Q-band high-performance notch filters at 56 and77 GHz notches for versatile fusion plasma
diagnostics
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言語: English
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公開日: 2021-12-20
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キーワード (En):作成者: NISHIURA, Masaki, SHIMIZU, T., Kobayashi, S.,TOKUZAWA, Tokihiko, Ichinose, K., KUBO, Shin
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http://hdl.handle.net/10655/00012782
This work is licensed under a Creative Commons
Attribution 3.0 International License.

Q-band high-performance notch filters at56 and 77 GHz notches for versatile fusion
plasma diagnostics
Cite as: Rev. Sci. Instrum. 92, 034711 (2021); https://doi.org/10.1063/5.0041243
Submitted: 21 December 2020 • Accepted: 24 February 2021 • Published Online: 12 March 2021
M. Nishiura, T. Shimizu,S. Kobayashi, et al.

COLLECTIONSPaper published as part of the special topic on Proceedings of the 23rd Topical Conference on High-Temperature
Plasma Diagnostics

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Rev. Sci. Instrum. 92, 034711 (2021); https://doi.org/10.1063/5.0041243© 2021 Author(s).

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scitation.org/journal/rsi

Q-band high-performance notch filters
at 56 and 77 GHz notches for versatilefusion plasma diagnostics
Cite as: Rev. Sci. Instrum. 92, 034711 (2021); doi: 10.1063/5.0041243
Submitted: 21 December 2020 • Accepted: 24 February 2021 •Published Online: 12 March 2021
M. Nishiura,1,2,a)T. Shimizu,1 S. Kobayashi,1T. Tokuzawa,1K. Ichinose,3 and S. Kubo1,3

AFFILIATIONS
1National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
3Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
2Note: Paper published as part of the Special Topic on Proceedings of the 23rd Topical Conference on High-Temperature PlasmaDiagnostics.
a)Author to whom correspondence should be addressed: nishiura@nifs.ac.jp

ABSTRACT

A six-pole Q-band waveguide filter with a notch frequency above the Q-band has been developed for plasma diagnostics. The previous paper[Nishiura et al., J. Instrum. 10, C12014 (2015)] reported that the notch frequency exists within the standard band. In this study, the newlyrequired notch filter extends the function, which prevents a thorny wave from being mixed into an instrument beyond the standard bandwidthof the waveguide. The mode control technique for cavities realizes a deep and sharp filter shape for Q-band notch filters with 56 and 77 GHz
notches, respectively. The former filter has an attenuation more than 50 dB at 56.05 GHz and a bandwidth of 1.1 GHz at −3 dB. The latter
filter has an attenuation more than 55 dB at 76.95 GHz and a bandwidth of 1.6 GHz at −3 dB. The electron cyclotron emission imaging andthe electron cyclotron emission (ECE) diagnostics for the Q-band implemented a pair of the fabricated filters and demonstrated the ECE
measurement successfully in the intense stray radiation from a 56 GHz gyrotron.
Published under license by AIP Publishing. https://doi.org/10.1063/5.0041243

I. INTRODUCTION
In fusion plasma experiments, intense millimeter waves forplasma production and heating are injected into a vacuum vessel
from a megawatt-class gyrotron. The power of the electromagnetic
waves is not completely absorbed by plasmas. Then, the stray radiation of the intense beams interferes with the signals of measurement
systems, for example, electron cyclotron emission (ECE),1 reflectometer, and collective Thomson scattering (CTS) diagnostics.2,3 In
most serious cases, intense millimeter wave radiation can damage amixer in receivers. To avoid such situations, we have started developing a notch filter with a narrow stop band and the wide pass bandin the frequency range from 74 to 80 GHz.4,5 The notch filter is tuned
for a mode, which is excited at the cavity resonators. In our case, thepurpose was the reduction of main stray radiation as well as spurious
radiations from the gyrotron associated with the on–off modulation
timing.

Rev. Sci. Instrum. 92, 034711 (2021); doi: 10.1063/5.0041243
Published under license by AIP Publishing

The electron cyclotron resonance heating (ECH) system of theLHD has implemented two gyrotrons for 77 GHz and three for
154 GHz. A gyrotron with a new frequency of 56 GHz has recently
been prepared for plasma experiments at a low magnetic field of∼1 T. The ECE imaging (ECEI) system has arrayed antennas.6 Eachantenna is connected to a mixer by Q-band waveguides. The strayradiation of the 56 GHz gyrotron above the Q-band range reaches
the mixers and is converted down to the intermediate frequency(IF) from 1 GHz to 10 GHz. The mixture of ECE signals and thestray radiation saturates and/or overloads the low noise amplifiers.
In some cases, the intense stray radiation can damage the down conversion mixer. Therefore, the reduction of more than 20 dB in thestop band and the low insertion loss are required to measure theclean ECE signals. The reduction level is estimated from the noise
signals of the ECEI at the injection timing of the 56 GHz ECH.
We have designed a notch filter composed of an E-band waveguide with mode-controlled cavities. Those designs are reported in92, 034711-1

