Zaigao Chen

Three-dimensional parallel UNIPIC-3D code for simulations of high-power microwave devices

This paper introduces a self-developed, three-dimensional parallel fully electromagnetic particle simulation code UNIPIC-3D. In this code, the electromagnetic fields are updated using the second-order, finite-difference time-domain method, and the particles are moved using the relativistic Newton–Lorentz force equation. The electromagnetic field and particles are coupled through the current term in Maxwell’s equations. Two numerical examples are used to verify the algorithms adopted in this code, numerical results agree well with theoretical ones. This code can be used to simulate the high-power microwave (HPM) devices, such as the relativistic backward wave oscillator, coaxial vircator, and magnetically insulated line oscillator, etc. UNIPIC-3D is written in the object-oriented C++ language and can be run on a variety of platforms including WINDOWS, LINUX, and UNIX. Users can use the graphical user’s interface to create the complex geometric structures of the simulated HPM devices, which can be automatic...

Three-dimensional simple conformal symplectic particle-in-cell methods for simulations of high power microwave devices

To overcome the staircase error in traditional particle-in-cell (PIC) method, simple conformal (SC) symplectic PIC method is presented. The SC symplectic finite-difference time-domain (FDTD) scheme is used to advance electromagnetic fields without reduction of time step. Particles are emitted from conformal boundaries with the charge conserving emission scheme and moved using the relativistic Newton-Lorentz force equation. The electromagnetic field and particles are coupled through the current term in Maxwells equations. One numerical example is used to verify the algorithms.

Novel high-power subterahertz-range radial surface wave oscillator

A novel high-power subterahertz-range radial surface wave oscillator (SWO), in which the electron beam is emitted radially and interacts with the slow wave structure (SWS) machined on a planar plate, is presented in this paper. Compared to the axial SWO where the electron beam is emitted axially and interacts with the SWS machined on the inner wall of a cylindrical waveguide, the radial SWO has two advantages. One is that fabrication of the radial SWS is much easier than that of the axial SWO. The other is that the radial SWO is a low-impedance device, it can produce much higher current than the axial SWO when they are driven by the same driven voltage, and hence, it may generate much higher output power. Particle-in-cell simulation results demonstrate that the proposed radial SWO driven by the voltage of 312u2009kV can produce the terahertz wave with the mean output power of 680u2009MW at the frequency of 0.142 THz, it has a very pure TM01 mode and the higher modes can be effectively suppressed.

A 0.14 THz relativistic coaxial overmoded surface wave oscillator with metamaterial slow wave structure

This paper presents a new kind of device for generating the high power terahertz wave by using a coaxial overmoded surface wave oscillator with metamaterial slow wave structure (SWS). A metallic metamaterial SWS is used to avoid the damage of the device driven by a high-voltage electron beam pulse. The overmoded structure is adopted to make it much easy to fabricate and assemble the whole device. The coaxial structure is used to suppress the mode competition in the overmoded device. Parameters of an electron beam and geometric structure are provided. Particle-in-cell simulation results show that the high power terahertz wave at the frequency of 0.14 THz is generated with the output power 255 MW and conversion efficiency about 21.3%.

Research on effect of emission uniformity on X-band relativistic backward oscillator using conformal PIC code

Explosive emission cathodes (EECs) are adopted in relativistic backward wave oscillators (RBWOs) to generate intense relativistic electron beam. The emission uniformity of the EEC can render saturation of the power generation unstable and the output mode impure. However, the direct measurement of the plasma parameters on the cathode surface is quite difficult and there are very few related numerical study reports about this issue. In this paper, a self-developed three-dimensional conformal fully electromagnetic particle in cell code is used to study the effect of emission uniformity on the X-band RBWO; the electron explosive emission model and the field emission model are both implemented in the same cathode surface, and the local field enhancement factor is also considered in the field emission model. The RBWO with a random nonuniform EEC is thoroughly studied using this code; the simulation results reveal that when the area ratio of cathode surface for electron explosive emission is 80%, the output powe...

