2 - 13 GHz. IF signals(spectrum) are separated into 11 signals at frequenciesof 2, 3, 4, · · ·, 12 GHz. These frequencies correspondto the radial location where ECE is emitted in plasma.

-28
4.61 GHz 6.01 GHz-30

8.81 GHz
7.21 GHz

-32
-34
4

5

6

7

8

9

IF signal (GHz)
Fig. 5 Frequency Responses of the MIR antenna element.

Conversion Loss (dB)

10
04.61GHz
6.01GHz
7.21GHz
8.81GHz

-10
-20
-30
LO Power : -16dBm
at the MIR optics

-40
-50
-80

-60

-40

-20

5. ECEI antenna array0

Diﬀerent points of the ECEI antenna array from
the MIR antenna array are the frequency range andaddition of a high-pass ﬁlter in front of the mixer element. The IF output characteristic of the MIR antenna array in the frequency range of ECEI is indicated in ﬁgure 8. This characteristic was obtained byilluminating 95GHz LO wave and w-band sweepableoscillator. Because of the ﬂat IF output in the ECEI
range, the mixer and IF circuits of the MIR antenna
can be applied to ECEI. The ECEI system employedsuper heterodyne detection with LO wave at a frequency of 95 GHz. ECE signals from plasma are not
only existing in the observing range but also under the
LO frequency. The downconverted signals are mixed
with higher side band and lower side band. Therefore,
to reject the unnecessary lower side band, a high-passﬁlter is required. The cutoﬀ frequency of the highpass ﬁlter is selected to be 93 GHz. There are two
methods to make the high pass ﬁlter.
One is to use the waveguide section as high-passﬁlter. The waveguide works as the high-pass ﬁlter.
The cutoﬀ frequency of a rectangular waveguide is determined from the size of the waveguide. The cutoﬀ
frequency of 93 GHz is correspond to a diameter of
1.588 mm. In a test, the cutoﬀ frequency response
was measured by changing the waveguide diameter.
In this test, the antenna with changed diameter of

LO power (dBm)
Fig. 6 Conversion losses of the MIR antenna element.

sponds to the normal direction from the front of theantenna. At 20.5 degrees and 17.5 degrees, the responses are decreased by 3 dB on H-Plane and EPlane, respectively.
Figure 5 shows frequency response of the antenna
element of the MIR antenna array. This data are obtained by injecting of four probing frequencies (60.41,
61.81, 63.01, 64.61 GHz) and LO wave at a frequencyof 55.8 GHz. The vertical axis indicates the IF output gain, the horizontal axis indicates the frequencyof the IF output. Four IF signals have the similar level(between 2.3 dB)
Figure 6 shows the characteristics of the conversion losses of the antenna element. The conversion loss
is plotted as a function of supplied LO power level.
In this ﬁgure, it was conﬁrmed that the four IF signals has the same tendency. The conversion loss curve
seems to be saturated at LO powers around -10 dB. In
MIR optics, supplying LO power is - 16 dB. Accordingly, the mixer can work under the highly eﬀective.

128

D. Kuwahara et al., Upgrade of 2-D Antenna Array for Microwave Imaging Reflectometry and ECE Imaging

Frequency separeter ArrayECEI Antenna Array
Local Wave
95 GHz

7 Channel

ECE Wave
97 GHz - 109 GHz

High-Pass Filter

IF Signal

2 GHz- 12 GHz

3-4 GHz
B.P.F

5-6 GHz
B.P.F

7-8 GHz
B.P.F

9-10 GHz
B.P.F

11-12 GHz
B.P.F

2-3 GHz
B.P.F

4-5 GHz
B.P.F

6-7 GHz
B.P.F

8-9 GHz
B.P.F

10-11 GHz
B.P.F

ADC

Fig. 7 Receiving Circuit of the ECEI system.