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Refs. 4 and 5. A notch filter with a different concept is also reported
in Ref. 7. However, a notch filter for the unwanted stray radiation,
which exists above the standard band of a waveguide, requires a newconcept. After the design of the basic structure, the level of detailis determined by the numerical simulation software Ansys HFSS.
The fabricated filter is characterized by using a vector network analyzer (VNA). Finally, the ECEI system implements the filters, whichsolved the previous problem.
II. DESIGN MODEL, SIMULATION, AND FABRICATIONOF NOTCH FILTER
A notch filter is designed to attenuate intense gyrotron strayradiation in the RF line of the ECE receiver with the rectangular
waveguide of WR-22 for the Q-band (33–50 GHz). The center frequency of the stop band corresponds to the nominal gyrotron frequency of 56.05 GHz within the bandwidth of 1 GHz. We used the E
plane tee junction as the mode-coupling technique for the individual cylindrical cavities that control the attenuation and the stop band
width of a notch filter.2–4 The details to determine the basic parameters are described in Ref. 4. When an electromagnetic wave with aspecific frequency passes through the main waveguide, it is reduced
at the exit port by a specific mode of TEmnξ excited in cylindrical cavities. Here, m, n, and ξ indicate integers, respectively. From possiblemodes in a cylindrical cavity, the lowest TE111 is selected to prevent
unnecessary notches from appearing in the Q band. For controllinga cavity mode, the cavity length l is determined byl=ξ

λg.
2(2)

where λ0 is the RF wavelength, λc is the cutoff wavelength of thewaveguide, and we assume that λ0 < λc . From the above equations′and the cut-off wavelength in the cylindrical cavity λcmn = πD/vmn,
the estimated cavity length becomes l = 3.089 mm at ξ = 1 for the′center frequency of the notch filter at 56.05 GHz. Here, vmn
is theroot of the derivative of the Bessel function Jm′ (x) = 0. The cavity
spacing Δl is calculated by the following equation:Δl =λg(1 + 2N).
4

Published under license by AIP Publishing

PurposeFrequency ranges
Notch frequency
Notch bandwidth
Cavity length

Old filters

New filters

Rejection andprotection of mainand spurious
gyrotron radiations for
CTS diagnostics
E bandE band∼0.2 GHz
Adjustable by screw

Rejection andfrom the protection
main gyrotron
radiation for ECE andECEI diagnostics
Q bandQ bandV bandV band
∼1.0 GHz
Fixed by fabrication

TABLE II. Design parameters for the Q-band 56 GHz notch filter (unit is in mm).

Center frequency ofthe notch filter
Cavity diameter
Cavity length
Cavity gap length distance
between two resonators
Slit width
Slit length
Slit depth

f center

56.0 GHzD

6.30
3.089 by Eq. (1) and 2.950
by the simulation

lΔl

4.554
1.0
2.2
0.2

slits and the number of cavities are optimized by the Ansys HFSS toobtain the required specification. The parameters are summarized inTable II. The cavity length was adjusted to set the center frequencyof 56.0 GHz in the simulation. Finally, the simulated cavity length
resulted in 2.95 mm for 56.0 GHz. The value was used for the filter fabrication. The distribution of the E-field in the notch filter for
56 GHz is shown in Fig. 1. The six cavities are arranged on both sidesof the main waveguide. The mode is excited at some cavities during
the wave propagation in the main waveguide.

(3)

Here, an integer N is set to one for compactness.
As noted above, the cavity modes are used to reject undesired
frequencies. The procedure of the design of the cavity mode, thenumerical simulation, and the fabrication is the same as the previous procedure. The main difference in the previous filters andthe new notch filters is summarized in Table I. The difference iscaused by the application of plasma diagnostic systems. A Q-band
waveguide notch filter with a notch frequency outside the Q-band
demonstrates efficient protection from a stray radiation of intense
millimeter waves, although the solution for the designed notch issensitive to the coupling structure of slits and other structures.
For the coupling between the main waveguide and the cavity,
the filter characteristics depend on rectangular slits. The coupling

Rev. Sci. Instrum. 92, 034711 (2021); doi: 10.1063/5.0041243

TABLE I. Features of old filters4,5 and new filters (this paper).

(1)

The guide wavelength λg is given byλ0,λg = √1 − (λ0 /λc )2scitation.org/journal/rsi

FIG. 1. The wave electric field in the Q-band notch filter.

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TABLE III. Specification of the Q-band notch filter.

f at centerf at 3 dB
S21 at min
S21 at 56.05 GHzInsertion loss

FIG. 2. The fabricated notch filter for the Q-band.

