24E06
Proof of High Power Oscillation of a Sub Terahertz Gyrotron
for Application to Collective Thomson Scattering Experiment on LHD
LHDにおける協同トムソン散乱への適用を目指した
サブテラヘルツ帯ジャイロトロンによる高出力発振の実証
Teruo Saito1, Yoshinori Tatematsu1, Naoki Yamada1, Ryosuke Ikeda1, Yuusuke Yamaguchi1,Isamu Ogawa1, Toshitaka Idehara1, V.N.Manuilov2, Shinya Ogasawara3, Takashi Shimozuma3,
Shin Kubo3, Masaki Nishiura3, Kenji Tanaka3, Kazuo Kawahata3
斉藤輝雄1，立松芳典1，山田尚輝1，池田亮介1，山口裕資1，小川勇1，出原敏孝1，V.N.Manuilov2
小笠原慎弥3，下妻隆3，久保伸3，西浦正樹3，田中謙治3，川端一男3
1

FIR FU, University of Fukui, 3-9-1 Bunkyo, Fukui, 910-8507, Japan
Nizhny Novgorod State University, Nizhny Novgorod, 603600, Russia
3
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, 509-5292, Japan1
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
2
ニジノブゴロド州立大学 603600 ロシア連邦ニジノブゴロド州
3
核融合科学研究所 〒509-5292 土岐市下石町322-6
2High power sub terahertz pulse gyrotrons are under development in FIR FU for application to CTS
measurement on a fusion plasma device, especially on high density plasmas produced in LHD. Veryrecently, a new record of pulse sub terahertz radiation exceeding 80 kW at about 390 GHz has beenobtained from a gyrotron with second harmonic oscillation. This paper reports detailed study of oscillation
characteristics of the gyrotron along with design consideration to realize high power single mode
oscillation at second harmonic. Discussion will also be given about the mode competition.

1. Introduction
Application of high frequency gyrotrons has been
intensified in various fields. Sub terahertz gyrotrons
are very attractive for plasma diagnostics of highdensity plasmas such as those in LHD. Althoughover one terahertz oscillation has been realized withboth second harmonic (SH) and fundamental
harmonic (FH) oscillations [1,2]. SH oscillation is
attractive
torelax therequirement
tosuperconducting magnets. On the other hand, SH
oscillation has a serious problem of mode
competition with FH modes, especially when weaim for high power oscillation. It is a very
challenging task to realize a 100 kW-level sub
terahertz gyrotron as a power source of a CTS
apparatus [3,4]. On the other hand, this study willbring a lot of problems to be studied.
We have demonstrated more than 50 kW SH
oscillations at 350 GHz and about 40 kW at 390
GHz with a demountable gyrotron in the first stage[5]. This paper reports the study for higher power inthe second stage.

numbers correspond to the SH and the FH
resonances, respectively.
Single mode oscillation of the SH TE3,7, TE1,8
and TE17,2 modes can be expected because they are
far from the neighboring FH TE4,3 mode.
The cavity radius Rc of 2.99 mm was determinedfrom a comprehensive consideration for radiation
frequency and the coupling between the RF-electricfield. For this radius, the resonance frequencies arearound 390 GHz.
3. Proof of High Power SH Mode Oscillation
We fabricated a new gyrotron of sealed-off typeto examine oscillation characteristics of the TE3,7,
TE1,8 and TE17,2 modes [6]. It operates in pulse
mode. The pulse width is several microseconds andthe repetition rate is less than 10 Hz. We began with
the TE1,8 mode and obtained a maximum power of
62 kW at the cathode voltage Vk of 65 kV and theχ'm,n (second harmonic)
23.8

24

TE13,3

24.4

24.6

TE17,2TE3,7 TE1,8 TE8,5
TE22,1TE5,6

2. Design Consideration
For realization of single mode SH oscillation,suppression of mode competition is very important.
Figure 1 shows the distribution of Bessel prime
zeros near candidate modes. Upper and lower

