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

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.