22P142-P
Development of fundamental oscillation 400 GHz sealed-off gyrotron
with continuous frequency tunablility
基本波発振による400GHz帯周波数連続可変ジャイロトロンの開発
Ryosuke Ikeda1, Toshitaka Idehara1, Isamu Ogawa1, Yoshinori Tatamatsu1,
Yukinori Yamada1, Yuusuke Yamaguchi1, Teruo Saito1, Yoh Matsuki2,
Keisuke Ueda2, Toshimichi Fujiwara2, T.H. Chang3 and N.C. Chen3
池田亮介1，出原敏孝1，小川勇1，立松芳典1, 山田幸毅1, 山口裕資1，斉藤輝雄1,松木陽2，植田啓介2，藤原敏道2，T.H. Chang3，N.C. Chen3
1

Research Center for Development of Far-Infrared Region, University of Fukui, 910-8507, Japan
2
Institute for Protein Research, Osaka University, 565-0871, Japan3
Department of Physics, National Tsing Hua University, Hsinchu, 300, Taiwan
1
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
大阪大学蛋白質研究所 〒565-0871 大阪府吹田市山田丘1-1
3
国立精華大学 台湾新竹市300

Gyrotron is required to enhance sensitivity of 600 MHz NMR spectroscopy for analysis of complex proteinsstructures. A fundamental oscillation 400 GHz range gyrotrons having boardband frequency tunability are being
designed and developed in the University of Fukui. The sealed-off gyrotron has been newly developed to realize higher
output power and more stable oscillation from results of a demountable tube type gyrotron. In preliminary experiments,
the output power reached about 50 W, which exceeds that of the demountable tube type gyrotron.

1. Introduction
Gyrotrons are powerful sources in millimeter to
sub-millimeter wavelength region. High frequency
gyrotrons the frequency of which exceeds about
200 GHz are applied for plasma diagnostics,
electromagnetic wave sintering, material sinteringand biological sciences and medical treatment. In
particular, high power irradiation of sub-millimeterwave is required to analyze the structures ofproteins by nuclear magnetic resonance (NMR)
spectroscopy [1]. The sensitivity of NMR
spectroscopy for more complex proteins is too low
and the analysis takes an immense amount of time.
A dynamic nuclear polarization (DNP) method is
expected to enhance the sensitivity [2]. The
enhancement of DNP depends on the oscillation
frequency of irradiation. The optimal frequencies
are fESR± fNMR, where fESR and fNMR are frequenciesof electron spin resonance (ESR) and NMR. Thevalue of fESR is 394.6 GHz for 600 MHz DNP-NMRspectroscopy. Continuous tunability of frequency isrequired to obtain the optimum enhancement.
Moreover, stability of radiation power is necessary.
Objectives of this study are the development of thefrequency tunable gyrotron for 600 MHz
DNP-NMR spectroscopy and the operation test of
the oscillation characteristics. In the university ofFukui, we have already developed a frequency
tunable gyrotron which oscillated fundamental
mode using a 15 T superconducting magnet [1].

This gyrotron was demountable type. The
frequency tuning was ranged from 394.65 GHz to395.27 GHz and the output power was ranged from10 to 30 W [3].
2. Experimental setup
A new fundamental oscillation gyrotron has beendeveloped to improve output power, the stabilityand CW operation. This gyrotron is of sealed-off
type as shown in Fig.1 The vacuum pressure in a
tube is kept under 10-6 Pa and the variation of thepressure is suppressed low. The liquid helium free 15
T superconducting magnet is used to excite afundamental oscillation at 394.6 GHz. The room
temperature bore of this magnet is 52 mm. The
Sapphire window

Collector

15 T superconducting
magnetCavity resonatorIon pump
Guncoil magnet
Magnetron injection gun

Fig.1 Sealed-off gyrotron (Gyrotron FU CW VIB) anda 15 T superconducting magnet.

