With the development of various applications,higher frequency,lower operating voltage,higher output power and miniaturization have been the requirements for TWTs [
Journal of Infrared and Millimeter Waves, Volume. 41, Issue 2, 437(2022)
Design and test of a low voltage suspended dual-microstrip meander-line slow wave structure at Ka band
A suspended dual- microstrip meander-line (SDMML) slow wave structure (SWS) was proposed. It consists of a metal enclosure as well as a suspended dielectric substrate, on the upper and lower surfaces of which are two metal meander-lines. As there are two electron beam tunnels locating above and below the dielectric substrate, and the electromagnetic wave has symmetrical distribution, it is possible to use one electron beam in each tunnel to drive the electromagnetic wave for a higher output power. The high frequency characteristics and the beam-wave interaction results of the SDMML SWS have been investigated by using simulation software. The hot performances of this SWS show that for two identical sheet beams with a voltage of 2050 V and a current of 0.2 A, the SDMML SWS has a maximum gain of 26 dB at 36 GHz and 3-dB bandwidth of saturation power of 8 GHz. To verify the simulation result, a SDMML SWS is fabricated by using new fabrication methods including the magnetron sputtering electroplate and laser ablation. The measured reflection loss of the SDMML SWS is better than -10 dB. The transmission loss of the fabricated slow wave structure is analyzed and verified through simulation and experimental results.
Introduction
With the development of various applications,higher frequency,lower operating voltage,higher output power and miniaturization have been the requirements for TWTs [
Generally,lower operating voltage means lower overall input power,and if the interaction efficiency is not improved,the output power of the SWS will also be reduced. Increasing the input power or interaction efficiency by taking certain measures is an effective method to increase the output power. One solution widely employed by researchers for improving interaction efficiency is using symmetrical meander-line SWSs [
In this paper,we proposed a SDMML SWS,which consists of two SDMMLs on the top and bottom surfaces of the suspended dielectric substrate. A metal enclosure is then employed to hold the suspended substrate. The odd mode with smoother dispersion characteristics is adopted as the operation mode,of which the operating voltage will decrease to 2 kV. By taking advantage of the double slow-wave structure with dual sheet electron beams,the input power can be increased and a larger output power can be achieved within the relatively short tube length. In order to meet the requirements of the vacuum devices,new fabrication method is tried,including magnetron sputtering,electroplate and laser ablation. The SDMML SWS is achieved with a metal layer of sufficient thickness to resist bombardment of electrons. We have completed the cold testing on the SDMML SWS. The cold testing results were analyzed and the improved fabrication method is provided.
This paper is organized as follows. The structure and dimension parameters of the SDMML SWS are described in Sect. 1,as well as the high frequency characteristics. The beam-wave interaction results are given in Sect. 2. The results of transmission characteristics are demonstrated. For verifying transmission characteristics of the SWS,the fabrication process and cold testing of that is outlined in Sect. 3. In the last,some useful conclusion is present in Sect. 4.
1 The high frequency analysis of the SDMML SWS
The unit periodic model of the SDMML SWS is illustrated in
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Figure 1.(a)The diagram unit model of the SDMML SWS,and (b) size view
Due to the presence of a double layer of metal meander-line strip in the SWS,the entire structure constitutes a three-conductor transmission line system. In the transmission mode of the three-conductor system there are odd mode and even mode,the electric fields of which are shown in
Figure 2.The electric field of (a) the odd mode, and (b) the even mode
Figure 3.The dispersion curves of the SDMML SWS
Figure 4.The interaction impedance curves of the odd mode of the SDMML SWS and conventional microstrip SWS
2 The hot performance results of the SDMML SWS
Based on the analysis of the high-frequency characteristics of this SWS,we determined the specific operating parameters for the hot performance simulation,as shown in
Figure 5.(a) Output power, gain and (b) saturation power, gain curves of the SDMML SWSs with single and dual beam
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Figure 6.Electron bunching above the SDMML SWS
Figure 7.Frequency spectrum of output signal(35 GHz)
3 Transmission characteristics simulation and test of the SDMML SWS
The transmission model of the SDMML SWS is established,as shown in
Figure 8.The model of the SDMML SWS with the input/output coupler
Figure 9.The simulation transmission characteristics curves of the whole structure
The bonding strength between the metal layer and the dielectric substrate of conventional microstrip is low,and it will peel off after the vacuum high temperature treatment. Therefore,we design a new fabrication method for the SDMML SWS. To strengthen the bonding strength between the metal layer and dielectric substrate,magnetron sputtering and electroplating are used for the manufacturing of metal layers. First,we perform magnetron sputtering on the cleaned alumina ceramic surface. The metal layers of magnetron sputtering are divided into two layers,the first layer is titanium and the second layer is copper. Titanium as the active metal has a good bonding effect and can effectively bond the copper layer to the alumina ceramic firmly. The thickness of metal layers by magnetron sputtering is very small,usually less than 1 μm. The thicknesses of the titanium layer and copper layer are 500 nm and 1 μm,respectively. Then we use electroplating to thicken the copper layer up to 10 μm. After electroplating,the solidity and thickness of the metal layer can meet the requirements. The picosecond laser is utilized to burn out the shape of the meander line.
