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The interest in millimeter and terahertz (THz) waves is on the rise due to their unique properties and suitability for a wide range of applications[1-3]. Thanks to advancements in the microfabrication technology, high-powered vacuum electron devices (VEDs) are capable of generating microwaves in the millimeter and THz range[4-5].
In a typical VED, the amplitude of the synchronous slow-wave z-component in the electric field (EZ) decreases as its distance from the structure increases. Only a thin layer of electrons moving close to the structure can effectively interact with this high-frequency EZ field. At the millimeter- and terahertz-wave band, the beam interacting with the high-frequency EZ field decreases. A number of methods have been proposed to improve VEDs, through increasing efficiency and decreasing starting current density[6-8]. There have been attempts at using sheet electron beam devices[9-11] to improve VED performance by enlarging interaction space and providing sufficiently high currents. High-operating current density reduces the lifespan of cathodes in millimeter-wave and THz VEDs, therefore, currents should be kept as low as possible.
This article explores the photonic column array slow-wave structure (SWS), in order to improve the performance of THz VEDs[12-18]. The study designed and simulated a 0.28-THz backward-wave oscillator (BWO). The photonic column array SWS allows the flow of sheet beams. A planar SWS is a multi-column structure, which collects a large number of electrons on the surface of its columns. More electrons are lost in column array structures than in comb structures. This allows for the reduction of the starting current density in multi-column structures through increasing the length or width of the structure. Higher operating voltages (10~20 kV) can then be used to provide significant power and frequency increases. This article focuses on the design and analysis of a 0.28-THz BWO. We show that photonic column array structures are effective in improving the operation of microwave VEDs, such as BWOs.
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A 0.28-THz BWO was designed and simulated using CHIPIC, a three-dimensional (3D) electromagnetic particle-in-cell (PIC) finite-difference time-domain (FDTD) code. The system used the photonic column array SWS.
The SWS and baseplate cover consisted of copper. The simulation parameters of the BWO are shown in Table 1. Fig. 5 shows the electron beam passing through the photonic column array SWS on the Y-Z and X-Z planes. The THz signal output through a coupler, which consisted of a slot coupled to a WR10 waveguide.
Table 1. Operating parameters of the BWO
Parameters Value Beam voltage/kV U = 12.5 Beam current/mA I = 78 Beam thickness/mm δ = 0.3 Current density/A·cm−2 10 Number of periods N = 80 External magnetic field/T 0.5 Baseplate-cover height/mm H = 0.45 The results of the PIC simulation are shown in Fig. 6. The average beam energy was reduced through absorption by the SWS, as shown in Fig. 6a. The total current in the SWS was 78 mA, and the interaction current was about 51.5 mA. About 29.7% of electrons were lost in the multi-column structures, because the sheet beam was immersed in the SWS, as shown in Fig. 5a. However, this increased the surface of the beam-wave interaction.
The output power as a function of time, and the corresponding FFT power spectrum are shown in Figs. 6c and 6d, respectively. In accordance with the power law P∝E2, the amplitude on the FFT power spectrum corresponding to 0.56 THz was much higher than that corresponding to 0.28 THz. The operating frequency of 0.28 THz matches the intersection in Fig. 3.
Fig. 7a shows the high-frequency field power as a function of SWS length. Most of the field power was retained in the BWO, unlike the output power in Fig. 6c. This shows that the output coupling efficiency of this device was very low. Fig. 7b shows the surface power loss in the BWO. Surface power loss was much higher than the output power loss shown in Fig. 6c. Power loss to structures cannot be ignored in simulations, especially for high-frequency devices.
Fig. 8 shows power as a function of various parameters. Fig. 8a shows that the power increased as the number of periods increased, and Fig. 8b shows that power increased as the number of rows increased. After a series of calculations and comparisons, the numbers of periods and rows were chosen to be 80 and 7, respectively.
Sweeping simulation of the beam voltage, as shown in Fig. 8c, shows that the power peaked at operating voltage U = 12.5 kV, and the frequency increased with increasing voltage. This suggests that the operating frequency of the oscillator can be changed by adjusting the operating voltage. Power increases with increasing current density, as shown in Fig. 8d. The BWO functioned at a current density of 6 A/cm2 and must maintain a relatively low operating current density to increase the lifespan of the cathode. Referring to Fig. 8e, the output power increased as the guiding magnetic field increased. Stronger magnetic fields increased the size and cost of the device. A 0.5-T guiding magnetic field was adopted for the device in this study.
Research on Terahertz Backward-Wave Oscillator Based on Photonic Column Array Slow-Wave Structure
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摘要: 研究了一种光子晶体的慢波结构,通过对该结构的色散、场分布和粒子模拟计算,设计和仿真了一个0.28-THz的带状注返波管。在阴极电流密度仅10 A/cm2(最小可低于6 A/cm2),电压12.5 kV,磁场0.5 T的情况下,该结构通过与带状电子注浸没式互作用,输出功率为435 mW。在此基础上,采用了LIGA加工技术制备了该慢波结构。研究表明,光子晶体结构能有效提高互作用效率和降低起振电流密度,有效提高太赫兹真空电子器件阴极的使用寿命,是提高太赫兹真空辐射源性能的一种有效途径。Abstract: This article explores a photonic column array slow-wave structure (SWS). A 0.28-THz sheet beam backward-wave oscillator (BWO) was designed and simulated by calculating the dispersion, field distribution and particle simulation. When the cathode current density is only 10 A/cm2 (the minimum is less than 6 A/cm2), the voltage is 12.5 kV and the magnetic field is 0.5 T, the structure interacts with the sheet beam by immersion and the output power is 435 mW. On the basis of previous work, the SWS was fabricated using Lithography-Galvanoformung-Abformung (LIGA) fabrication technology. The results show that the column array structure can effectively improve the interaction efficiency and reduce the starting current density; and effectively improve the lifespan of terahertz (THz) vacuum electron device (VED) cathodes, which is a viable means of increasing the performance of THz vacuum radiation sources.
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Key words:
- column array slow-wave structure /
- THz BWO /
- starting current /
- vacuum electron device
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Table 1. Operating parameters of the BWO
Parameters Value Beam voltage/kV U = 12.5 Beam current/mA I = 78 Beam thickness/mm δ = 0.3 Current density/A·cm−2 10 Number of periods N = 80 External magnetic field/T 0.5 Baseplate-cover height/mm H = 0.45 -
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