Proceedings of ITC/ISHW2007

Development of Microwave Imaging Reflectometer at NIFS
Y. Nagayama1), S. Yamaguchi1), Z. B. Shi2), Y. Kogi3), A. Mase3), S. Sugito1), N. Ohyabu1)1)2)National Institute for Fusion Science, Toki 509-5292, JapanThe Graduate University for Advanced Studies, Toki 509-5292, Japan
3)
KASTEC, Kyushu University, Kasuga 816-8580, Japan(Received 15 October 2007 / Accepted 15 October 2007)

The Microwave Imaging Reflectometer (MIR) is under development at NIFS. The first MIR system isinstalled in Large Helical Device (LHD). The optics of this system is made of Aluminum mirrors and Plexiglas
plates. In this system, the primary mirror is remotely controlled to optimize the reflection power. The
illumination wave is generated by 3 IMPATT oscillators, and the local oscillator (LO) is a Gunn oscillator. Thissystem uses wide band-pass filters. The second MIR system is installed in TPE-RX, which is a large RFP. The
optics of this system is made of Aluminum mirrors, Teflon lenses and Plexiglas plates in order to reduce size. In
this system the frequencies are so stabilized that noise can be reduced by using narrow band filters. The 4x4 2-D
mixer array and phase detection system have been also developed.
Keywords: microwave, imaging, reflectometry, MIR, LHD, turbulence, fluctuation, diagnostics

1. Introduction
It is considered that plasma confinement in a fusion
devise is governed by turbulence and instabilities, which
often appear in the electron density fluctuations. The
microwave reflectometry is a sensitive measurement ofturbulent density fluctuations because it uses reflection of
the microwave by the plasma with cut-off density [1].
The density fluctuation causes the amplitude modulationand the phase modulation in the reflected beam. Recently
the Microwave Imaging Reflectometer (MIR) has been
intensively developed because MIR may provide the2D/3D image of density fluctuation to reveal the physics
of turbulence [2]. The short wave fluctuation diverge thereflected wave. MIR takes advantage of large aperture
optics to form an image of the reflecting layer onto anarray of detectors located at the image plane, enabling
localized sampling of small plasma areas.
Issues in the development of MIR are as follows: (1)improvement of sensitivity or the signal to noise ratio; (2)improvement of number of channels in the 2D detector
array; (3) reducing the cost.
The MIR is underdevelopment at National Institute for Fusion Science(NIFS) by collaborating with Kyushu University [3].
The first MIR system has been installed in the Large
Helical Device (LHD) [3,4]. The second MIR system
has been installed in TPE-RX, which is a large RFP
device at National Institute of Advanced Industrial
Science and Technology (AIST) [5]. This paper will
present how to solve the above issues at NIFS.
author’s e-mail: nagayama.yoshio@nifs.ac.jp

2. MIR System in LHD
Figure 1 shows a schematic diagram of the MIR
system in LHD.
The illumination wave (RF) isgenerated by 3 high power (0.5 W) IMPATT oscillators,
whose frequencies are 53, 66 and 69 GHz, respectively.
These oscillators used to be installed 5 m far from LHD.
However, the operation of the IMPATT oscillators wasinterfered by the leakage of the magnetic field. Actually,
the output power of the IMPATT oscillators reduces as themagnetic field increases, and finally no output power areobserved. After enclosing in 5 mm thick soft-iron shieldcase, the power was dropped by 30 %. Therefore themicrowave sources are installed 15 m far from LHD andthe microwave is transferred using oversized rectangular
waveguides (X-band, WR-90) and 90 degree H bends.
By using optical system, the illumination wave radiatedfrom a horn antenna (WR-15) is formed to a parallel
beam with a diameter of 20 cm at the reflection layer inthe plasma. The illumination wave is reflected by thethree cutoff layers, which are determined by the local
density and the magnetic field in the case of X-mode
reflection.
The primary imaging mirror (M1) is installed in theLHD vacuum vessel. This is an elliptic concave mirror
with the size of 43x50 cm and the focal length of 106.5 cm.
The distance between the mirror M1 and plasma is 210 cm.
The mirror M1 is remotely manipulated by the use ofultrasonic motors (USMs). Since the LHD plasma is

Proceedings of ITC/ISHW2007

Fig. 1 Schematic view of MIR system in LHD. M1: Adjustable concave mirror, M2: Plane mirror, DP: Dichroic plate(70GHz), BS: Plexiglass beam splitter, US: Ultrasonic motor, ES: ECCOSORB CV3.

twisted, toroidal and poloidal angles of the main mirror
should be controlled within 1 degree in order to obtainreflection from plasma [4].
The reflected wave is accumulated by the primaryimaging mirror. ECE and the reflected wave are separated
by a dichroic plate (DP), which is a high pass filter with asharp cut-off frequency of 70 GHz. This dichroic plate isan 8 mm thick aluminum alloy plate with many circularholes. The diameter of each hole is 2.5 mm and they areseparated by 2.8 mm. The hole has the angle of 45 degree
to the plate so that ECE wave more than 70 GHz
efficiently passes through the dichroic plate.
B =2.87T, R =3.575m, γ=1.254, B =100%
ax

(a)ax

ctr-NBI
19 -3(10 m ) (keV)#74422

ICHco-NBI

4 (b)

Te

202 (c)qperp NBI

Tin1eβ00(d)

MIR01234(e)MIR#2MIR#8
mm#27
1.55

1.551
t (sec)1.552

Fig. 2 (a) Heating, (b) temperatures, (c) electron density andbeta, (d) MIR signal in slow scale, (e) MIR and magnetic
probe signals in fast time scale in LHD.

