22P169-B
Mode Identification of High Power 77 GHz Gyrotron in LHD
LHDにおける高出力77 GHzジャイロトロンの発振モード同定S. Ogasawara1), S. Kubo1,2), M. Nishiura2), Y. Tatematsu3), T. Saito3), K. Tanaka2).
T.Shimozuma2), Y. Yoshimura2), H. Igami2), H. Takahashi2), S. Ito2), Y. Takita2),
S. Kobayashi2), Y. Mizuno2), K. Okada2), R. Minami4), T. Kariya4) and T. Imai4)
小笠原慎弥1)、久保伸1,2)、西浦正樹2)、立松芳典3)、斉藤輝雄3)、田中謙治2)、下妻隆2)、吉村泰夫2)、伊神弘恵2)、高橋裕己2)、伊藤哲2)、夛喜田泰幸2)、
小林策治2)、水野嘉識2)、岡田宏太2)、南龍太郎4)、假家強4)、今井剛4)
1)Department of Energy and Technology, Nagoya University, Nagoya 464-8463, Japan2)National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi, 509-5292, Japan
3)Research Center for Development of FIR , University of Fukui, Fukui 910-8507, Japan
4)Plasma Research Center, University of Tsukuba, Tsukuba 305-8577, Japan
1)名古屋大学大学院工学研究科エネルギー理工学専攻 〒464-8463 名古屋市千種区不老町
2)核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
3)福井大学遠赤外領域開発研究センター 〒910-8507 福井市文京3-9-1
4)筑波大学プラズマ研究センター 〒305-8577 つくば市天王台1-1-1
Collective Thomson scattering (CTS) system has been developed at the Large Helical Device (LHD) using
electron cyclotron resonance heating (ECRH) system with a high power 77 GHz gyrotron. The gyrotron
output is modulated to subtract the back ground electron cyclotron emission (ECE) from received signals.
Spurious mode power is excited during the turning on/off phase of the gyrotron power. Such spurious mode
oscillation can make the CTS analysis complex or even damage the sensitive CTS receiver. Therefore, it isimportant to suppress or minimize spurious mode oscillation. As the first step to suppress spurious modes,
we measured the oscillation frequency of 77 GHz gyrotron output and observed the spurious radiation
modes. The frequency of one of the spurious modes was 74.7 GHz. The spurious mode is supposed to be
TE17,6 from cavity mode and starting current calculation.

1. Introduction
Collective Thomson scattering (CTS) diagnostic
in the Large Helical Device (LHD) has beendeveloped as a method for measuring bulk and fastion velocity distribution functions in high density
and high temperature plasmas. Recent developmentof high power radiation sources, like mega-wattclass gyrotrons and relevant transmission/antennasystem, enable to detect the scatteredelectromagnetic wave by small electron density
fluctuations associated with the thermal ion motion inplasmas [1, 2]. The CTS diagnostic has been
applied utilizing the existing electron cyclotron
resonance heating (ECRH) system with high power
mega-watt gyrotrons at the frequency of 77 GHz [3,4].
2. Spurious radiation of 77 GHz gyrotron
The power of the CTS probe beam is modulated
to subtract the background electron cyclotron
emission (ECE). Fig. 1 shows (a) an example of theRF signal of 77 GHz gyrotron and (b) one of theCTS signals. CTS signals were obtained by a highsensitive heterodyne radiometer. The details of theCTS receiver are described in the reference [5].
During the turning on/off phase of the modulation,
the spurious mode radiations are superimposed on
the measured signal which is the sum of CTS

Fig. 1. Time evolution of (a) the RF signal of77 GHz gyrotron, (b) CTS signals obtained ina high sensitivity heterodyne radiometersystem [5]. Arrows indicate the spikes due to aspurious mode oscillation in Fig. 1 (b).

component and background ECE. For the precise
estimation of the CTS spectrum, such spikes due tothe spurious mode and background ECE have to becorrectly subtracted from the received signal.
Additionally, the spurious radiation signals evensmall in power can make saturation or overload of
one or several IF amplifiers. Therefore, it isimportant for precise CTS measurement to develop
a gyrotron operation method to suppress or
minimize such spurious mode oscillation without
degrading the main mode power.
3. Frequency measurement of the 77 GHz gyrotron
and identification of the spurious radiation mode
As the first step for suppressing the excitationof the spurious mode, we measured the temporal
evolutions of the frequency of the oscillations in theoutput of 77 GHz gyrotron by using a heterodyne
receiver with 40 channel filter banks and a fastsampling oscilloscope. The receiver has a narrow
band notch filter in the range of 76.95±0.2 GHz,
to suppress the stray radiation at the main frequency
for CTS diagnostic. The anode voltage relative tothe cathode is optimized to give a stable andefficient gyrotron oscillation under given beam
current and magnetic field condition. The time
evolution of the anode voltage at the starting up
phase and half modulation period is shown in Fig. 2(a). Optimum anode voltage, 45 kV, is settled at 3
ms after the starting up. It is modulated between 45
kV and 35 kV after that with 50 Hz. The radiation
frequency was measured at the anode voltage
phases of (A) rising, (B) stable and (C) falling. The
spurious mode frequency was observed at about
74.7 GHz when the anode voltage transits from 24
kV to 45 kV and 45 kV to 35 kV. Fig. 2 (b) showsthe spectrogram during the anode voltage risingphase ( indicated by a hatched region (A) in Fig. 2(a)). Relatively strong oscillation at 74.7 GHz
appeared during the anode voltage range between
27 kV to 40 kV. The main output TE18,6 mode at
77.0 GHz was observed when the anode voltage
settled at 45 kV. The spurious mode frequency isclose to the calculated one of the TE17,6 mode for agiven cavity. Furthermore, The starting current
calculation code [6] shows that the TE17,6 mode isrelatively easy to start oscillation as compared withthe other modes under the measured condition.
Therefore, the spurious mode is considered to beTE17,6. This mode is considered to oscillate during
the transition phase of the anode voltage and theelectron beam current of the gyrotron. The mainand powerful TE18,6 mode starts to oscillate after theanode voltage settled at 45 kV by mode competition
with TE17,6.

Fig. 2. Time evolution of (a) the anode voltageand (b) spectrogram of the output power
frequency of 77 GHz gyrotron during theanode voltage range between 27 kV to 40 kV.

4. Conclusion
We measured the frequency evolution of 77
GHz gyrotron output power for a modulated pulse.
As the result, the spurious mode frequency wasabout 74.7 GHz at the transition of the anodevoltage. We consider that the observed spurious
radiation is the TE17,6 from the calculation of theresonance condition of the cavity and the starting
current of the gyrotron. We will search the way tosuppress or reduce the spurious radiation of TE17,6
mode by optimizing the magnetic field strength
and/or other gyrotron operation parametersexperimentally with the help of mode competition
calculation code [6].
References[1] H. Bindslev, S.K. Nielsen et al., Phys. Rev. Lett. 97,205005 (2006).
[2] S. B Korsholm, M. Stejner et al., Phys. Rev. Lett.
106, 165004 (2011).
[3] M. Nishiura, S. Kubo et al., Journal of Physics:
Conference Series 227, 012014(2010)[4] S. Kubo, M. Nishiura et al., Rev. Sci. Instrum. 81,10D535 (2010)[5] M. Nishiura, K. Tanaka et al, Rev. Sci. Instrum. 79,10E731 (2008)[6] Y. Tatematsu, T. Saito, J. Plasma. Fusion. Res.
Vol.84, No.12 (2008), pp855-860