24.2

1224.825

TE14,312.2
12.4
χ'm,n (fundamental)

25.2

25.4

TE6,6 TE23,1

TE11,4TE4,312.6

Fig. 1. Distributions of SH and FH modes

FP_ 636_ FP_ 7.6117T_ HV66kVIgun140A_ Ib10A_ pyro_ exp

FP_ 816_ FP_ 7.6000T_ HV66kV_ Igun140A_ Ib10A_ pyro_ shr

Intensity(V)Intensity( V)

3210681210

Displacement between Meshes( mm)Fig. 2. An Example of FPI signal of single mode
oscillation

beam current Ib of 11 A with no simultaneous
oscillation with the FH TE4,3 mode. However, theelectron beam radius Rb corresponding to theemission belt radius Rk of the electron gun lies in
the range of low coupling coefficients.
Then, Rk of the electron gun was changed torealize the maximum coupling with the TE17,2 mode.
The new power record has attained to 83 kW at Vkof 60 kV and the beam current Ib of 10 A. Figure 2shows a signal transmitted through a Fabry-Perotinterferometer (FPI). The pulse train corresponds tothe TE17,2 mode with the frequency of 388,8 GHz.
Single mode oscillation is clearly shown.
Figure 3 plots new power records of SH gyrotron
in the sub terahertz region. Power records obtainedwith the previous gyrotron of demountable type arealso plotted.
100FIR(Pulse)Power (kW)80FIR(CW)CPI
IAP

60MITTOSHIBA
Sydney Univ.

40200200

300
400500Frequency (GHz)600

Fig. 3. Record powers against frequency

4. Observation of Mode Competition
We have proven high power single mode
oscillation of SH modes. However, a little changesof the magnetic field strength Bc at the cavity or thecurrent of the auxiliary coils installed near theelectron gun result in mode competition. Figure 4
represents an example of such a case. A small pulse
train in between each peak of the main TE17,2 modeindicates simultaneous oscillation of theneighboring SH TE8,5 mode. The combination ofsimultaneously oscillating modes changes withvariation of the operation conditions.

0.5

0.0681012Displacement between Meshes(mm)Fig. 4. An Example of FPI signal with mode
competition

5. Design of a Fundamental Harmonic Gyrotron
We have succeeded to demonstrate single mode
oscillation of SH modes at power levels reaching
100 kW. However, it is rather sensitive to theoperation condition.
Then, a 300 GHz FH gyrotron has been designed.
Mode selection is also important for FH mode.
Compatibility with the mode converter is anotherfactor because the new gyrotron will be equippedwith a built-in mode converter. We have selected
the TE14,2 mode. An electron gun has been designed
for this gyrotron [7]. A mode competition
calculation predicts more than 200 kW oscillation.
6. Summary
SH oscillation of the TE3,7, TE1,8 and TE17,2
modes has been confirmed with a newly
manufactured sealed-off gyrotron. The maximum
power obtained with the TE1,8 mode was 62 kW at388 GHz. A further higher power of 83 kW has
been attained at 389 GHz with the TE17,2 mode. We
have succeeded in high power single mode
oscillation of SH modes.
A FH gyrotron with a 12 T magnet has been
designed. It is now under fabrication.
Acknowledgments
The present study was supported by programs of
the Grants-in Aid for Scientific Research of Japan
Society for the Promotion of Science. It was also
supported by the NIFS Collaboration Research
programs.
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015101.
[3] S. Kubo et al.: Rev. Sci. Instrum. 81 (2010) 10D535.
[4] T. Saito et al.: J. Phys. Conf. Series 227 (2010)
012013.
[5] T. Notake et al.: Phys. Rev. Lett. 103 (2009) 225002.
[6] N. Yamada et al.: Plasma 2011, 22P140-P.
[7] Y. Yamaguch et al.: Plasma 2011, 22P141-P.