Plasma and Fusion Research: Regular Articles

Volume 14, 3402111 (2019)

Prototype Phased-Array Patch Loop Antennae for Electron
Cyclotron Emission Diagnostics∗)
Masaharu FUKUYAMA, Hiroshi IDEI1) , Miu YUNOKI, Ryuichi ASHIDA, Daichi OGATA,
Ryuya IKEZOE1) and Takumi ONCHI1)Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan
1)
Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan(Received 9 January 2019 / Accepted 9 April 2019)
A two-dimensional (2D) phased-array-antenna (PAA) for electron cyclotron emission is one of the diagnostics for finding the window area of mode conversion from electron cyclotron waves to electron Bernstein wave.
Spatial resolution of the reconstructed emission field pattern is improved with a lot of the antenna elements. The
study proposes a phased-array patch loop antenna (PAPA) as a promising system to increase the elements of PAA.
A 2D image reconstruction of a 6 GHz radio frequency source emission shows that the source position is detected
within millimeter range by applying the prototype PAPA. This result indicates that the 2D image reconstruction
with multi-element PAPA is feasible.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: electron cyclotron emission, phased array antenna, patch antenna, adaptive-array technique
DOI: 10.1585/pfr.14.3402111

1. Introduction
A non-inductive method to heat plasma and start-up
current solely by electromagnetic waves is required in thespherical tokamaks (STs) with limited space for the central solenoid Ohmic-heating coil. Even though the electromagnetic wave faces its cut-oﬀ regarding high plasma
density, an electrostatic wave so-called electron Bernstein
wave (EBW) can be propagated in such over-dense plasma
with no density limits. The obliquely injected ordinarymode (O-mode) wave from the magnetic low-field side
(LFS) would be converted to extra-ordinary-mode (Xmode) wave at the O-mode cut-oﬀ layer and then, it is converted to EBW at the upper hybrid resonance. The optimal
incident angle for the O-X-mode conversion depends onthe external magnetic field and the plasma density. The
electrostatic EBW emission can be detected as the electromagnetic wave at LFS by the inverse mode conversion
from EBW to O-mode [1].
In Q-Shu University Experiment with steady-state
Spherical Tokamak (QUEST), the EBW heating and current drive are performed with major and minor radii as
0.64 m and 0.36 m, respectively. When the O-mode cutoﬀ layer was positioned at a normalized plasma radius ρ =0.78, the optimum parallel (to magnetic field) refractive inopt
dex was evaluated as N// = 0.61 [2]. To search the mode
conversion window, the phased-array waveguide antenna(PAWA) has been developed [3]. Then, the low-power experiment with 3 × 3 PAWA was conducted in QUEST vesauthor’s e-mail: fukuyama@triam.kyushu-u.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

sel [4]. A thermal noise source (NS) mimicking incoherent emission was detected using the adaptive-array technique, and the location of the NS was identified with a
two-dimensional (2D) image reconstruction of the emission. Although the way to increase the element of thephased-array antenna (PAA) eﬀectively works to suppress
the side lobe and precisely identify the NS location, thefabrication of multiple-PAWA requires complex processes
with high cost. Since the patch loop antenna is suitable formass production due to compact, simple, and lightweightfeatures, the fabrication of multiple-element phased-arraypatch loop antenna (PAPA) is eﬀective to increase the element of the PAA [5]. This paper demonstrates a 2D image reconstruction using prototype PAPA as the proof-ofprinciple experiment for the multi-element PAPA.
The rest of this paper is organized as follows. In Section 2, a fabricated patch loop antenna benchmarked for
assembling the PAPA is introduced. Section 3 presents test
results of the prototype PAPA at low-power facilities, andits 2D emission image reconstruction for finding the location of radio frequency (RF) and emission source (ES) with
PAPA. A brief conclusion is given in Section 4.