Novel low-voltage subterahertz-range radial backward wave oscillator

To increase the output power of compact low-voltage subterahertz-range vacuum electron devices (VEDs) to hundreds of watts and overcome the difficulties of the fabrication and assembly, a novel low-voltage subterahertz-range radial backward wave oscillator (BWO), in which the electron beam emits radially inward and interacts with the slow wave structures (SWSs) machined on a planar plate, is presented in this paper. Compared to the VEDs where the axial electron beam or planar sheet electron beam is adopted, the dispersion curve of the proposed radial BWO is independent of the radius and azimuthal coordinate. Hence, the power capacity is increased by using the overmoded SWSs, and the azimuthal asymmetrical modes cannot be excited.

An optimization method of relativistic backward wave oscillator using particle simulation and genetic algorithms

Optimal design method of high-power microwave source using particle simulation and parallel genetic algorithms is presented in this paper. The output power, simulated by the fully electromagnetic particle simulation code UNIPIC, of the high-power microwave device is given as the fitness function, and the float-encoding genetic algorithms are used to optimize the high-power microwave devices. Using this method, we encode the heights of non-uniform slow wave structure in the relativistic backward wave oscillators (RBWO), and optimize the parameters on massively parallel processors. Simulation results demonstrate that we can obtain the optimal parameters of non-uniform slow wave structure in the RBWO, and the output microwave power enhances 52.6% after the device is optimized.

Study on the stability and reliability of Clinotron at Y-band

To improve the stability and reliability of Clinotron at the Y-band, some key issues are researched, such as the synchronous operating mode, the heat accumulation on the slow-wave structure, and the errors in micro-fabrication. By analyzing the dispersion relationship, the working mode is determined as the TM10 mode. The problem of heat dissipation on a comb is researched to make a trade-off on the choice of suitable working conditions, making sure that the safety and efficiency of the device are guaranteed simultaneously. The study on the effect of tolerance on devices performance is also conducted to determine the acceptable error during micro-fabrication. The validity of the device and the cost for fabrication are both taken into consideration. At last, the performance of Clinotron under the optimized conditions demonstrates that it can work steadily at 315.89u2009GHz and the output power is about 12u2009W, showing advanced stability and reliability.

Research on the effect of cathode plasma expansion on x-band relativistic backward wave oscillator using moving-boundary conformal PIC method

The cathode plasma expansion has been widely investigated and is recognized as impedance collapse in a relativistic backward wave oscillator (RBWO). However, the process of formation and expansion of cathode plasma is very complicated, and the thickness of plasma is only several millimeters, so the simulation of cathode plasma requires high temporal and spatial resolutions. Only the scaled-down diode model and the thin gas layer model are considered in the previous hybrid simulation, and there are few numerical studies on the effect of cathode plasma expansion on the RBWO. In this paper, the moving-boundary conformal particle-in-cell method is proposed; the cathode plasma front is treated in this novel method as the actual cathode surface, and the explosive electron emission boundary moves as the expansion of cathode plasma. Moreover, in order to accurately simulate the electromagnetic field near the cathode surface, the conformal finite-difference time-domain method based on the enlarged cell technique i...

Dual-frequency low-voltage subterahertz radial clinotron based on fan-shaped splitter

To further improve the output power and tuning bandwidth of compact low-voltage subterahertz vacuum electron devices, a novel dual-frequency low-voltage subterahertz radial clinotron oscillator, in which two fan-shaped radial electron beams transmit inward along the axial direction and interact with two fan-shaped slow wave structures (SWSs) machined on the azimuthally symmetric planar plate individually, is studied in this paper. Compared to the clinotron where the planar SWS composed of the rectangular grooves is adopted, the dispersion curve of proposed fan-shaped radial SWSs is independent of radius and azimuthal coordinates. The particle-in-cell simulation results indicate that the designed clinotron can simultaneously produce the electromagnetic waves at 0.305u2009THz and 0.34u2009THz, the corresponding output powers are 9.1 W and 24u2009W, and the tuning range of bandwidth can be significantly broadened.