Linlin Hu

Optimization of the watt-scale 220-GHz folded waveguide BWO

The latest development of 220 GHz folded waveguide (FW) backward wave oscillator (BWO) is presented here. The pill-box window system, beam optical system (BOS), slow wave structure (SWS) have been designed, machined and tested. However, the collector current of the prototype tube can just reach 13 mA with the emission current of 22 mA due to the imperfection of the structure and magnetic field, where no microwave signal detected. In previous SWS design, the threshold current is roughly estimated as 12 mA, which may become higher in real case. Apart from improving the assembly and brazing precision of the structure, the total number of periods is considered to be optimized to decrease the threshold current to 3 mA. The optimization result shows that the BWO can still output watt-scale power around 220 GHz.

Theoretical and Experimental Study of the Modified Pill-Box Window for the 220-GHz Folded Waveguide BWO

The modified pill-box window is designed and manufactured as a power-coupler part of the 220-GHz folded waveguide backward wave oscillator in the Institute of Applied Electronics. Owing to the modified structure, both the fringe area and the end area of the thin dielectric window piece can be metalized, thus the reliability of vacuum seal and strength can be greatly improved compared with its traditional counterpart. An analytical method is developed from the previous equivalent circuit theory of the traditional pill-box window. According to the analytical expressions, the inner parameter of the modified pill-box window is roughly designed. The 3-D computer code is used to verify and further optimize the analytical design. The calculation shows good consistency between the theoretical and the numerical methods. The resonant modes in this oversized window system are also numerically calculated to demonstrate that it is difficult to remove all the modes from the operating frequency range. Fortunately, the error analysis shows that these possible spurious modes are hardly stimulated in the window system with TE10 mode injected, when the misalignment of each transmission section can be limited to the practical mechanical tolerance. For this requirement, an innovative mechanical structure is then proposed to guarantee the precision of the parameters of window system which can nicely restrain the variations caused by the brazing process. By carefully controlling the errors in machining, assembly, and brazing process, the brazed and nonbrazed samples are finally manufactured and tested. The experimental result indirectly demonstrates that the microwave circuit design and the mechanical design is reasonable and reliable, where the reflection ratio is <;0.1 between the frequency range of 215-225 GHz.

An Improved Analytical Model for Fast Evaluating Absorbing Material Properties for 220-GHz FW BWO

Y-band devices, including folded waveguide, backward-wave oscillator, and traveling wave tube, are to be developed in our group, which use absorbing material in its attenuator to suppress self-oscillation or match the boundary conditions. This paper describes a developed analytical model of a rectangular waveguide system based on the voltage scattering matrix analysis to fast calculate the relative permittivity and electric loss angle tangent of a ceramic sample in consideration of waveguide wall loss. Compared to the high-quality factor cavity method and quasi-optical cavity method, this transmission/reflection measuring method in consideration of ohmic loss is of low cost and more convenient to obtain the material properties in a large-frequency range, by sacrificing some accuracy. 3-D numerical model is established to verify our method. The result shows that our method is accessible to give a fast message of material properties in a certain frequency range. The error analysis of this equivalent method is also discussed in this paper, which shows that this method is too error sensitive to be a practical tool in consideration of the controllable machining and assembly errors. Then, a simplified method is proposed to make the calculation result less sensitive to the parameter errors. The evaluation shows that the relative measuring error of the attenuating material to be tested can be confined to 25% around the frequency of 220 GHz. In advantage of its configuration, the absorber with this material to be used guarantees the small reflection ratio when the value of permittivity and loss tangent of the attenuating material varied by 25%.

Theoretical models for designing a 220-GHz folded waveguide backward wave oscillator

In this paper, the basic equations of beam-wave interaction for designing the 220 GHz folded waveguide (FW) backward wave oscillator (BWO) are described. On the whole, these equations are mainly classified into small signal model (SSM), large signal model (LSM), and simplified small signal model (SSSM). Using these linear and nonlinear one-dimensional (1D) models, the oscillation characteristics of the FW BWO of a given configuration of slow wave structure (SWS) can be calculated by numerical iteration algorithm, which is more time efficient than three-dimensional (3D) particle-in-cell (PIC) simulation. The SSSM expressed by analytical formulas is innovatively derived for determining the initial values of the FW SWS conveniently. The dispersion characteristics of the FW are obtained by equivalent circuit analysis. The space charge effect, the end reflection effect, the lossy wall effect, and the relativistic effect are all considered in our models to offer more accurate results. The design process of the FW BWO tube with output power of watt scale in a frequency range between 215 GHz and 225 GHz based on these 1D models is demonstrated. The 3D PIC method is adopted to verify the theoretical design results, which shows that they are in good agreement with each other.

Design and test of 220GHz folded-waveguide backward-wave oscillator

The design of 220GHz folded waveguide (FWG) backward-wave oscillator (BWO) is presented in this paper. A prototype tube has been fabricated successfully though technical study and the backward oscillation has been observed in the hot test. The signal of peak power 36mW, frequency 229GHz is achieved with beam voltage 17.5kV, collected current 12.2mA, transmission rate 77% and working duty 1/20.

Design and experiment of the electron-optical system for 0.14THz TWT

This paper described the 3D simulation method and results of electron gun and beam transport for the 0.14THz TWT with the help of CST. The hot test results show that the design of the electron beam optics system satisfied the need of the TWT. It bases the manufacture of the 0.14THz travelling wave tube.

Optimization design of high efficiency in 140GHz folded waveguide slow wave structure

The paper puts forward a design thought of phase velocity tapering in the design of 140GHz folded waveguide traveling wave tubes. It can make electron with traveling wave field synchronization again in output segment of interaction circuit. By the phase velocity analysis and optimized design, the big signal simulation is calculated in operating voltage 14.05kV and current 25mA. The efficiency is improved 12% in 140GHz, output power in improved 1.4W. The efficiency is improved 42% in 142GHz, output power is improved 2.5W. -1dB bandwidth is improved 7GHz from 5GHz. The design improves the bandwidth and work efficiency in D band folded waveguide traveling wave tubes.

Study on the Increased Threshold Current in the Development of 220-GHz Folded Waveguide Backward-Wave Oscillator

Backward-wave oscillator (BWO) is a promising candidate as a millimeter-wave and submillimeter-wave (MMW/SMMW) power source and has a compact configuration, moderate output power, and frequency-tuning abilities. The physical conditions for increasing the threshold current of a typical MMW/SMMW-BWO are theoretically studied in this paper. We demonstrate the greater effect of axial phase velocity variation of deformed slow-wave structures on the threshold current in a 220-GHz folded waveguide (FW) BWO in comparison with other insensitive factors. For further verification, a prototype tube of FW BWO is developed. This tube exhibited increased threshold current during experiments. This finding is consistent with the error analysis proposed by our theory.

High-frequency structure design of the 310GHz folded-waveguide extended interaction klystron and cold test of input cavity

The preliminary design and particle simulation of high-frequency structure for 310GHz extended interaction klystron (EIK) with folded waveguide (FWG) as the resonant cavity was presented in this paper The validity of FWG slow-wave structure (SWS) as EIK resonant cavity was demonstrated. The simulation indicated that FWG cavities have high characteristic impedance and can generate high gain amplification. FWG-EIK could provide a new technical solution for THz-EIK study. The designed FWG-EIK can be fabricated by matured processing. The prototype of input and output cavities have been cold tested and the resonant frequency and reflection coefficient accord with the design.