Dual-Comb Microwave Doppler Reflectometer System in LHD ...
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Proceedings of IRW14
Dual-Comb Microwave Doppler Reflectometer System in LHD and
Feasibility Study for a JT-60SA Doppler Reflectometer
T. Tokuzawa1,2, S. Inagaki3, A. Ejiri4, H. Idei3, R. Imazawa5, N. Oyama5, M. Yoshida5,
K. Tanaka1, H. Tsuchiya1, K. Ida1, K. Y. Watanabe1,6, and H. Yamada1,4
1National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
2SOKENDAI, 322-6 Oroshi-cho, Toki 509-5292, Japan 3Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan.
4Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa 277-8561, Japan 5National Institutes for Quantum and Radiological Science and Technology, 801-1 Mukoyama, Naka,
Ibaraki, 311-0193, Japan 6Dept. of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya,
464-8601, Japan
E-mail contact of the corresponding author: [email protected]
Introduction
New Ka-band frequency comb Doppler reflectometer system which is called a “dual
comb Doppler reflectometer” has been installed in LHD. Previous frequency comb system
requires a very high sampling rate (40 GS/s) digital storage system to obtain all frequency
comb components in the probing beam, and the observable time is limited by the memory size.
This time, dual frequency comb sources with a 20 MHz difference are used to reduce the IF
frequency components to less than 2 GHz, leading to lower sampling rate digitizers. In
addition, the new receiver circuit requires the lower radio frequency range components and a
circuit board due to the IF reduction. Cost saving and better performances are achieved. Some
experimental results are successfully observed in LHD.
Feasibility study of Doppler reflectometer to JT-60SA has been carried out. The
preliminary results of a full-wave 3D numerical simulation using launching / receiving
antenna are obtained.
In the following proceedings, the above dual comb Doppler reflectometer is reported.
Concept of dual comb operation
Frequency comb Doppler reflectometers [1] have been applied in LHD, and the
14th Intl. Reflectometry Workshop - IRW14 (Lausanne) 22-24 May 2019 O.203___________________________________________________________________________
precise radial profiles of the perpendicular velocity are obtained up to 20-40 radial points,
simultaneously, by using high sampling ratio (40 GS/s) data acquisition system [2]. This is a
very useful technique, but the observation time is limited because of the limitation of the
stored memory. For resolving this problem, we try to reduce the intermediate frequency (IF)
components in the mixer output.
There are some techniques below for reducing the IF. Those are
(1) The local frequency (LO) is set to almost center of the probing comb frequency (RF)
range which is ka-band (26 - 40 GHz).
(2) LO is used in another frequency comb source. That is called a “dual comb” system.
These conditions are shown in Fig. 1. In the case of (1), the IF frequency range is reduced to 7
GHz which is one half of the original value. However, this value is still high enough to
observe the long discharge. In the case of (2), the IF frequency can be lowered to 0.5 GHz,
which may store a long discharge.
Fig. 1 schematic of the generating IF frequency. (left) LO sets the outside RF, (center) LO sets the
center of RF, (right) dual comb cases.
14th Intl. Reflectometry Workshop - IRW14 (Lausanne) 22-24 May 2019 O.203___________________________________________________________________________
The concept of the dual comb operation is shown in Fig. 2. When the source 1 of the
frequency comb is operated by a 190 MHz clock and the source 2 is operated by 200 MHz,
these two comb sources generates discrete IF components in each frequency step. The output
frequencies of mixed signal should be the multiple of the frequency difference (∆f =10 MHz)
as shown in Fig. 3. The test of the dual comb operation is carried out and the spectrum of the
mixer output is shown in Fig. 4. The low frequency IF signals are successfully observed.
Fig. 3 schematic of each mixed frequency
component in dual comb operation Fig. 4 Generated If components in dual comb
operation
Dual comb Doppler reflectometer system in LHD
Ka-band dual comb Doppler reflectometer system is shown in Fig. 5. Each comb
source is operated by 710 and 730 MHz. RF frequency is matched to the former Doppler
reflectometer system [1], because of the correlation measurement. One of the frequency
comb components is selected to use the LO of the Mixer 1 by the band-pass-filter (BPF) of
26.27 GHz. Output of Mixer 1 is combined with the frequency comb components of source 2.
Fig. 2 schematic of dual comb operation concept
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For the precise heterodyne detection, a part of source 1 and 2 components are mixed and
generates the frequency chain which is used for the reference signal for IQ detection. Both
outputs from Mixer 2 and Mixer 3 are then led to electro-optical converter in order to direct
the signal to IQ detection in another room 100 m away. A part of the traveling RF probe
signal components are separated and led to the high sampling data acquisition (currently, it is
oscilloscope). The other part of the signal is led to 8ch IQ detection system to measure the
whole plasma discharge utilized by the real time data acquisition system. Thanks to the
reduced IF frequency range, the filter bank and the IQ detection could be created on one
circuit board, as shown in Fig. 6.
Fig. 5 Schematic of ka-band dual comb Doppler reflectometer system
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Fig. 6 Photograph of 8ch BPF and IQ detection circuit board
System test for Doppler shift by rotating grating and first observation in
LHD plasma
For the system test of the measurement of the Doppler shifted frequency, the grating
drum whose rotation speed is controllable is prepared. The drum size is the diameter of
500mm and the grid spacing is 10mm, as shown in Fig. 7. The rotating speed is up to 4000
rpm. The test layout is shown in Fig. 8. The observed spectrogram, when the rotating speed is
changed, is shown in Fig. 9. Each peak is corresponds to the grating. The change of the
Doppler shift is clearly observed when the rotation direction reverses. The relationship
between the rotation speed and Doppler shift frequency is shown in Fig. 10. The linear
relationship is clearly obtained.
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Fig. 7 Photograph of rotating grid drum Fig. 8 Schematic layout of Doppler shift
measurement test
Fig. 9 Spectrogram of the Doppler shifted IF signal during the drum rotating test
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Fig. 10 Observed Doppler shifted frequency as a function of rotating speed
The system is applied to the LHD plasma experiment. Figure 11 shows the temporal
behaviors of the Doppler shifted frequency (fD). The response of the fD is almost agreement
with the other Doppler reflectometer signal which is installed at the same port. Dual comb
Doppler reflectometer is found to be well working.
Acknowledgments
This work was partially supported in part by KAKENHI (Nos. 19H01880, 17K18773, 17H01368, 15H02335, and 15H02336), by a budgetary Grant-in-Aid from the NIFS LHD project under the auspices of the NIFS Collaboration Research Program (ULPP027 and KLPH024), by the collaboration programs of the RIAM of Kyushu University, by the Asada Science foundation, and by the collaboration programs of the QST. Additional support was provided by Japan / U.S. Cooperation in Fusion Research and Development.
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Fig. 11 Temporal behaviors of Doppler shifted frequencies of new dual comb system (upper) and the
different Doppler reflectometer system.
References
[1] T Tokuzawa, et al., "Ka band Microwave Frequency Comb Doppler Reflectometer System for the Large Helical Device", Plasma and Fusion Research 9 (2014) 1402149.
[2] T. Tokuzawa, et al., "Microwave frequency comb Doppler reflectometer applying fast digital data acquisition system in LHD", Review of Scientific Instruments 89 (2018) 10H118.
14th Intl. Reflectometry Workshop - IRW14 (Lausanne) 22-24 May 2019 O.203___________________________________________________________________________