2. Patch Loop Antenna
The patch loop antenna in which the loop is squareshaped was designed for 6 GHz-wave detections by applying a three-dimensional (3D) electromagnetic simulator “COMSOL Multiphysics”. The simulation results showthat the patch loop antenna with a quarter-wavelength side
has higher antenna gain than that of the eighth-wavelength
side. However, the directivity of the quarter-wavelength-3402111-1

c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 14, 3402111 (2019)side antenna is lower than that of the eighth-wavelengthside antenna. Since antenna gain is relatively low compared to that of waveguide antenna in general, the antenna
gain of patch loop antenna as high as possible is requiredat the patch loop antenna. Thus, an antenna with a quarterwavelength side was selected. A patch loop antenna wasmade by milling 1.6 mm thickness FR1. Figure 1 depicts
a photo of the fabricated antenna. The performance of thisantenna was verified experimentally.

2.1

Experiment using one patch loop antenna

ferential phase between the reference and the propagatingsides. The RF and LO synthesizers, and the network analyzer were phase-locked using a 10 MHz reference clock
signal.
Figures 3 (a) and (b) show the horizontal x- and vertical y- phase profiles at the normal propagating distance ofz = 0.182 m where the coordinates of (x, y, z) are illustrated
in Fig. 2 (b). The solid lines in Fig. 3 show the calculated
phase profiles from the Kirchhoﬀ integral at the launcher
antenna center with a point image source. Figure 3 (c)illustrates the phase evolution along the propagation z at

The property of a fabricated antenna was tested using a low-power test system. Figures 2 (a) and (b) show
a block diagram of the heterodyne system with a patch
loop antenna, a launcher antenna, and a patch loop antenna. 6 GHz RF-wave was generated by applying a synthesizer. The RF-wave was launched from a quad-ridge antenna and its radiation field was received by the patch loop
antenna on a 3D stage. The propagating-wave fields were
measured as a 70 MHz intermediate frequency wave (IFwave) with a 5.93 GHz local oscillator wave (LO-wave) at
the heterodyne detection. As shown in Fig. 2 (a), the reference, 70 MHz IF-wave, was detected before the launcher
antenna. Using a two-channel network analyzer, the IF
phase of the propagating wave was measured as the dif-

(a)

(b)

(c)

Fig. 1 A photo of the patch loop antenna fabricated by milling
1.6 mm thickness paper phenolic copper clad laminates(FR1). The side of the square loop is about quarter-wave.

Fig. 3 Measured and calculated phase profiles along x- and ydirections, and the evolution in propagating z-direction
with a patch loop antenna. The directions of coordinates(x, y, z) are illustrated in Fig. 2 (b).

Fig. 2 (a) Block diagram of the heterodyne system with a quad ridged launcher antenna and a patch loop antenna to observe if the patch
antenna can properly detect propagating-wave field-phase. Two synthesizers are used to excite test RF-wave and local oscillator(LO) wave at heterodyne detection, with phase-locking using a 10 MHz clock reference signal. (b) Configuration of a launcher
antenna and a patch loop antenna for the phase detection with the patch loop antenna. The position of the receiver loop antenna is
shifted by movable 3D stage in x-, y- and z-directions for measuring radiated field-phase profiles or evolution.

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Fig. 4 (a) Block diagram of the heterodyne system with two path loop antennae for checking the interference eﬀect between them. (b)Configuration for checking the interference between the two antennae in phase measurements. The loop antennae are arrayedvertically and horizontally to check the interference in their diﬀerent directions. The one loop antenna is moved using the 3D stage
for the other fixed loop antenna to change the distance between the antennae.

x = y = 0. The origin point was the center of the squareloop. As shown in Fig. 3, the profiles and evolution ofphase are in good agreement with the calculated results
and the expected linear dependence, respectively. These
results show that the fabricated patch antenna is present
to correctly detect the emitted phase for the adaptive-array
analysis.