Figure 10.The SDMML SWS after laser processing (a) top view, (b) bottom view.
Figure 11.The cold testing tube model and the pictures of the SDMML SWS and test tube
Figure 12.The cold testing result of the SDMML SWS
We subsequently investigated and confirmed the reasons for the unexpected high insertion loss. The first reason is attributed to the conductor loss of the meander line. The fabricated meander line consists of two layers:500 nm thick titanium and 10 μm thick copper. The conductivity of titanium,which is 2.38 × 106 S/m,has not been considered in the original simulation. The second reason is the dielectric loss of the oxidation of the copper layer,which is caused by the heat of laser processing,as shown in
Figure 13.(a) Diagram of oxidation area of the copper layer processed by laser, and (b) the model of oxidation area in simulation
Further simulations have been carried out to verify the reasons of high insertion loss,and the results are shown in
Figure 14.The comparison of transmission loss S21 of experiment and some kind of simulated models
In view of the above discussion,there are two issues that need our attention in the subsequent processing. In order to reduce the transmission loss,on one hand,high-conductivity metals should be used instead of the low-conductivity metal layer as bonding layers. On the other hand,the metal layer should be protected by inert gases during the laser cutting process to prevent the generation of the oxidation area.
4 Conclusion
This paper has described the design,simulation,fabrication and cold testing the SDMML SWS at Ka-band. As a three-conductor transmission line structure,the SDMML SWS have odd and even modes. The odd mode has a lower operating voltage and wider bandwidth compared to the even mode. The suspension feature of this structure makes it suitable for double electron beam interactions and can effectively increase the electron beam input power. The operating voltage and current of this SWS are 2050 V and 0.2 A. From the particle-in-cell simulation,the maximum output power is 20 W at 36 GHz,corresponding gain and efficiency of 26 dB and 5%,respectively. The 3-dB bandwidth of saturation power is 8 GHz.
The transmission characteristics of the SDMML SWS have been studied and the cold test results have been analysis. The reflection loss(S11)of cold testing results has a good match with the ideal simulation results,which is below -10 dB from 30 to 40 GHz. But the transmission loss(S21)cannot match with the ideal simulation results. The low conductivity metal and some oxidation area have a serious impact on transmission loss. Therefore,the selection of a high conductivity metal as the bonding layer and the protection of the metal layer from oxidation should be focused in the processing of planar microstrip SWS.
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Teng-Long HE, Shao-Meng WANG, Xin-Yi LI, Duo XU, He-Xin WANG, Ning-Jie SHI, Hua-Rong GONG, Zhi-Gang LU, Zhan-Liang WANG, Zhao-Yun DUAN, Yu-Bin GONG. Design and test of a low voltage suspended dual-microstrip meander-line slow wave structure at Ka band[J]. Journal of Infrared and Millimeter Waves, 2022, 41(2): 437
Category: Research Articles
Received: Apr. 22, 2021
Accepted: --
Published Online: Jul. 8, 2022
The Author Email: Yu-Bin GONG (ybgong@uestc.edu.cn)