23P174-B
Interdigital backward-wave oscillator for 100 GHz and 1 W class
100 GHz帯1 W級出力を目指したインタデジタル型後進波発振管
Masashi Kato, Yukihiro Soga, Ryousuke Tsuchiya, Yasutada Kato, Ritoku Ando, Keiichi Kamada,
Mitsuhiro Yoshida1 and Satoshi Ohsawa1
加藤真志, 曽我之泰，土屋良輔，加藤泰禎, 安藤利得, 鎌田啓一, 吉田光宏1, 大澤哲1
Graduate School of Natural Science and Technology, Kanazawa University
Kakumam-machi, Kanazawa-shi, 920-1192, Japan
金沢大学自然科学研究科 〒920-1192 金沢市角間町1

High Energy Accelerator Research Organization, KEK
1-1 Oho, Tsukuba, Ibaraki 305-0801 Japan高エネルギー加速器研究機構 〒305-0801 茨城県つくば市大穂1-1
A backward-wave oscillator (BWO) with an interdigital slow-wave structure for 100GHz band has beendeveloped. Simulation has shown that the BWO produces radiation at output power of 700 mW at afrequency of 102 GHz. We designed and constructed the thermionic electron gun that produces anelectron-beam that has sufficient energy and emission current for the radiation. A distortion of the beam
trajectory was suppressed by a strong homogeneous magnetic field and a focus electrode placed around theemitter.

1. Introduction
Terahertz (THz) radiation, which is located inthe spectral region of 0.1-10 THz, is expected tohave very wide applications. There are a numberof candidates for terahertz sources, such as solidstate oscillators, quantum cascade lasers,
optically pumped solid state devices and novel
free electron devices like Klystrons, Backward
Wave Oscillators (BWO) and Gyrotrons [1].
However, it is difficult to achieve a stable andhigh-power oscillation in the THz frequency
range.
In this paper we focused on BWO which iscompact and a broadband oscillator. In general
BWO covers the frequency range below 1 GHz toover 100 GHz, but the maximum output power is
too small in the THz frequency range. For
example, output power of continuous wave at 100
GHz is tens of milli-watt at most. So we developed
the interdigital BWO with power output of about 1
Watt at 100 GHz.

follows; L 150 um; h 440 um. We made theinterdigital metallic pattern on a Teflon substrate.
A characteristic feature of the interdigital SWS isthat the ω-β diagram is shifted by π rad ascompared with that of a single comb SWS as shownin Fig 2. In fact, in the range 0 < βL < π, the wave isbackward wave. The circuits are referred to as
fundamental backward wave circuits. The opposites
are referred to as fundamental forward wave
circuits. Interaction in the fundamental backward
wave structure occurs with electrons at transit angle
near π/2. It should be noted that the transit angle ismuch smaller than for the fundamental forward
wave structure (~3π/2). As a result, BWOs with
fundamental backward wave structure are operated
with faster and, therefore, higher voltage electron
beams than fundamental forward wave BWOs, sothat a high power radiation is expected.
200
Fundamental mode
n=1 Spatial harmonic
Beam mode

2. Slow wave structure
The interdigital slow wave structure (SWS) isshown in Fig. 1. We designed the interdigital SWS
for 100 GHz band
Lradiation stimulated byan electron beam withenergy of 10 keV,hcurrent of 100 mA, andbeam radius of 1 mm.
Dimensions of the: Metal
interdigital SWS are as
Fig.1. Interdigital

Frequency [GHz]150

100500π2πPhase [rad]Fig. 2. Dispersion of interdigital slow-wave circuit
and a beam mode at 10kV

3. Simulation
CST Studio software includes several solver
modules applicable to vacuum electronics design.
We used PARTICLE STUDIO in CST Studiowhich includes the capability to simultaneously
model the time-dependent RF fields, magnetic
focusing field, and a particle in cell (PIC) electronbeam. The software is used to analyze microwave
tubes [2,3].
In the simulation the electron beam has
cylindrical shape of radius 1 mm, beam voltage of
10kV, beam current of 50–100 mA, and themagnetic field is applied by a stack of daughnut
type magnets. The distance between SWS and thebottom of the electron beam is 0.1 mm.
Figure 3 shows that the time dependence ofradiation and Fourier analysis of the signal. Theradiation has a strong peak at a frequency of 102
GHz, which is in good agreement with the design
goal of 100 GHz. The output power is estimated at
700 mW in the beam condition of 10 kV and 100mA. The output power as a function of a beam
current is shown in Fig. 4. A start current for theradiation is 60 mA (7.6 A/cm2).
0.12The electron emitter which consists of the Ir-coateddispenser cathode is capable of generating currentof over 100 mA [4]. A focus electrode placed at
adjacent to the emitter and tipped toward the anodestraightens the equipotential profile between thecathode and the anode. The trajectory simulation of
the electron-beam emitted form the gun containing
the focus electrode shows a rectilinear electron flow
as shown in Fig.5. The special variation of the beam
radius is less than 6%.
In a preliminary experiment of the electron gun,the beam current of 60 mA is observed atacceleration energy 10 keV in the test chamber.

Fig. 5 Axial magnetic field of cylindrical magnet
and trajectory of electron beam at 10 kV

FFT of signal

Intensity [a.u.]

0.1

5. Summary

We designed the interdigital SWS for 100 GHz
band radiation stimulated by an electron beam withenergy of 10 keV. In the simulation of the BWO,
we observed the radiation which is in good
agreement with the design.
The electron gun for the BWO has been designed
for the generation of the cylindrical beam withbeam radius 1 mm, beam voltage 10 kV. A low
variation of beam radius less than 6% is obtained bythe trajectory simulation.

0.08

0.06

0.04

0.02

0020

406080100
Frequency [GHz]120140160

Fig.3. FFT analysis of the signal
output
time

0.8
0.6
0.4
0.2
05060

7080
90
Current [mA]

100

100
9080
7060
50
403020100110

Time [nsec]

Output power [a.u.]

1Fig.4 Current vs output power and start time

4. Design of electron gun
We developed an electron gun for the 100GHz
band interdigital BWO. The electron gun has been
designed for the generation of the cylindrical beamwith beam radius 1 mm, beam voltage 10 kV, beam
current 100 mA and 3 mm cathode-anode distance.

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Electronics Conference, 2008. IVEC 2008. IEEEInternational, pp.390-391 (2008)[3] Carol L. Kory and James A. Dayton, Jr : Vacuum
Electronics Conference, 2008. IVEC 2008. IEEEInternational, pp.392-393 (2008)[4] S. Kimura, T. Yakabe, S. Matsumoto, D. Miyazaki, T.
Yoshii, M Fujiwara, and S. Koshigoe : IEEE
TRANSACTION ON ELECTRON DEVICES,Vol.37, No.12 pp.2564-2567 (1990)