22P140-P
Development of a high power sub-THz pulse gyrotron
サブテラヘルツ帯の高出力パルスジャイロトロンの開発
Naoki Yamada1, Teruo Saito1, Yoshinori Tatematsu1, Shinji Ikeuchi1, Yuusuke Yamaguchi1,Ryosuke Ikeda1, Isamu Ogawa1, Toshitaka Idehara1, Shinya Ogasawara2, Shin Kubo2,
Takashi Shimozuma2, Kenji Tanaka2, Masaki Nishiura2 and V.N.Manuilov3
山田尚輝1, 斉藤輝雄1，立松芳典1，池内真司1，山口裕資1，池田亮介1，小川勇1，出原敏孝1，小笠原慎弥2，久保伸2，下妻隆2，田中謙治2，西浦正樹2，V.N.Manuilov3
FIR FU, University of Fukui1, NIFS2, Nizhny Novgorod State University3
3-9-1 Bunkyo, Fukui, 910-8507, Japan1 , 322-6 Oroshi-cho, Toki, 509-5292, Japan2,Nizhny Novgorod, 603600, Rossia3
福井大学遠赤外領域開発研究センター 1，核融合科学研究所2，ニジノブゴロド州立大学3〒910-8507 福井市文京3-9-11, 〒509-5292 土岐市下石町322-62, 603600 ニジノブゴロド州3
We are developing a high power sub terahertz gyrotron for application as a power source of Collective Thomson
scattering (CTS) diagnostics of fusion plasmas, especially of LHD high density plasmas. The frequency is in the 400
GHz band and the target output power is 100 kW. An output power of 56kW was obtained at a frequency 387.8GHz
using the second harmonic oscillation. In order to achieve much higher output power, the oscillation mode has beenchanged to that with a higher coupling coefficient. As a result, an output power of 83 kW has been so far obtained. Inaddition, we have been designed a 300 GHz gyrotron with the fundamental oscillation. A design calculation predicts
more than 200 kW. It will be equipped with an internal mode converter.

2. Means for obtaining a higher output power
Two means for obtaining a high output power are
as follows.
The first mean is to increase the cathode voltage
Vk to suppress deterioration of the quality of theelectron beam. The maximum value of Vk at which
we can operate has increased from 60 kV to 65 kV
by improving the switching element of the power
supply.
The second means is to change the oscillation
mode from the TE1,8 mode to the TE17,2 mode thecoupling coefficient CRB of which is two times ashigh as that of the TE1,8 mode. In fact, a mode
competition calculation predicts a higher oscillation
power by using the TE17,2 mode. However,modification of the electron gun is necessary
corresponding to the mode change. The beam radius
Rb at which CRB has the maximum value is 2.09 mm
in the cavity of the present design. It is slightly
larger than that for the TE1,8 mode. Then, the radiusof the emission belt Rk was increased from 14 mm
to 15 mm to meet the following relation between Rband Rk,
Bk
Rb  Rk,
Bc
where Bk and Bc are the cathode magnetic field andthe cavity magnetic field, respectively.
3. Experimental result (cathode voltage 65 kV)The output power increased by increasing Vk to65kV. The maximum power reached 62 kW at thebeam current of 11 A. The oscillation efficiency is8.7 %. Figure1 plots the maximum power obtainedat each beam current. A decrease in power still
exists.

Power (kW)

1. Background
FIR-FU has proved high power 400 GHz band
gyrotron oscillation at second harmonic resonance.
So far the maximum power has attained to 56 kW at
387.8 GHz, with the cathode voltage of 60 kV, andthe beam current of 9 A. The oscillation mode wasidentified as the TE1,8 mode from frequency
measurement.
However, the output power is lower than 100 kW
as the objective power. Then, we have adopted tow
means to realize much higher power.

7060504030
20100

Vk 60 kV Power●
Vk 65 kV Power ■246810 12 14Ib (A)Fig.1. Maximum power for each beam current isplotted.

The oscillation frequency was measured by using
a heterodyne receiver system. The measured
frequency of 387.8 GHz is almost equal to 388.2
GHz, the resonant frequency of the TE1,8 mode, and
we identified the oscillation mode as the TE1,8 mode.
The small frequency difference about 0.4 GHz isdue to manufacturing error of about 2.5μm.
4. Experimental result (mode change)
Figure 2 shows the beam current dependence ofthe output power obtained after modification of theelectron gun. The maximum output power of 83 kW
as a new power record was obtained at 10 A. The
output power slightly decreases at the beam current
larger than 10 A. This is likely due to deteriorationof the beam quality at large Ib.

100HV66kV_115/350_MaxPowerのIb依存性

Power(kW)Vk = 60 kV60402025002000246 8 10 12 14Ib(A)Fig.2 Maximum power is plotted for each beamcurrent.

The oscillation frequency was measured with a
Fabry-Perot interferometer. Figure 3 indicates a
waveform of the interferometer. It proves nooscillation of fundamental modes. The measured
frequency is 388.8 GHz. The design frequency of
the TE17,2 mode is 389.3 GHz. The measured
frequency is again slightly less than the designvalue. Then, the oscillation mode was identified as
the TE17,2 mode
FP_815_FP_7.6100T_HV66kV_Igun140A_Ib10A_pyro_exp

Vk = 60 kV
Ib = 10 A
B = 7.6100 T
f = 388.8 GHz

4Intensity(V)

5. Design of a 300 GHz fundamental harmonic
gyrotron
We designed 300 GHz fundamental harmonic
gyrotron, in parallel with experiments using thesecond harmonic oscillation. In order to obtainfundamental oscillation of 300 GHz, a new liquid
He free 12 T superconducting magnet will beinstalled. The TE14,2 mode was selected. The design
parameters are frequency 295 GHz, the cavity
radius is 3.40 mm, the cavity length is 9 mm, andthe beam radius is 2.42 mm.
Figure 3 shows the oscillation power calculatedwith the mode competition code. The calculation
predicts that shows that the TE14,2 mode oscillatesalone and the output power is larger than 200 kW.
Oscillation power stays larger than 200 kW for a
possible change in Rb.

20024681012Displacement between Meshes(mm)Fig.3 An example of the waveform obtained with a
Fabry-Perot interferometer.

Power(kW)Power(kW)80

The oscillation efficiency at 83 kW with Vk of 60
kV and the beam current of 10 A is 13.8 %.
Oscillation of only second harmonic mode was
not obtained with Vk of 65 kV. At this voltage,
simultaneous oscillation of the fundamental TE4,3
mode was observed in a wide operation range.

218kW

TE3,6cntr
TE14,2coTE1,7cntr
TE8,4cntr

150

Vk = 65 kV
Ib = 10 A

100

α = 1.2
Rb = 2.42 mm
L = 9 mm

500
11.0

11.2

11.4 11.6
B(T)

11.8

12.0

Fig.4 Power calculation with the mode competitioncode.

6. SummaryHigh power second harmonic gyrotron
oscillation has been obtained. The recorded power
was 62 kW at 387.8 GHz with the TE1,8 mode. For
realization of further higher power with the TE17,2mode, the electron gun was modified. The
recorded power with this mode has so far been 83
kW at 388.8 GHz. In this experiment, we haveachieved a new record in the sub terahertz region.
As another way getting higher power, afundamental harmonic gyrotron with a 12 T
magnet has been designed. An output power of
200 kW class can be expected.
References[1] T. Saito et al., : 15th Int. Sym. on LASER-AIDED
PLASMA DIAGNOSTICS, Korea, 2011, T11.