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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Q-band high-performance notch filters at 56 and 77 GHz notches forversatile fusion plasma diagnostics
M. Nishiura,1,2 T. Shimizu,1 S. Kobayashi,1 T. Tokuzawa,1 K. Ichinose,3 S. Kubo,1,31National 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, 2778561, Japan
3
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan
2(Presented XXXXX; received XXXXX; accepted XXXXX; published online XXXXX)(Dates appearing here are provided by the Editorial Office)A 6-pole Q-band waveguide filter with a notch frequency above the Q-band has been developed for plasma
diagnostics. The previous paper [M. Nishiura, et al., Journal of Instrumentation, Vol. 10, C12014 (2015)]
reports that the notch frequency exists within the standard band. In this study, the newly required notch filter
extends the function, which prevents a thorny wave from being mixed into an instrument beyond the standard
bandwidth of 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 the attenuation more than50 dB at 56.05 GHz and the bandwidth of 1.1 GHz at -3 dB. The latter filter has the attenuation more than 55
dB at 76.95 GHz and the bandwidth of 1.6 GHz at -3 dB. The electron cyclotron emission imaging (ECEI) andthe 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.
I. INTRODUCTION
In fusion plasma experiments, intense millimeter waves
for plasma production and heating are injected into avacuum vessel from a mega-watt class-gyrotron. The powerof the electromagnetic waves is not completely absorbed byplasmas. Then, the stray radiation of the intense beams
interferes with signals of measurement systems, forexample, electron cyclotron emission (ECE) [1],reflectometer and collective Thomson scattering (CTS)
diagnostics [2, 3]. In most serious cases, intense millimeterwave radiation can damage a mixer in receivers. To avoid
such situations, we have started developing a notch filter
with a narrow stop band and the wide pass band in thefrequency 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, the purpose was the reduction of main strayradiation as well as spurious radiations from the gyrotron
associated with the on – off modulation timing.
The electron cyclotron resonance heating (ECH) systemof the LHD has implemented two gyrotrons for 77 GHz, andthree for 154 GHz. The new frequency of 56 GHz has
recently been prepared for plasma experiments at low
magnetic field of ~ 1 T. The ECE imaging (ECEI) system
has arrayed antennas [6]. Each antenna is connected to amixer by Q-band waveguides. The stray radiation of 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
Author to whom correspondence should be addressed:a)nishiura@nifs.ac.jp

amplifiers. In some cases, the intense stray radiation candamage the down conversion mixer. Therefore, thereduction of more than 20 dB in the stop band and the low
insertion loss are required to measure the clean 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 in the references [4, 5]. A notch filter with adifferent concept is also reported in the reference [7].
However, a notch filter for the unwanted stray radiation,
which exists above the standard band of a waveguide,
requires a new concept. After the design of the basicstructure, the detail is determined by a numerical simulation
software Ansys HFSS. The fabricated filter is characterized
by a vector network analyzer (VNA). Finally, the ECEI
system implements the filters, which solved the previousproblem.
II.
DESIGN
MODEL,SIMULATION,ANDFABRICATION OF NOTCH FILTER
A notch filter is designed to attenuate intense gyrotron
stray radiation in the RF line of the ECE receiver with therectangular waveguide of WR-22 for Q-band (33-50 GHz).
The center frequency of the stop band corresponds to thenominal gyrotron frequency of 56.05 GHz within thebandwidth of 1 GHz. We used E plane tee junction as themode-coupling technique for the individual cylindricalcavities that control the attenuation and the stopband widthof a notch filter [2-4]. The detail to determine the basic
parameters is described in the reference [4]. When anelectromagnetic wave with a specific frequency passes
through the main waveguide, it is reduced at the exit port bya specific mode of TEmn excited in cylindrical cavities.
Here, m, n, and  indicate integers, respectively. Frompossible modes in a cylindrical cavity, the lowest TE 111 is
selected to prevent unnecessary notches from appearing in
the Q band. For controlling a cavity mode, the cavity lengthl is determined by
𝜆𝑔(1)
𝑙=𝜉
2
The guide wavelength g is given by
𝜆0(2)
𝜆𝑔 =
√1 − (𝜆0 /𝜆𝑐 )2where 0 is the RF wavelength, c is the cutoff wavelengthof the waveguide and we assume that 0  c. From theabove equations and the cut-off wavelength in the′cylindrical cavity cmn = D/ 𝑣mn, the estimated cavity
length becomes l = 3.089 mm at  = 1 for the center′frequency of the notch filter at 56.05 GHz. Here, 𝑣mn
is the′ (𝑥)root of the derivative of the Bessel function 𝐽𝑚= 0. Thecavity spacing l is calculated by the following equation,𝜆𝑔(3)Δ𝑙 = (1 + 2𝑁)
4
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 thecavity mode, the numerical simulation, and the fabricationis the same as the previous procedure. The main differencein the previous filters and the new notch filters issummarized in TABLE 1. The difference is caused by theapplication of plasma diagnostic systems. A Q-band
waveguide notch filter with a notch frequency outside theQ-band demonstrates efficient protection from a strayradiation of intense millimeter waves, although the solution
for the designed notch is sensitive to the coupling structureof slits and other structures.
TABLE 1. Features of old filters4, 5 and new filters (thispaper)
PurposeFrequency
rangesNotchfrequency
Notch
bandwidth
Cavity length

