Research on a Ka-band large-orbit gyro-TWT with periodic
dielectric-loaded structure
Yang Jintao1, Wang Efeng1, Lei
Chaojun2, Zhao Qixiang3,
Fengjinjun1, Lei Zihan1 and Zeng
Xu1
1 Beijing Vacuum Electronics Research Institute,
Beijing,100020, China
2 China People’s Police University,
Langfang,065000, China
3 Guilin University Of Electronic Technology,
Guilin, 541004, China
Email: 1214945800@qq.com
This paper presents a design of a gyro-TWT operating in the large-orbit
electron beam mode, with the aim of reducing the working magnetic field
while ensuring operational efficiency. The periodic dielectric-loaded
structure is adopted as the high-frequency interaction circuit, which
has not been reported in the application of large-orbit gyro-TWT in
literature, while this structure has been successfully applied in the
development of small-orbit devices. This paper conducts a comprehensive
study and analysis of this structure and achieves stable operation in
the Ka-band after the optimization of the tube. This tube works at
second harmonic of electron frequency in the mode of large-orbit
electron beam. The required magnetic field is only 5100 Gauss, which can
be generated using electromagnetic coils instead of superconducting
magnets. The operational parameters include voltage of 75 kV, current of
9A, and velocity spread of 3.5%. Under these conditions, the device
presents stable operation, with -3 dB bandwidth of 4.3 GHz, and maximum
output power of 165 kW. This result meets the expected requirements for
magnetic field and operational efficiency, thus validating the
feasibility of practical fabrication of large-orbit gyro-TWT with
periodic dielectric-loaded structure.
Introduction: The gyrotron traveling wave tube (gyro-TWT) is
capable of generating high-power, wide-band coherent radiation in the
millimeter and submillimeter wavebands. It has significant applications
in millimeter-wave radar, communication, electronic warfare, deep space
exploration, and other fields [1-2].
The high-intensity magnetic field in gyro-TWT is used to guide the
electron beam. In the Ka-band gyro-TWT, the working magnetic field is
around 1.2 Tesla. Due to issues related to heat dissipation and power
consumption, the maximum magnetic field achievable with electromagnetic
coils is currently around 7000 Gauss, which falls short of the 1.2 Tesla
magnetic field that only superconducting magnets can offer. However,
superconducting magnets bring limitations in terms of flexibility,
maneuverability, and quick startup, significantly impacting the
practical applications of gyro-TWTs.
To overcome the mentioned drawbacks, it is necessary to reduce the
working magnetic field, which can be achieved by using higher harmonic
number. However, higher harmonic number leads to a sharp decrease in
output power, efficiency, and bandwidth. The large-orbit electron beam
working mode offer a promising solution to this problem. Compared to the
small-orbit mode, the large-orbit mode has the following characteristic:
(1) High interaction efficiency: Even when reducing the working magnetic
field, the interaction efficiency remains nearly on par with that of the
fundamental harmonic. It has been reported that the interaction
efficiency can still reach 22% when a 94 GHz large-orbit gyrotron
operates in the 6th harmonic mode [3]. (2) good mode selectivity: In
the large-orbit mode, the electron beam only interacts with modes where
s equals m, where s represents the harmonic number, and m represents the
azimuthal mode index.
The Institute of Applied Physics in Russia (IAP) has successfully
developed a prototype of a Ka-band large-orbit gyro-TWT. This tube
operates at second harmonic with working magnetic field of 0.6 Tesla,
which can be achieved using conventional electromagnetic coils. Under
the conditions of 70 kV voltage and 10A current, it achieves a maximum
peak output power of 160 kW with -3 dB bandwidth of 2.1 GHz and gain of
20-30 dB [4]. The tube employs a three-fold helically corrugated
waveguide as the high-frequency interaction structure. This structure
ensures that the waveguide’s dispersion curve is nearly linear in the
region where the axial wave number is zero. This characteristic is
advantageous for bandwidth extension, and the structure exhibits lower
sensitivity to velocity spread. In addition to IAP, the University of
Strathclyde in the United Kingdom has also been dedicated to research on
helically corrugated waveguides [5]. Up to this point, reports on
large-orbit gyro-TWTs have only mentioned the helically corrugated
waveguides used as their high-frequency interaction circuit.
The Beijing vacuum electronic research institute(BVERI) has developed a
series of gyro-TWTs utilizing periodic dielectric-loaded structures as
high-frequency interaction circuits, employing small-orbit electron
beams, and has achieved good results in the Ka, Q, and W bands
[6-8]. The lossy material of the periodic dielectric-loaded
structure is made of high thermal conductivity AlN-SiC ceramics. This
structure is simple, well-established in terms of manufacturing, and
provides easy control of loss with good thermal conductivity. Building
on the research of Ka-band gyro-TWTs with periodic dielectric-loaded
interaction structures in the laboratory, this paper has shifted to
using a large-orbit electron beam while retaining the periodic
dielectric-loaded interaction structure. The tube in this paper operates
in TE21 mode, designed at the second harmonic to reduce
magnetic field.
Model design: For the
Ka-band, the interaction structures adopted by the existing small-orbit
gyro-TWTs in the laboratory are periodic dielectric-loaded structures,
as shown in Figure 1a. In this structure, conductor rings and dielectric
rings are alternately placed. The lossy material used for dielectric
rings is the high thermal conductivity AlN-SiC ceramic series. By
adjusting the axial spacing ratio and radial thickness of the rings,
effective suppression of non-working modes can be achieved. In the
Ka-band, the lossy characteristics of the ceramic material for various
modes are depicted in Figure 1b.
The complete tube with periodic
dielectric-loaded structure is depicted in Figure 2a. During testing, it
operates with magnetic field of 1.24 Tesla, working voltage of 66 kV,
and working current of 13A, achieving a maximum peak power of 293 kW
[9]. Figure 2b illustrates the relationship between output power and
frequency. Additionally, the test results and PIC simulation results are
presented on the same graph. The -3 dB bandwidth is calculated as 2.1
GHz. The maximum efficiency is 34.2%, and the maximum gain reaches 56
dB. It is evident from the graph that the simulation results closely
match the test results. The high-frequency interaction circuit of the
Ka-band large-orbit TWT studied in this paper adopts the above-mentioned
periodic dielectric-loaded structure.