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Abstract—Microfabricated vacuum-tube THz sources are of
great interest for numerous applications such as communications,
radar, sensors, imaging, etc. Recently, miniaturized sheet-beam
traveling-wave tubes for THz and sub-THz operation have
attracted a considerable interest. In this paper, we present the
results of modeling and development of medium power (10-100
W) G-band traveling-wave tube amplifiers with a sheet electron
beam.
I. INTRODUCTION
ICROFABRICATED traveling-wave tubes (TWTs) have
attracted a considerable interest as wideband power
amplifiers at THz frequencies. Using high-aspect-ration sheet
electron beam in such devices allows reduce of electron
current density and thus facilitates cathode development and
beam focusing. Several designs of sheet-beam TWTs with
different slow-wave structures (SWS) have been suggested
[1]-[6]. In this paper, we present the results of development
and modeling of a G-band (0.22 THz) sheet-beam TWT with
medium-power (10-100 W). Beam current density is about
100 A/cm2, which is attainable for existing thermionic
cathodes. We are developing sheet-beam TWTs with SWSs of
two different kinds, namely, a metallic grating SWS in a
rectangular waveguide [4], [5] and a planar meander-line
SWS on a dielectric substrate [6].
II. TWT WITH DOUBLE GRATING SWS
Among the metallic grating structures, the half-period
staggered double grating is known as a promising wideband
SWS for THz TWT amplifier [1], [2]. For modeling of
electromagnetic parameters of the SWS, we use commercially
available 3D software such as ANSYS HFSS [7] and
COMSOL [8], as well as fast and accurate code developed at
Saratov State University (SSU) [4]. The SWS has a passband
as wide as 70 GHz. The coupling impedance is rather low and
does not exceed 1 Ω in the most part of the passband, except
the vicinity of the cutoff/stopband frequencies [[5]]. The
coupling impedance can be increased by decreasing the height
of the beam tunnel, as well as the vane thickness, but the
capability of such a technique is limited by technological
reasons.
Small-signal and large-signal [9] beam-wave interaction in
the G-band TWT with 100 mA, 20 kV sheet electron beam
and half-period-staggered grating SWS with optimized
dimensions (Table I) is simulated. In Fig. 1, an example of
simulation is presented. About 60 W saturation power is
attained at 195 GHz in the 80-period (40 mm) SWS circuit. In
Fig. 2, gain vs. frequency curves for different input powers are
presented.
Fig. 2. Nonlinear gain vs. frequency at different input powers.
TABLE I
OPTIMIZED DIMENSIONS OF THE DOUBLE-GRATING SWS
Period of the structure (um) 500
Vane thickness (um) 100
Beam tunnel height (um) 200
Slot depths (um) 300
Width of the structure (um) 850
Relative shift between the gratings (um) 250
The copper SWS circuit is designed and fabricated using
the electrical discharge machinery technique. A wedge-shape
impregnated tungsten cathode producing a 700 100
um
intensive sheet electron beam with up to 140 A/cm2 current
density is developed and tested [5]. Beam focusing by a
uniform magnetic field was simulated.
Tatiana A. Karetnikova1,2, Andrei I. Benedik1,3, Andrey G. Rozhnev1, Nikita M. Ryskin1,2,
Gennadiy V. Torgashov2, Nikolay I. Sinitsyn2, and Pavel D. Shalaev3
1Department of Nonlinear Physics, Saratov State University, 83 Astrakhanskaya st., Saratov, 410012 Russia
2Saratov Branch, Institute of Radio Engineering and Electronics, RAS, 38 Zelenaya st., Saratov, 410019 Russia
3“Almaz” R&D Co., 1 Panfilova st., Saratov, 410033 Russia
Development and Modeling of G-band Vacuum Tube Power Amplifiers
with Sheet Electron Beam
M
Fig. 1. Interaction power vs. longitudinal coordinate z at different frequencies.
III. TWT WITH MEANDER-LINE SWS ON A DIELECTRIC
SUBSTRATE
The microstrip meander SWS on a dielectric substrate has
been recently proposed for using THz band amplifiers and
oscillators [3], [6]. In Fig. 3, schematic of the SWS is presented.
It is assumed that the SWS is placed in a rectangular metallic
waveguide.
The G-band (180-230 GHz) SWS was designed and
simulated. For the simulations, 3D ANSYS HFSS software
package [7] was used. Dimensions of the SWS are listed in
Table II. The SWS exhibits normal dispersion with slow-wave
factor ~7 11
ph
cv in the operating frequency range.
Several samples of the meander SWS circuit with
input/output couplers were fabricated. An example is presented
in Fig. 4. For the fabrication, copper films of 1 um thickness
were sputtered over sapphire or silica substrates. After that,
meander was patterned by photolithography. The SWS has a
wide passband (50 GHz) with central frequency around 210
GHz.
Fig. 3. Schematic of the meander SWS.
TABLE II
DIMENSIONS OF THE G-BAND MEANDER-LINE SWS
Waveguide cross section, a×b (mm) 2×1
Substrate thickness H (um) 500
Period of the structure L (um) 200
Meander period, d (um) 50
Metallized strip width, s (um) 15
Metallized strip thickness, t (um) 1
Due to the limited conditions for cold-test measurements in
G-band, lower-frequency V-band (50-70 GHz) prototypes on
silica and polycor (Al2O3) substrates were fabricated. The
results of simulations and measurements reveal rather high
coupling impedance (1-10 Ohm).
IV. ACKNOWLEDGMENT
This work is supported by the Russian Foundation for Basic
Research grant No. 14-02-00976-a and 16-08-00450-a.
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Fig. 4. Photo of the copper meander SWS on the silica substrate.
