Qian-Zhong Xue

Loss-Induced Modal Transition in a Dielectric-Coated Metal Cylindrical Waveguide for Gyro-Traveling-Wave-Tube Applications

The mode identification principles, mode structures, and propagation characteristics of electromagnetic modes in a metal cylindrical waveguide coated with an inside layer of lossy dielectric have been investigated for gyro-traveling-wave-tube applications. For the first time, the loss-induced modal transition is revealed, in which the dispersion curves of a pair of nearby modes cross each other, and their mode structures interchange. The relations among the dispersion curves, mode structures, and propagation attenuations are also presented. The distinctive discriminations of propagation properties between different modes enable us to explore many promising applications using lossy dielectric-coated waveguides.

Effect of a backward wave on the stability of an ultrahigh gain gyrotron traveling-wave amplifier

A systematic stability analysis method using theoretical tools combining linear and self-consistent nonlinear theory is presented to analyze an ultrahigh gain gyrotron traveling-wave (gyro-TWT) amplifier operated in the fundamental TE11 mode in the Ka-band. It characterizes the role that the backward-wave component plays in the internal feedback physical processes of two major kinds of self-induced oscillations associated with TE11(1) absolute instability and TE21(2) gyrobackward-wave oscillation. For the first time, self-induced constriction in TE11(1) absolute instability caused by a strong backward-wave component is revealed through simulation. Both the thickness and resistivity of the distributed wall loss loaded on the inside of the interaction waveguide have obvious effects on stabilizing both kinds of oscillations. Following the stability analysis, a multistage interaction circuit is proposed by nonlinear analysis which shortens the length of the entire structure and enables the ultrahigh gain gyro...

Two-Dimensional Metallic Photonic Crystal with Point Defect Analysis Using Modified Finite-Difference Frequency-Domain Method

We have derived a modified finite-difference frequency-domain (FDFD) algorithm for two-dimensional (2-D) metallic photonic crystal (MPC) analysis. Using this method, the numerical results for the transverse-electric (TE) and transverse-magnetic (TM) modes in square and triangular lattices are in excellent agreements with those from other method. Then the correspondence of the band gaps between a unit cell and a supercell is demonstrated. Furthermore, by comparing the field distributions of the defect modes in a point defected MPC and a point defected dielectric photonic crystal (DPC), it is found that the defect MPC has a higher degree of localization, which means that MPC is preponderant for resonator and waveguide applications in millimeter wave and sub-millimeter wave bands.

Modal Transition and Reduction in a Lossy Dielectric-Coated Waveguide for Gyrotron-Traveling-Wave Tube Amplifier Applications

A metal cylindrical waveguide coated with an inside layer of lossy dielectric which affects the propagation characteristics of a guided electromagnetic mode is investigated for gyrotron-traveling-wave tube (gyro-TWT) amplifier applications. This paper reveals a series of novel phenomena. The dispersion curve of a higher order mode has a turning point during its evolvement from the fast wave region to the slow wave region. An electromagnetic mode in the lossy dielectric-coated waveguide exhibits a transverse partial-standing-wave distribution. The dielectric loss induces modal transition which results in the dispersion curves of a pair of nearby modes crossing each other and interchanging mode structures. Modal reduction caused by strong dielectric loss merges a pair of nearby modes into one. In this one merged mode, the dielectric-coated waveguide is equivalent to a conventional cylindrical waveguide with imperfect conducting wall. This improved understanding of lossy dielectric-coated metal cylindrical waveguide is of value and usefulness for application toward gyro-TWTs capable of high-power and wide bandwidth.

Particle simulation of a ka-band gyrotron traveling wave amplifier

The design of a ka-band gyrotron traveling wave (gyro-TWT) amplifier is presented. The gyro-TWT amplifier with a severed structure operates in the fundamental harmonic TE01 circular electric mode. The beam-wave interaction is studied by using a particle-in-cell (PIC) code. The simulations predict that the amplifier can produce an output peak power of over 155 kW, 22% efficiency, 23 dB gain, and a 3 dB bandwidth of 2 GHz for a 70 kV, 10 A electron beam with an axial velocity spread Δvz/vz=5%.

