01pC03
LHDにおける協同トムソン散乱への適用を目指した
300 GHz帯高出力ジャイロトロンの開発
Development of 300 GHz Band Gyrotron for Practical Use
in Collective Thomson Scattering Diagnostics in LHD
斉藤輝雄１、山口裕資１、立松芳典１、福成雅史１、廣部 匠１、田中俊輔１、新林竜志１、
下妻 隆２、久保 伸２、田中謙治２、西浦正樹３、O. Dumbrajs4
T. Saito1, Y. Yamaguchi1, Y. Tatematsu1, M. Fukunari1, T. Hirobe1, S. Tanaka1, R. Shinbayashi1,
T. Shimozuma2, S. Kubo2, K. Tanaka2, M. Nishiura3 and O. Dumbrajs4
１

福井大遠赤センター, ２核融合研, ３東大新領域
４
The Institute of Solid State Physics, Univ. of Latvia
12
FIR UF, NIFS,3 Graduate School of Frontier Sciences, Univ. of Tokyo,
４
The Institute of Solid State Physics, Univ. of Latvia
Introduction
Gyrotrons with frequencies around 100 GHz forelectron heating are currently used in collective
Thomson scattering (CTS) diagnostics [1, 2].
However, the probe waves suffer from severe
refraction and/or absorption in plasmas. Moreover,strong electron cyclotron emission becomes a large
noise source. Sub-THz waves are almost free fromthose problems. Therefore, a high power sub-THz
gyrotron is required in CTS diagnostics of highdensity plasmas in LHD. We set the power target ofthe probe wave higher than 100 kW to obtain asufficiently large signal to noise ratio. Firstly
developed second harmonic gyrotron demonstrated
single mode oscillation approaching 100 kW at 389
GHz [3]. However, a competing fundamental
harmonic mode prevented to further enhance thepower [4]. We have then developed a fundamental
harmonic prototype gyrotron and verified thedesign concept for stable high power oscillation [5].
Based on this successful result, a gyrotron forpractical use in CTS diagnostics in LHD has been
designed with the same concept as that of theprototype gyrotron and fabricated. High power
oscillation test of the practical gyrotron is reported.
Design of and Fabrication of Gyrotron
The sub-THz gyrotron for CTS is operated in apulse mode. The design frequency is 303.3 GHz.
The design concept is the same as that of theprototype gyrotron [5]. A moderately over-moded
cavity is used to realize simultaneously single mode
oscillation and a low Ohmic loss on the cavity
surface. The oscillation mode is the TE22,2 mode
belonging to the same whispering gallery modefamily as the TE14,2 mode of the prototype gyrotron.
The mode number has increased to extend the pulse

width and the duty ratio
up to 10%.
Design calculations
were carried out about
the dependence of theoscillation power andthe efficiency on thebeam current. For the
beam voltage Vk of 65
kV and the beam
current IB of 15 A,
oscillation power higher
than 300 kW wasexpectedwiththevelocity pitch factor of 1.2. A newly
designed electron gun
was optimized for theTE22,2 mode by using Fig. 1. Picture of thepracticalthe design principle fabricatedgyrotron. A Gaussian
reported in Ref. [6].
beamisradiatedElectron beams of high through the vacuum
qualitywiththe window at the left side.
expected  values andsufficiently small velocity spreads can be generated
up to IB = 20 A [7].
Figure 1 shows the picture of the fabricatedgyrotron. This gyrotron is mounted on a liquid
He-free 12 T SC magnet. The diameter of the room
temperature bore is 100 mm. An internal mode
converter composed of a helical-cut Vlasov typelauncher and four mirrors is installed into a rather
narrow room. Performance test of the mode
converter is also an important task of the presentstudy. The vacuum window is made of a c-axis cut
crystal sapphire disk. Its thickness is optimized for303.3 GHz.

400151Wave Form #0671, VR = 200/356

Vk = 65 kV

100.75200

0.5Efficiency

1000.25Signal (a.u.)Power

Pyroelectric detector signal(Integration mode)

5

Efficiency

Power (kW)300

0-5-10Beam voltage-15
Beam current

0051015
Beam Current IB (A)020Fig. 2. Output power and efficiency as functions ofthe beam current. The beam voltage is 65 kV. Otherparameters such as the modulation anode voltage areoptimized for each beam current.

Results of Oscillation Test
Figure 2 plots the results of the high power
oscillation test in addition to the already reported
data [7]. In particular, the highest power > 320 kW
was newly attained. The efficiency is higher than
30% except the low beam current. The obtained
powers are consistent with the design calculation
with  from 1.1 to 1.3. Further higher power
approaching 400 kW is expected with increasing IB
after conditioning.
The radiation pattern was measured with an
infrared camera as the profile of temperature
increase on a 1 mm-thick polyvinyl chloride plate
put in front of the vacuum window A Gaussian likeradiation pattern similar to the calculated pattern
was obtained. Therefore, good performance of themode converter was confirmed.
Measurement of oscillation frequency with a
Fabry-Perot interferometer showed single mode
oscillation around 303 GHz. More accurate
frequency measured with a heterodyne receiversystem was 303.3 GHz for Vk = 62 kV and themagnetic field at the cavity B = 11.59 T. It is equal
to the design frequency within an accuracy of theorder of 10 MHz. The frequency variation due tovariation of the operation conditions was less than±100 MHz. This frequency stability is very
important for the CTS experiment.
The frequency spectrum was very narrow andstable during the pulse width. Moreover, nopossible competing mode was observed throughout
the pulse width including the phases of turn on andthe turn off of the oscillation pulse. Oscillation
other than the TE22,2 mode was not observed withina frequency band from 290 GHz to 312 GHz.
Figure 3 represents an oscillation pulse with thepulse width of 50 s. The output power is about 200-200102030Time (us)405060Fig. 3. Oscilloscope traces of a 50-s oscillation. The
beam voltage and the beam current were set at 60 kV
and 15 A, respectively. The magnetic field was 11.59 T.

kW. The oscillation signal was measured with apyroelectric detector in time integration mode. The
linear increase in this signal indicates stationaryoscillation during the whole pulse width.
SummaryA 300 GHz band pulse gyrotron was fabricatedfor actual use as a power source of CTS diagnostics
in LHD. The results of the oscillation test has
confirmed single mode oscillation of the TE22,2
mode as the design mode. The measured frequency
is almost equal to the design value 303.3 GHz,
which corresponds to a minimum of ECE for thestandard operation of LHD. The radiated beam has
a Gaussian like pattern. More than 320 kW has beenobtained.
The oscillation test results have proved that thisgyrotron can be practically used in CTS diagnostics
in LHD.
Acknowledgement
This work has been performed with the supportand under the auspices of the JSPS KAKENHI
Grant Number 25247094, the NIFS Collaboration
Research program (NIFS13KOAR014) and thespecial fund of University of Fukui.
References[1] M. Nishiura et al., Nucl. Fusion 54, 023006, 2014.
[2] J. Rasmussen et al., Nucl. Fusion 56, 112014,2016.
[3] T. Saito et al., Phys. Plasmas 19, 063106, 2012.
[4] T. Saito et al., Phys. Rev. Lett. 109, 155001, 2012.
[5] Y. Yamaguchi et al., Nucl. Fusion, 55, 013002,
2015.
[6] Y. Yamaguchi et al., Phys. Plasmas 19, 113113,
2012.
[7] Y. Yamaguchi et al., J. Instrumentation 10,
C10002, 2015.