Old filters
Rejection andprotection of mainand spurious gyrotron
radiations for CTS
diagnostics
E bandNew filters
Rejection andprotection from themain gyrotron
radiation for ECE
and ECEI
diagnostics
Q bandQ bandE bandV band~ 0.2 GHz~ 1.0 GHz

Adjustable by screw

Fixed byfabrication

parameters are summarized in TABLE 2. The cavity length
was adjusted to set the center frequency of 56.0 GHz in thesimulation. 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 filterfor 56 GHz is shown in Fig. 1. The six cavities are arranged
on both sides of the main waveguide. The mode is excited
at some cavities during the wave propagation in the mainwaveguide.
TABLE 2. 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
betweentwo resonators
Slit width
Slit length
Slit depth

fcenter

56.0 GHzDl

6.30
3.089 by Eq. (1)2.950 by the simulation
4.554

l

1.0
2.2
0.2

The designed notch filter is fabricated using theoptimized parameters from the simulation. The outerdimensions of the filter are 20 mm × 20 mm × 50 mm (seeFig. 2). The filter is made of free cutting copper. The
entrance face and the exit face need adaptors to connect theWR-22 waveguide. In the previous paper [4, 5], we used aplunger to vary a cavity length, which requires the precise
adjustment for the notch frequency. We were able toeliminate the tuning procedure of each individual cavity byletting the cavity lengths be fixed. We made the first version
with the cavity length designed by the Ansys HFSS. Afterthe notch frequency of the first version was measured, thecavities 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.

V bandFor the coupling between the main waveguide and thecavity, the filter characteristics depend on rectangular slits.
The coupling slits and the number of cavities are optimizedby the Ansys HFSS to obtain the required specification. TheFIG. 1. The wave electric field in Q-band notch filter.

The characteristic of S parameters S21 is measured by aVNA in Fig. 3. The designed TE111 mode appeared at thecenter 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 less than 1.5 dB
around 41 GHz. In the simulation, the insertion losses are
0.8 dB for 2 pairs of cavities and 1.6 dB for 3 pairs ofcavities. The surface condition of the free cutting copper
walls would not cause the linear increase. Therefore, theinsertion loss around 40 GHz would be caused by the slit orresonator structure. The numerical result reproduces twounknown notches. The closest cavity modes correspond toTE112 at 65.7 GHz and TE311 at 79.2 GHz. The reason forthe large frequency gaps is not clearly understood. Because
the redundant extra notches exist out of the Q-band, thepresent performance satisfies our specification.

attenuation is more than 55 dB at 76.95 GHz, and thebandwidth at -3 dB is 1.6 GHz.

(a)

(b)

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

TABLE 3. Specification of the Q-band notch filterf @center

55.85 GHzf @3dB

55.3, 56.4 GHz

S21@minS21@56.05GHzInsertion loss

> 60 dB
50 dB< 2 dB (in Q-band)

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

FIG. 3. Characteristic of the Q-band notch filter (see TABLE 3).
The gyrotron frequency of 56.05 GHz, which should be rejected, islocated inside the notch. The solid and broken curves aremeasurement and simulation results, respectively.

(b)

ECH56GHz

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 ECEI system. The design concept is thesame as that described above. The outer dimensions of thecuboid shape are 20 mm × 20 mm × 42 mm. The maximum

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

The diagram of the ECEI system is shown in Fig. 4 (a).
A pair of Q-band 56 GHz notch filter and Q-band 77 GHz
notch filter is mounted in a series on the RF part of the ECEI
diagnostic. The apparent system mounted on the LHD isshown in Fig. 4 (b). The time evolution of the ECE signals
before and after the insertion of the notch filters are plottedin Figs. 5 (a) and (b), respectively. In Fig. 5 (a), the ECE
signal does not behave correctly at t = 3.1-3.6 s due to theintense stray radiation inside the vacuum vessel caused by56 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 in this experimental campaign.
IV. SUMMARYThe Q-band waveguide notch filters with 55 and 77
GHz notches have been developed for versatile millimeterwave-diagnostics (ECE, ECEI, CTS, etc.). For signal
conditioning and component protection of plasma
diagnostics, the notch filters require a low insertion loss in
Q-band (33-50 GHz) and a narrow stopband width at Vband (50-75 GHz) or E-band (60-90 GHz). The fabricatednotch filters work well for the intended purpose ofmonitoring the ECE signals. The fabricated filters areimplemented in the arrayed ECEI system and the ECE
system of the LHD. The ECE diagnostics are available forresearch on heating efficiency, MHD instability, andenergetic particle physics in plasmas.
ACKNOWLEDGMENTS
This work is supported by JSPS KAKENHI grant number
19KK0073.
Data Availability
The data that support the findings of this study are available fromthe corresponding author upon reasonable request.
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