10

0

93GHz

Insertion Loss (dB)-5

IF output (dB)

-10
-15
-20
-25
-30
Local Frequency

-35
-40

85

90

95

100

#5
#4
#3
#2
#1
MIR #3

105

0
-10
-20
-30
-40
-50
80

110

85

90

95

100 105 110

Frequency (GHz)

Illumination Frequency (GHz)

Fig. 9 Cutoﬀ response of dichroic ﬁlter.

0-5(LO) wave can propagate by high transmission. At
a frequency of 92 GHz, the transmission is under -25
dB. The required performance was achieved. But ata frequency of 97.5 GHz, the gains of ♯5 - ♯1 antennas
decreased by about 10 dB.
The other method is the application of a dichroicﬁlter. In the frequency range of microwave, thedichroic ﬁlter is a metal plate that a lot of holes. Holes
work as circular waveguides, and it can use as planer
high-pass ﬁlter. Figure 9 shows a cutoﬀ response of a
93 GHz dichroic ﬁlter. The diameter of the hole (circular waveguide) is 1.9 mm. This ﬁlter rejects signals at
lower than 93 GHz signal by about 20 dB. Although
there are many sharp undulations in a range higher
than 93 GHz (pass-band), the performance is better
than that of a waveguide-section-high-pass-ﬁlter. Finally we decided that the dichroic ﬁlter and the MIR
antenna array would be employed as ECEI detector.

IF output (dB)

-10
-15
-20
#5 93.0GHz#4 90.6GHz#3 88.4GHz#2 86.8GHz#1 85.8GHz
MIR #3

-25
-30
-35
84

86

88

90

92

94

96

Illumination Frequency (GHz)
Fig. 8 Cutoﬀ response of the test antenna.

waveguide section from the MIR antenna was used.
Figure 8 shows the cutoﬀ response of tested antennas. The vertical axis indicates the gain, the horizontal axis shows illumination frequency. The upper
ﬁgure is plotted in the whole frequency and the lower
ﬁgure is a blow-up in a cutoﬀ range. In the ﬁgures,♯5 - ♯1 curves obtained from antennas with waveguidediameters of 1.613, 1.656, 1.697, 1.729, 1.748 mm correspond to 93.0, 90.6, 88.4, 86.8, 85.8 GHz. ♯5, thediameter of 1.613 mm gave a good response. 95 GHz

6. Summary
A 1-D antenna array for the MIR and the ECEI
was upgraded as follows: 1) channel number, 2) shapeof the mouth of antenna element, 3) employment of
more wide-band IF ampliﬁer, 4) gold plating of PCB.
For MIR, antenna array has ﬂat IF signal response.

129

D. Kuwahara et al., Upgrade of 2-D Antenna Array for Microwave Imaging Reflectometry and ECE Imaging

And the characteristics of the conversion losses were
measured. It turned out that our MIR system hasenough LO power. For ECEI, development of thehigh-pass ﬁlter was described. Two methods for applying the high-pass ﬁlter were presented. One is using the waveguide section of the antenna element as
waveguide-high-pass ﬁlter. The cutoﬀ response wasmeasured and it determined the best waveguide diameter. The other method, the application of thedichroic ﬁlter was also introduced. And a dicrhoic
ﬁlter and the third generation MIR antenna array are
employed as ECEI detector.
This work was carried out as one of
the NINS Imaging Science Project (Grant
No. NIFS08KEIN0021).
This work was also
supported by NIFS (Grant No. NIFS08ULPP525).
[1] C. Laviron, A. J. H. Donne, M. E. Manso, J. Sanchez,
Plasma Phys. Control. Fusion, 38 905 (1996).
[2] E. Mazzucato Rev. Sci. Instrum. 69, 2201(1998).
[3] H. Park et al., Rev. Sci. Instrum. 74, 4239 (2003).
[4] S. Yamaguchi et al., Rev. Sci. Instrum. 77, 10E930(2006).
[5] S. Yamaguchi et al., Plasma Fusion Res. 2, S1038(2007).
[6] D. Kuwahara et al., Plasma Fusion Res. SERIES. 8,
649-654 (2009).
[7] Z. Shi et al., Plasma Fusion Res. 5, S1019 (2010).

130