Dual-Comb Microwave Doppler Reflectometer System in LHD ...

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.


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


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


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.


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


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.


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.


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