The reflected beam and the illumination beam areseparated by the beam splitter (BS), which is a 3 mm thickPlexiglas plate. The reflected illumination beam by BS
and the transmitting illumination beam through DP areabsorbed by microwave absorbing forms (ECCOSORB
CV-3) in order to reduce background noise. Because of
microwave absorbers and beam separation, the leakage ofthe illumination wave to the receiver is significantly
reduced, so that the background level in the signal isdrastically reduced.
The plasma image is formed on the front end ofreceiving horn antenna (WR-15) array by the mirror optics.
The pair of antennas is separated by 8.4 cm in the poloidal
direction, and another pair by 10.7 cm in the toroidal
direction on the cutoff layer. Received reflection wave istransmitted to the heterodyne receiver with oversizedwaveguides (X-band).
In the heterodyne receiver,reflected wave is mixed with the wave from local oscillator(LO) with the frequency of 63 GHz to make intermediate
frequency (IF) signals.
IF signals selected byband-pass-filters with the bandwidth of 10% of the central
frequency are amplified by modular RF amplifiers.
Finally they are rectified by Schottky barrier diodes,
amplified by DC amplifiers and digitized by PXI digitizer
modules.
Figure 2 shows an example of MIR signals in LHD.
Clear MIR signals are obtained after adjusting the main
mirror. Using this system, novel MHD modes areobserved in the case of low density, as shown in Fig.2(e).

3. MIR System in TPE-RX
The second generation of MIR system has beendeveloped for TPE-RX. Main features of the second
generation are as follows: (1) narrow IF bandwidth; (2) 2D
receiver array; (3) phase detection. Figure 3 shows theProceedings of ITC/ISHW2007
schematic view of the MIR system in TPE-RX. In thisMIR system a Gunn oscillator generates the RF wave withfrequencies of 20 GHz (ω). IMPATT oscillator is not usedbecause its output has many modes with slightly different
frequencies.
The primary mirror (M1), which is an elliptic concave
mirror with the size of 40x43 cm, makes the RF beam. Theillumination beam designed as its diameter is 9 cm at thewindow and 10 cm in plasma. As the diameter of thequartz window of the TPE-RX viewing port is 10 cm, theillumination wave can pass through the window. Thiswindow size is similar to the open mouse (10.5x8.7 cm) of
the 20 dB standard gain horn antenna for 20 GHz (WR-42).
Since the magnetic field is very low in TPE-RX, theSchematic view of MIR system in TPE-RX.

600
Intensity (AU)Fig. 3ch.1

z=800 mmch.3

400

200

0400

450500550x (mm)600

Fig. 4 Test beam profile at the plasma.

illumination wave is in the O-mode, of which cutoff
frequency is the plasma frequency. The reflected wave iscollected by M1 and is separated from the illumination
beam by the first beam splitter (BS1). The LO wave and thereflected wave is mixed with the second beam splitter(BS2). These beam splitters are made of 3 mm thickPlexiglas plate. RF wave reflected by BS1 and LO wave
passing through BS2 are absorbed by foam absorber(ECCOSORB CV-3) in order to reduce background noise.
An image of reflection layer is made onto the 2-D mixer
array by the Teflon lens (L1). As a test, the emission fromthe position of detector is measured at the position that is80 cm far from the frame. This position is 133 cm far
from the primary mirror M1, and is 38 cm far from thewindow of the viewing port. This position corresponds tothe position with the minor radius of 30 cm inside theplasma. Fig. 3 shows the resolution at the plasma is 8 cm.
The channels 1 and 3 are separated by 4.5 cm at thedetector position, and they are separated by 5 cm at theplasma position.
In the 2-D mixer array, four mixer elements are set ona circuit board with a distance of 12 mm, and 4 circuitboards are stacked with a distance of 15 mm. Eachelement consists of a planer Yagi-Uda antenna, a balun, abeam lead type Schottky barrier diode (SBD) and an IF
amplifier. In this system, the balun is improved from theoriginal planer Yagi-Uda antenna system [6]. On thedesign of antenna system, a computer code forelectro-magnetic field (Microwave Office) is employed.
Our Yagi-Uda antenna has 3 guiding elements, a pair ofdipole and a reflector element. The IF amplifier consistsof SAW filters (Murata SAFCC110MCA1T00) and RFamplifiers (RF Micro Devices RF3396). Also the mixerelement has two voltage regulators (On Semiconductor
MC78LC40NTR) for the power supply of amplifier andthe DC bias of SBD. The signal output is a MMCX
straight PCB jack connector (HUBER-SUHNER
82_MMCX-S50-0-2/111_KE). The circuit board is a thin
Teflon circuit board (Nippon Pillar Packing
NPC-F220A(18/18)) with a thickness of 0.18 mm. The
circuit is made by the micro strip line technology. Theshield box that contains 2-D mixer array has 16 signal
output with SMA connector. The MMCX and SMA areconnected with a coaxial cable inside the shield box.
The LO wave (ω+ω1) is made by mixing the RF wave
and the 110 MHz (ω1) at an up-converter (HittiteHMC523). The lens L1 and lens L2 makes a spot of LOwave with the diameter of 10 cm on the 2-D mixer array.
By mixing the reflected wave with the frequency of ω andthe LO wave, the 2D mixer makes IF signal the frequencyof ω1. Since the IF frequency ω1 is well stabilized, thenoise can be significantly reduced by amplifying with band
pass filter (BPF) with a narrow bandwidth (4 MHz).