The designed notch filter is fabricated using the optimized
parameters from the simulation. The outer dimensions of the filterare 20 × 20 × 50 mm3 (see Fig. 2). The filter is made of free cutting
copper. The entrance face and the exit face need adapters to connect
the WR-22 waveguide. In the previous paper,4,5 we used a plunger
to vary a cavity length, which requires the precise adjustment for thenotch frequency. We were able to eliminate the tuning procedure ofeach individual cavity by letting the cavity lengths fixed. We madethe first version with the cavity length designed by the Ansys HFSS.
After the notch frequency of the first version was measured, the cavities in length were drilled additively to fill the frequency gap between
the measurement and the design. The Q-band 77 GHz notch filter
requires the procedure, and the Q-band 56 GHz does not require it.
The characteristic of S parameters S21 is measured by a VNA in
Fig. 3. The specification is summarized in Table III. The designed
TE111 mode appeared at the center frequency of 55.85 GHz. The
bandwidth at −3 dB is 1.1 GHz. The attenuation at 56.05 GHz
achieved more than 50 dB. The measured insertion loss was lessthan 1.5 dB around 41 GHz. In the simulation, the insertion losses
are 0.8 dB for two pairs of cavities and 1.6 dB for three pairs ofFIG. 3. Characteristics of the Q-band notch filter50 (see Table III). The gyrotron
frequency of 56.05 GHz, which should be rejected, is located inside the notch. The
solid and dashed curves are the measurement and simulation results, respectively.

Rev. Sci. Instrum. 92, 034711 (2021); doi: 10.1063/5.0041243
Published under license by AIP Publishing

55.85 GHz
55.3, 56.4 GHz>60 dB
50 dB<2 dB (in Q-band)

cavities. The surface condition of the free cutting copper walls would
not cause the linear increase. Therefore, the insertion loss around40 GHz would be caused by the slit or resonator structure. The
numerical result reproduces two unknown notches. The closest cavity modes correspond to TE112 at 65.7 GHz and TE311 at 79.2 GHz.
The reason for the large frequency gaps is not clearly understood.
Because the redundant extra notches exist out of the Q-band, thepresent performance satisfies our specification.

III. ECEI SYSTEM WITH Q-BAND NOTCH FILTERSThe ECH in the LHD uses the 77 GHz gyrotrons. For the same
reason, Q-band 77 GHz notch filters are also prepared for the ECEIsystem. The design concept is the same as that described above. The
outer dimensions of the cuboid shape are 20 × 20 × 42 mm3 . The
maximum attenuation is more than 55 dB at 76.95 GHz, and thebandwidth at −3 dB is 1.6 GHz.
The diagram of the ECEI system is shown in Fig. 4(a). A pairof Q-band 56 GHz notch filter and Q-band 77 GHz notch filter
is mounted in series on the RF part of the ECEI diagnostic. The
apparent system mounted on the LHD is shown in Fig. 4(b). The
time evolution of the ECE signals before and after the insertion ofthe notch filters is plotted in Figs. 5(a) and 5(b), respectively. InFig. 5(a), the ECE signal does not behave correctly at t = 3.1–3.6 s
due to the intense stray radiation inside the vacuum vessel caused

FIG. 4. ECEI system in the LHD. (a) The schematic diagram of the ECEI system.
The heterodyne technique is used to down convert the RF signal. The notch filters
are inserted in the RF line. (b) A pair of notch filters with notch frequencies of 56
and 77 GHz is mounted on the antenna array.

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IV. SUMMARYThe Q-band waveguide notch filters with 55 and 77 GHz
notches have been developed for versatile millimeter wavediagnostics (ECE, ECEI, CTS, etc.). For signal conditioning andcomponent protection of plasma diagnostics, the notch filters
require a low insertion loss in Q-band (33–50 GHz) and a narrowstop band width at V-band (50–75 GHz) or E-band (60–90 GHz).
The fabricated notch filters work well for the intended purpose ofmonitoring the ECE signals. The fabricated filters are implemented
in the arrayed ECEI system and the ECE system of the LHD. The
ECE diagnostics are available for research on heating efficiency,
MHD instability, and energetic particle physics in plasmas.
ACKNOWLEDGMENTS
This work was supported by the JSPS KAKENHI (Grant No.
19KK0073).

DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
REFERENCES
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Korsholm, M. Salewski, and LHD Experiment Group, Nucl. Fusion 54, 023006(2014).
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M. Nishiura, S. Kubo, K. Tanaka, S. Kobayashi, K. Okada, K. J. Okada,T. Nishimura, T. Mushiake, T. Shimozuma, T. Mutoh, T. Saito, Y. Tatematsu, andY. Yamaguchi, J. Instrum. 10, C12014 (2015).
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2

FIG. 5. ECE signals in the ECEI diagnostic (a) before and (b) after the insertion ofthe Q-band waveguide notch filter with the 56 GHz notch.

by 56 GHz ECH. On the other hand, in Fig. 5(b), we found that
the Q-band notch filter suppresses the unexpected signal accordingly at t = 3.1–3.6 s. The diagnostics are operated successfully inthis experimental campaign.

Rev. Sci. Instrum. 92, 034711 (2021); doi: 10.1063/5.0041243
Published under license by AIP Publishing

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