2.2

Experiment with two patch loop antennae

It is important to accurately measure the phase profile and evolution for identifying the ES location throughthe adaptive-array analysis. Phase measurements would be
diﬃcult because of interference when the distance between
the two antennae is close. Two patch loop antennae wereused to check the interference eﬀect between the antennae. Figure 4 (a) shows a block diagram of the heterodyne
system with two antennae for checking the interference effect. The same 6 GHz RF-wave was utilized with the same
5.93 GHz LO-wave. As shown in Fig. 4 (b), these antennae were arrayed vertically and horizontally in their directions to check the interference. One patch loop antenna
was fixed and the position of the other antenna was shiftedusing the 3D stage. The phase diﬀerence (Δθ) between theantennae was measured using a network analyzer.
Δθ was evaluated as a linear dependence on the diﬀerential propagation distance (z − z) between the antennae;Δθ = k(z − z), where z denotes the distance between the
launcher and the swept antennae defined as x2 + y2 + z2 .
Figure 5 demonstrates the measured Δθ on (z − z). As
shown in Fig. 5 (a), the interference eﬀect between the antennae was not detected in the vertical y-direction, since themeasured Δθ was linearly proportional to (z −z). However,as shown in Fig. 5 (b), the measured Δθ was distorted fromthe calculated line when the (z − z) was smaller than 5 mmin the horizontal x-direction. When (z − z) = 9.6 mm, i.e.,the distance between the antennae is 60 mm, the interfer-

Fig. 5 Dependence of the phase diﬀerence (Δθ) on diﬀerential
propagation distance (z − z) between two patch loop
antennae, where z denotes the distance between the
launcherand the swept antennae which can be definedby x2 + y2 + z2 . The coordinates of (x, y, z) are shownin Fig. 4 (b). The marked distance with broken lines of(z − z) = 9.6 mm corresponds to the two antennae distance of 60 mm.

ence eﬀect was not observed even in the x-direction as wellas in the y-direction, and there was no significant ghost or
side lobes on 2D image reconstruction. The configuration
with the antenna distance of 60 mm in both x- and y- directions has been adopted for the PAPA.

3. PAPA
Figure 6 presents a photo of the fabricated fourelement PAPA based on the bench test results. Each patch
antenna was fixed using sub miniature type a (SMA) cabling. The SMA cables were fixed on acrylic plates. This
PAPA comprises four (2 × 2) elements.

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3.1

Volume 14, 3402111 (2019)

Experiment using PAPA

The assembled PAPA for the adaptive array analysis
was bench-tested to check accuracy. Figure 7 (a) depicts
a block diagram of the heterodyne system with a fourelement PAPA. The same RF and LO waves were utilized
for the PAPA test. The four-element PAPA was fixed, while
the launcher antenna position was shifted in the normal
propagating z-direction using the 3D stage. The phase evolution on z for each loop antenna was measured (a totalof four times), where z denotes the propagation distance
between the swept
 launcher antenna and the PAPA, and
it is defined by x2 + y2 + z2 in the coordinates shown in
Fig. 7 (b).
Figure 8 shows each phase evolution of the fourelements on z measured at x = y = 0. Solid lines present
the linear dependencies of kz for the various antenna elements. The measured phase evolutions on z correspondto the linear dependencies. This result shows that the effect of interference is relatively small enough. While phaseoﬀsets from the diﬀerence in length of RF circuits are not

Fig. 6 Photo of fabricated four-element PAPA. The distances between centers of the square loops in horizontal and vertical coordinates are 60 mm along the interference check
between the antenna elements described in section 3.1.

negligible. Therefore, calibration to eliminate the oﬀset isrecommended. A method for this is given below.

3.2 A 2D image reconstruction with PAPAThe 2D image reconstruction was demonstrated withthe four-element PAPA to identify the ES location. The
beam-forming adaptive-array technique was used to reconstruct the emission pattern. The measured profile of intensity, I(x, y), in the beam-forming can be given byI(x, y) =4| exp(θi − θi oﬀset − k(ri − zES ))|2 /ri2 ,i=1ri = z2ES + (x − xi )2 + (y − yi )2 ,

(1)
(2)

where θi denotes the phase diﬀerences between the i-th
PAA port against port 1. θi oﬀset denotes the phase oﬀset
at each port. The coordinates (xi , yi , zi ) and (xES , yES , zES )are the port positions of the PAPA elements and the ES
position, respectively.
Figure 9 (a) shows a block diagram of the heterodyne

Fig. 8 Measured phase evolution of each port on propagation
distance z where z denotes the propagation distance between swept
 launcher antenna and patch loop antenna defined by x2 + y2 + z2 . The coordinates of (x, y, z) are
shown in Fig. 7 (b).

Fig. 7 (a) Block diagram of the heterodyne system with four-element PAPA to observe whether the phase evolution is correctly measured
along the propagation for the adaptive array analysis. (b) Configuration to measure the field-phase evolution along the propagationfor each element of the PAPA.