Recent Results in the Development of a Ka-Band Second Harmonic Gyroklystron Amplifier

High-power coherent millimeter-wave sources have been extensively studied for many applications including high-resolution radars, millimeter-wave electronic counters measures, material processing, and high- energy particle accelerators. This paper presents some recent experiment results in the development of this gyroklystron at IECAS.

Nonlinear study on the terahertz free electron laser amplifier with elliptical waveguide

The use of an elliptical waveguide and a planar wiggler with parabolically tapered pole pieces as the terahertz free electron laser (FEL) amplifier model is proposed. A set of self-consistent differential equations for the FEL amplifier is derived by using nonlinear theory, and the characteristics of this amplifier are numerically analyzed. Our numerical simulations are conducted to the 1000GHz amplifier with an electron beam energy of 1.74MeV. The results indicate that the peak power of 180kW and frequency bandwidth of 13.5GHz can be obtained.

An Extended Theoretical Method Used for Design of Sheet Beam Electron Gun

An extended theoretical method based on Pierce theory is investigated in this paper to provide a reliably support and direct approaches for the design of the general sheet beam electron gun. First, the general characteristics of the sheet beam electron gun are discussed by dividing the gun into three investigation parts, and the design curves are derived theoretically, which are quite different from the curves for circular beam gun. The valid range of curvature radius ratio of cathode to anode Rc/Ra for sheet beam gun is achieved as 1.879 <; Rc/Ra <; 2.79 with the compression factor M of 1.879 <; M <; 30, which can make the sheet beam gun more stable and reliable. Then, the theoretical design method based on the design curves for sheet beam gun is deduced, and the reconstruction of anode aperture and the correction of cylindrical aberration are considered thoroughly, which may improve the design method more accurate and practical. Based on the theoretical calculation and parameters analysis, two types of sheet beam gun models with different M of 4 and 10 are constructed and simulated by the 3-D software, respectively, with the same beam voltage of 22 kV, a beam current of 3.50 A, and the beam cross section of 6 mm×0.24 mm. The results achieve the good agreement and verification with the theoretical expectation for the both the two sheet beam gun.

Design of plane-distributed three-beam electron gun

A design study for plane-distributed three-beam electron gun using theoretical method combined with 3D simulation software is described in this paper, which will be used in wideband W-band TWT. The gun is characteristic with the beam voltage of 22kV, beam current of 0.15×3 A, and the radius of beam tunnel of 0.18mm. The simulation results conform well to the theoretical designed parameters.

Research progress of Ka band Gyro-TWTs in IECAS

A high power Gyrotron Traveling-Wave Amplifier(Gyro-TWT) operating in the low-loss TE01 mode has been designed and demonstrated. The gyro-TWT consists of a double-anode magnetron injection gun(MIG), TE01 mode interaction circuit, a 36mm diameter collector, and a three disk sappire output window. A double-anode magnetron injection gun is designed to operate at 70 kV and 10A using the EGUN trajectory code and particle in cell program MAGIC. The electron beam with v⊥/vz =1.0 has a predicated axial velocity spread of 3-5%. The TE01 mode interaction circuit is made up of a section of alternating metal and lossy ceramic (BeO+TiO2) followed by a unloaded, metal-walled output region. The lossy property of the periodically loaded ceramic relieves the worries of the potential Bragg resonance arising from the periodicity of the interaction circuit, bring high attenuation to the potential competing mode, and enhances the stability of a gyro-TWT. The nonlinear self-consistent simulation code evaluated the operating characteristics of the gyro-TWT amplifier. For an axial velocity spread of Δvz/vz=3%, The predicted peak power is 180kW with -3dB bandwidths1.75GHz.