12 kV and an anode voltage Va = 4.7 kV, a beam
current Ib ~ 250 mA and the gun coil current Ig =-150 A. The peaks of radiation power correspond tominimums of the starting current for which the
expected cavity modes are oscillated. Therefore,
TE0,6,1 mode was oscillated from Bc ~ 14.35 T. It is
expected that the oscillation of TE0,6,q modes
continued until Bc ~ 14.6 T without stopping
radiation. The output power was about 30 W at q =1 and the output powers were about 5 – 20 W in
higher axial modes. The frequencies tuning is
expected in this region and the oscillation
frequencies will be measured very soon.
The dependencies of the output powers on the
beam current and the voltage between the cathodeand the anode were measured at Bc = 14.37 T. The
output power linearly increased with increasing
beam current, as shown in Fig.3. The maximum
output power reached about 50 W. And the output
powers increased gradually by increasing a pitch
factor as a function of the voltage between thecathode and the anode as shown in Fig.4. The pitch
factors were calculated by electron beam analysis
code, EGUN [5]. Output power reached about 40 W
in the region of pitch factor α ~ 2.0. The mostsuitable operating condition is being searchedaiming at improvement of further output power.

TE7,4

TE2,650

Output power P [W]
Oscillation signal Vpyro [a.u.]

TE0,6TE10,3400.030
20
100
14.0

14.2

14.4

14.6

14.8Starting current Ist [A]TE4,5-0.5
15.0

Magnetic field strength Bc [T]Fig.2 Oscillation signal, output power and calculated
stating current as a function of magnetic field strength.

60

Bc = 14.37 T50

Vk = 12.0 kV
Va = 4.7 kV40Ig = -150 A

30
201000100

200300400500Beam current Ib [mA]

Fig.3 Output power as a function of beam current.
5040304Bc = 14.37 TIg = -150 A3Vk = 12.0 kV
Ib = 250 mA2201

1005.5

Pitch factor power and the calculated stating current as afunction of magnetic field strength. The oscillation
signal and output power were measured by apyroelectric detector and a water load respectively.
This operation condition is a cathode voltage Vk =0.560

Output power P [W]

3. Preliminary experiment results
Figure 2 shows the oscillation signal, the output

References[1] V.S. Bajaj et al.: J Magn Resonance 160, (2003) 85.
[2] A.W. Overhauser: Physical Review 92 (1953) 411.
[3] T. Idehara et al.: Journal of Infrared, Millimeter andTerahertz Waves 31 (2010) 775.
[4] T.H. Chang et al.: Journal of Applied Physics 105(2009) 063304-1.
[5] W. B. Herrmannsfeldt: Stanford Linear Accelerator
Center Report SLAC-331-UC-28 (1998).

Output power P [W]operating mode was designed as TE0,6,q around theESR frequency of 394.6 GHz. The index, q, stands
for the axial mode number. The shape of the cavity
resonator is a conventional cylindrical cavity withlinear tapers on both end. The length and diameterof the cavity are 25.0 mm and 4.750 mm,respectively. Calculated Ohmic, diffractive andtotal quality factors are QΩ = 22375, Qd = 77734
and Qt = 17374. We can expect excitation of axial
modes with q > 1 owing to the long cavity. In
addition, a backward wave interaction with theelectron beam will continuously cover the gap
between neighboring axial modes without stopping
radiation [4]. A magnetron injection gun is a triode
type and the emitter diameter is 9.0 mm. Cathodevoltage is up to 30 kV and the maximum beam
current is 500 mA. The electron beam is injected in
the 2nd electric field peak region (beam radius :0.65 mm) of TE0,6,q mode at the cavity entrance byadjusting a gun coil magnet.

6.0

6.5

7.0

7.5

0
8.0

Voltage between cathode and anode Vak [kV]Fig.4 Output power and calculated pitch factor as afunction of the voltage between cathode and anode.