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Fig. 9 (a) Block diagram of the heterodyne system with the four-element PAPA to observe whether the emitted-field image from thelauncher antenna is 2-dimensionally reconstructed and its emitted source position is properly detected with the adaptive arrayanalysis. (b) Photo of the launcher antenna and the four-element PAPA. The launcher antenna is located at (0.030 m, −0.030 m,
0.140 m). Here the coordinate origin is the center of the four-element PAPA.

Table 1 Phase diﬀerences between the PAA ports against port 1.

port
port2
port3
port4

θi oﬀset
25.1 [deg]−163.3 [deg]−4.9 [deg]θi23.8 [deg]−54.3 [deg]76.6 [deg]Table 2 Phase diﬀerences between the PAA ports against port 1
regarding measurement and calculation.

port
port2
port3
port4

Fig. 10 Field intensity pattern reconstructed through beamforming analysis for ES radiation from (xES , yES ) =(0.030 m, −0.030 m). The peak point of the reconstructedemission pattern is (0.034 m, −0.024 m).

system on PAPA and the ES source position detection. A
photo of the launcher antenna and the four-element PAPA
is shown in Fig. 9 (b). Each port phase oﬀsets stemmingfrom the diﬀerence among the lengths of the RF circuit
were evaluated from the symmetric positions on the four
elements for the PAPA center. The launcher antenna wasset at (x, y) = (0,0), which is the PAPA center. The coordinate system is defined as shown in Fig. 9 (b). Here, the normal propagating distance from PAPA center to the launcher
antenna was z = 0.140 m. The measured phases were described by the phase oﬀsets of the antenna ports. The
launcher antenna was re-located at (0.030 m, −0.030 m,
0.140 m), while θi were measured again. Table 1 summarizes the phase diﬀerences between the PAA ports against
port 1.
Figure 10 shows the reconstructed emission patternfrom equations 1 and 2 with the measured phase after
the oﬀset calibration. The cross (+) and closed rectangle() denote the ES point and PAPA positions, respectively.

measurement (θi − θi oﬀset )−1.3 [deg]109.0 [deg]81.5 [deg]calculation
0.0 [deg]
86.5 [deg]86.5 [deg]The peak point of the emission pattern was (0.034 m,−0.024 m) against the ES point of (0.030 m, −0.030 m) in
the x and y planes. The phase diﬀerences between the PAA
ports against port 1 could be calculated using kz . Table 2
illustrates the list of the phase diﬀerences through the measurement and the calculation. The measured phases werein good agreement with the calculation except for the port
3. Since the patch antenna was fixed only with SMA cabling in the setting of PAPA, port 3 would be misaligned
because of gravity while conducting the experiment. The
proper fixtures for PAPA are required in our future works.
Consequently, the location of ES can be properly identified through 2D image reconstruction with the prototype
PAPA. The proof-of-principle of the prototype PAPA wassuccessfully shown, proving that the 2D image reconstruction with multi-element PAPA is feasible.

4. Conclusions
The fabricated patch loop antenna is simple and compact. It also has an advantage for practical mass produc-

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Volume 14, 3402111 (2019)tion. The fabricated patch antenna can measure the phaseprofile and evolution properly at the low-power bench test.
The prototype PAPA comprised four-elements, in which
the distance between the antennae is 60 mm was assembled. The 2D imaging measurement was demonstrated using the four-elements PAPA. The ES position could be detected within a few mm range when the ES was located at
the normal propagating distance of z = 0.140 m.

Acknowledgments
This research was partially supported by Grant-in-Aid
for Challenging Exploratory Research (JSPS KAKENHI
Grant Number JP16K14529).
[1] H.P. Laqua et al., Phys. Rev. Lett. 78, 3467 (1997).
[2] H. Idei et al., J. Plasma Fusion Res. Series 8, 1104 (2009).
[3] H. Idei et al., J. Instrum. 11, 4, C04010 (2016).
[4] K. Mishra et al., Rev. Sci. Instrum. 85, 11E808 (2014).
[5] A. Pal et al., IEEE Trans. Antennas Propag. 65, 576 (2017).

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