Wenxin Liu

Three-dimensional theory of Smith-Purcell free-electron laser with dielectric loaded grating

A dielectric loaded rectangular grating for Smith-Purcell devices is proposed in this paper. Regarding the electron beam as a moving plasma dielectric, a three dimensional (3D) linear theory of beam-wave interaction is developed. The first and second order growth rates are calculated, which are obtained by expanding hot dispersion equation at synchronous point. The results show that the cutoff frequency is affected by grating width. The dispersion curve becomes flatter and shifts towards lower frequency by loading dielectric in grooves. The simulation results, which are obtained by a 3D particle-in-cell code, are in good agreement with theoretical calculations. Compared the first and second order growth rate, it shows that the discrepancy is large when beam parameters are selected with high values. In this case, it is necessary to apply the second order growth rate, which can accurately describe the process of beam-wave interaction.

Dispersion Characteristics of Two-Beam Folded Waveguide for Terahertz Radiation

A novel type of slow-wave structure, two-beam folded waveguide (FW) operating at terahertz (THz) band, is presented in this paper. The propagating characteristics of two-beam FW are investigated. Results of different methods including field matching and equivalent circuit are compared for the calculating dispersion of two-beam FW for THz radiation. The equivalent circuit model considers the circular bends and beam tunnels without electron beams. Field matching analysis considers both of the electron beams and the circular bends. Moreover, results from CST microwave studio simulations are compared with above different methods to find a sufficiently accurate prediction of dispersion of two-beam FW.

Dispersion characteristics of three-dimensional dielectric-loaded grating for terahertz Smith-Purcell radiation

In this paper, a dielectric-loaded grating for Smith-Purcell device is proposed. The three-dimensional (3D) analytical theory for hot dispersion relation is obtained by using field matched method, which is solved by numerical simulations. The first and second order growth rates for the proposal model are analyzed, which is obtained by expanding hot dispersion equation at the operating point. The results show that the dispersion can be effectively weakened by introducing dielectric-loaded grating, in which the cutoff frequency is affected by the grating thickness. The dispersion curve becomes flatter and shifts towards lower frequency at the optimum grating parameters. The 3D particle-in-cell (PIC) simulation is also performed and the results are in good agreement with theoretical calculations. Comparing the first order growth rate with the second one, it reveals that the discrepancy is small when electron beam parameters are selected with small values. Otherwise, the discrepancy is large and cannot be ignored. To accurately describe the process of beam-wave interaction, the second order growth rate is necessary to apply.

Two-Section Folded-Waveguide Slow-Wave Structure for Terahertz Extended Interaction Oscillator

Extended interaction oscillator (EIO) is a promising terahertz (THz) power source, which has many potential important applications, such as military radar, medical imaging and material science research, and so on. To improve the operation performances of the THz EIO, a novel type of two-section folded waveguide (TSFW) slow wave structure (SWS) is proposed in this paper. The dispersion characteristics for TSFW obtained by an equivalent circuit model are analyzed, and the output characteristics for TSFW EIO are studied and optimized with the help of 3-D particle-in-cell simulation. Comparing with the single section of FW SWS, the remarkable enhancement of output power in the TSFW SWS is observed from 50 up to 130 W. Based on the proposal model, the TSFW is fabricated with a high speed numerical control milling machine and its transmission characteristics are tested. The results of the cold test are in agreement with theoretical predications.

Enhancements of Terahertz Radiation From a Grating Waveguide by Two-Stream Instability

An approach to the enhancement of terahertz radiation from a grating waveguide by two-stream instability (TSI) is presented in this paper. The grating waveguide composed of a planar metallic grating and an opposite plate is driven by two electron beams with a velocity separation. The dispersion equation is derived with the linearized fluid theory and solved numerically. Also, the numerical results show that a considerable enhancement of growth rate is possible at an optimized velocity separation. With the help of 2-D particle-in-cell simulations, the improvement of radiation performance induced by the TSI is demonstrated.

Design of Photonic Crystal Resonant Cavity Using Overmoded Dielectric Photonic Band Gap Structures

An overmoded photonic crystal resonant cavity with two dimensional dielectric lattice structures is proposed and simulated. The dominant mode is a higher-order TM03-like at the frequency of 31.14GHz, the fundamental mode and most other modes are not supported by the cavity. The structure would be potential for application in accelerator, gyrotron and klystron in Ka-band. DOI: 10.2529/PIERS061007043414

The Investigation of Thermal Plasma-Loaded Free-Electron Laser: Linear Analysis

The dispersion equation of thermal plasma-loaded FEL is derived employing linear fluid theory. The characteristic of this thermal plasma FEL is analyzed in detail. FEL growth rate varying with plasma density and plasma temperature is investigated. We find there is an optimum plasma temperature where FEL growth rate can be enhanced considerably. The FEL growth rate is decreased accompanying plasma density raising.

Enhancements of Cherenkov Radiation by Two Electron Beams

An approach to the enhancement of Cherenkov radiation source driven by two electron beams (TEBs) is presented in this paper. The Cherenkov radiation source is composed of a cylindrical grating, and TEBs with a velocity separation are concentric and separated. By a field matched method, the dispersion equation is derived with a linearized fluid theory and solved by numerical calculation. The numerical results show that a considerable enhancement of growth rate is realized at an optimized voltage ratio due to two stream instability. Moreover, with a help of 2-D particle-in-cell simulations, the improvement of output power induced by the two stream instability is demonstrated, and the decreased of output power is observed as the increased distance of two electron beams.

Research on Three Types of Rhombus Lattice Photonic Band Structures

The gap-midgap ratio for three types of rhombus lattice photonic band structures are analyzed by plane wave expansion method, which is confirmed by the HFSS simulation. Firstly, general wave vectors in the first Briliouin zone are derived. Secondly, the gap-midgap ratios as a function of filling factor and background material are investigated, respectively. These results would provide theoretical instruction for designing arbitrary bend waveguides or other optical integrated devices using photonic crystal with a rhombus lattice.

Start current of dielectric-loaded grating in Smith-Purcell radiation

In this paper, a three-dimensional dielectric loaded grating (DLG) is proposed for the Smith-Purcell (SP) device. Taking into the considerations of thickness and width of electron beam, the dispersion equation is derived by using field matches method. The complex frequency is obtained by the numerical solution of dispersion equation, in which the imaginary part represents linear growth rate. The impacts of the electron beam filling factor (EBFF) on growth rate are discussed under the condition that the beam current and beam current density are kept as constants, respectively. In addition, the start current for SP oscillator is obtained by using the dispersion relation combined with boundary conditions. The relationship between the start current and other parameters is discussed and compared with the conventional metal grating. The results show that with the increasing of EBFF, the peak growth rate increases rapidly firstly and then decreases slowly, in which the current and current density are kept as con...