22P139-P
Development of a high frequency gyrotron with an internal mode convertor
モード変換器内蔵高周波数ジャイロトロンの開発
Yoshinori Tatematsu, Yuusuke Yamaguchi, Toshitaka Idehara, Tatsuru Kawase, Junki Aiba,Hideaki Kato, Takanori Ozeki, Ryosuke Ikeda, Isamu Ogawa, Tomohiro Kanemaki, Teruo Saito
立松芳典，山口裕資，出原敏孝，川瀬樹，相場旬基，加藤秀明
尾関隆則，池田亮介，小川勇，印牧知廣，斉藤輝雄
Research Center for Development of Far-Infrared Region, University of Fukui,Fukui 910-8507, Japan
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
Gyrotron FU CW GII has been developed in Research Center for Development of Far-Infrarred Region,
University of Fukui (FIR FU). It is the second gyrotron with an internal mode convertor, developed in FIR
FU. It operates at 393 GHz at the second harmonic resonance. Measured output power was 74 W. An
almost linearly polarized Gaussian-like beam was successfully delivered from the vacuum window. In
addition, fundamental resonance oscillations with more than one hundred W output power were observedin the frequency range of 200 GHz. This gyrotron will be a power source radiating useful Gaussian-likebeams with two different frequency bands around 200 and 400 GHz.

1. Introduction
In FIR FU, a lot of high-frequency gyrotrons
have been developed as sources in the millimeterand sub-millimeter wave range. However, they
were a linear type with axial output coupling. In
this type of gyrotron, the electromagnetic wave
which oscillates in the cavity resonator transmits
to the top window through the waveguide,maintaining the complex spatial structure of thefield (so-called, circular waveguide mode) andradiates through the window with the samestructure. Such a beam is unsuitable as a powersource for various applications. Then we havestarted development of another type of gyrotrons
with an internal quasi-optical mode convertor.
Our first gyrotron of such a kind is Gyrotron FU
CW GI [1], which operates at the fundamental
cyclotron resonance with a frequency of 203 GHz.
The measured maximum power was 0.5 kW. The
results have been reported at annual meetings ofthe plasma-fusion society of Japan and thephysical society of Japan. The second gyrotron ofthis type has been developed. It is called Gyrotron
FU CW GII. The designed frequency is 394.6
GHz, corresponding to the ESR frequency for 600
MHz DNP-NMR.
2. Gyrotron FU CW II
The photograph of Gyrotron FU CW GII isshown in Fig. 1. It is installed on an 8T
superconducting magnet. It is a sealed-off typegyrotron. Selection of the oscillation mode anddesign of the cavity dimension were carefully
carried out. As the oscillation mode, TE8,3 mode has

been selected. Although it has a competing mode
TE3,5, it can oscillate as long as the electron incident
radius is properly set. Cavity radius and length were
determined to 2.15 and 17 mm, respectively. An
existing electron gun with 4.5 mm cathode radius isreused. A quasi-optical mode convertor consists ofa conventional helical cut waveguide and fourmirrors.
A radiation pattern of the power launched fromthe gyrotron window was measured as shown in Fig.
2. A circular pattern was observed. Polarization
measurement of this beam was carried out. Wavesignal intensity through a polarizer, consisting ofmetal wires, was observed with a pyro-electricdetector. Figure 3 shows the signal intensity against

Fig. 1 Gyrotron FU CW GIIthe rotation angle of the polarizer, , where  is theangle between the wire direction and the verticalline. The dependence of the observed signal on agrees with cos2. This guarantees the polarizationis almost linear. The direction of the electric field ishorizontal in Fig. 2.
Operation test was carried out for some range ofthe magnetic field strength B at the cavity. Signal
intensity measured with a power-meter which isusually used for infrared lasers is shown in Fig. 4.
A peak around B = 7.2 T represents the signal ofTE8,3 mode. There appear several signals from othercavity modes. The measured frequencies
corresponding to the four peaks in Fig. 4 are 189.1,393.4, 398.9, and 205.6 GHz, respectively. Basedon the measured frequencies, oscillation modes
have been identified as TE1,3, TE8,3, TE1,6 and TE4,2,
respectively, as labeled in Fig. 4. TE8,3 and TE1,6modes operate at second harmonic and TE1,3 andTE4,2 at fundamental. In spite that the internalconvertor has been designed for the co-rotating
TE8,3 mode, circular radiation patterns are observed
for all of these modes. In particular, the wave beamof TE4,2 is radiated through the almost center
position of the window. We showed that the beam
direction is related with the quantity m/'mn of thecavity mode, where m is the azimuthal number and'mn is n-th zero of J'm(z). The values of m/'mn for
TE8,3 and TE4,2 are 0.45 and 0.43, respectively.
Thus the present result agrees with the previous
observation [2].
Radiation power from the window was evaluatedwith a water load. The maximum observed power
was 74, 140 and 110 W for TE8,3, TE4,2 and TE1,3modes, respectively. The operation cathode voltage
and the beam current are around -18 kV and 0.5 A,respectively. It is reasonably to consider that theactual powers radiated from the window are higher
than these measured values because we neglected
the heat capacity of the water vessel and assumingthat all the power was absorbed by the water load.
Since the cavity length has been optimized for thesecond harmonic oscillation, the efficiency ofpower conversion is lower for fundamental
oscillations. However, the power of thefundamental modes exceeds 100 W. Thus, thisgyrotron can radiate useful Gaussian-like beams
with two different frequencies around 200 and 400
GHz. Much higher powers are expected if we use
electron guns specially optimized for the gyrotron.
3. Summary
We have developed Gyrotron FU CW GII, oursecond gyrotron with the internal mode convertor.
It radiates a circular shaped beam with 100 W

output power. This gyrotron can be used as aradiation source with two different frequency bands
around 200 and 400 GHz.
References[1] Y. Tatematsu, Y. Yamaguchi, T. Idehara et al: Proc.
36th IRMMW-THz, Th2A.5 (2011) .
[2] Y. Tatematsu, T. Saito, T. Ozeki, S, Hashimoto, Proc.
35th IRMMW-THz, We-P.14 (2010).

Fig.2. Radiation pattern from FU CW GII measured
with an IR camera.

Fig.3. Polarization measurement of the radiation
beam. Signal intensity was observed with apyro-electric detector through a polarizer, whosewire direction rotates by Fig.4. Measured signal intensity at the window.
Attached labels indicate oscillation modes in thecavity.