Proceedings of ITC/ISHW2007
The reflective wave contains the amplitude A and thephase φ, as, Aexp(iωt+φ), where the amplitude A and thephase φ is generated by a density fluctuation in the plasma.
The phase φ is important because it indicates the vibration
motion of the reflection layer. The IF signal also contains
the amplitude and the phase, as, Aexp(iω1t+φ). The
amplitude is obtained by rectifying the IF signal withMMIC (Analog Devices AD8362). The phase is obtainedby comparing the differential wave (ω1) and the signal
wave with the IQ demodulator (Analog Devices AD8348).
I and Q signals correspond to cosφ and sinφ, respectively.
Figure 5(a) shows plasma parameters of high Θ RFP
plasma in TPE-RX. Here, Ip is the toroidal plasma
current, and ne is the line averaged electron density. Fand Θ are the toroidal and poloidal fields normalized by
the average toroidal field at the plasma boundary, defined
as F=Bt(a)/<Bt>, and Θ=Bp(a)/<Bt>, respectively.
Example of MIR signals from this plasma is shown in Fig.
5(a,b). Fig. 5(c) shows MIR channel numbers at thedetector position schematically. Here p and t denote thepoloidal and the toroidal directions, respectively. Since
the imaging optics makes up-side down image, the p(a)TPE52990

Ip (kA)200

1ne (1019m-3)

Θ0
1.5

0F0Θ
F-0.5

1ch.6
A0(b)

0.02

0.04 0.06
t (sec)0.08
pch.11
A0.1(c)12345

678

In conclusion, MIR is under development at NIFS.
MIR is expected as a new imaging diagnostics ofturbulences and instabilities in plasmas.
The firstgeneration of MIR has been installed in LHD. Thissystem uses imaging optics and waveguide antenna arrayas a detector. The second generation of MIR has been
installed in TPE-RX. In this system, the differential
frequency between the reflected wave and the LO wave is
so stabilized that sensitive measurement becomes possible.
So, the 2D imaging detector array with planar Yagi-Uda
antenna works well. Also preliminary operation of phasedetection system has been successful. This result is very
encouraging in the development of MIR.

Acknowledgements

13 14 15 16ch.6A

4. Conclusion

This works is supported by National Institute forFusion Science (No. NIFS07ULPP525) and by National
Institute of Natural Sciences (No. NIFS07KEIN0021).

9 10 11 12ch.10
A

direction corresponds to the positive sign in the plasma andthe t direction corresponds to the clockwise direction in thetop view. As waveforms are similar to other channels, the2D receiver works well.
Fig. 5(c) also shows I-Q signals of channel 6 of the2D receiver. Fig. 5(d) shows Lissajous’ curve of I-Q
signals of channel 6 between t=28.06 and 28.10 msec. If
I and Q signals are proportional to cosφ and sinφ,respectively, the Lissajous’ curve should be circle.
In
this case, we can find amplitude of I and Q signals areabout two times different. May be I and Q signals
represent phase because the trajectory is rotating. So thistrajectory indicates that the reflection layer moves backand forth. Closed circle indicates a data point, which istaken every 1 µs. At the point (I, Q)=(0.3, 0.2), whichcorresponds to t=0.028078 sec, the direction of trajectory isturned over. This indicates that the reflection layer
motion is turned over at this time.

t

TPE52990_0.02806-0.02810

(d)

ch.6
I

[1]
[2]
[3]
[4]

0ch.6
Q
0.02805

0.0281t (sec)References

ch.6

Q

0.02815
0I

Fig. 5 (a) Plasma parameters and MIR signal in high Θ
operation in TPE-RX. (b) Signals of the 2D mixer
array of MIR in fast time scale. (c) Schematic view ofthe 2D mixer array. (d) Lissajous' curve of I-Q signals

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