Yunxian Tian

A 3-D Large-Signal Model of Folded-Waveguide TWTs

A steady-state 3-D large-signal model of the beam-wave interaction (BWI) in folded-waveguide (FW) traveling-wave tubes (TWTs) has been developed. The model used a three-port network representation of the circuit and a set of discrete rays representation of the 3-D electron beam. Values of the impedance elements that characterize the model are determined by a 3-D finite-element code, high-frequency circuit simulator (HFCS). Besides, the RF fields inside the beam tunnel are represented with the digitized field profile obtained from HFCS. A new circuit attenuation model is described by introducing only one parameter. The model has been implemented in the BWISFW-3-D code. The results of the code predictions agree well with the measured data for an FW-TWT operating in the W-band. The effect of uniform focusing magnetic field on BWI is also studied.

A three-dimensional nonlinear beam–wave interaction theory for common traveling wave tubes

A three-dimensional (3-D) nonlinear theory model of beam–wave interaction for common traveling wave tubes (TWTs) is described. The equation governing the amplitude of electromagnetic wave is derived analogously to Poynting’s theorem. The electron dynamics are treated using the 3-D Lorentz force equations. RF space charge fields are obtained from solutions of the Helmholtz equations. In the model, the RF field profiles for cold structure are represented by the digitized RF field calculated by a finite-element software, HFSS. Because the digitized RF field can be obtained in the same way, the 3-D simulation code can be used to simulate common TWTs, such as helix TWTs, coupled-cavity TWTs, and even the folded waveguide TWTs. Results from the 3-D code are compared with those from 1-D code, experiment and the existing analytical theories for three types of slow wave structures.

Study on the attenuation characteristic of terahertz wave through a non-uniform magnetized plasma

Abstract In this paper, the interaction between terahertz wave and a non-uniform magnetized plasma slab is investigated. The electric field distribution in non-uniform plasma is presented and used to calculate the attenuation of the electromagnetic wave through plasma. Several types of electron density profiles are proposed and discussed. Effects of different plasma parameters (electron density, collision frequency, plasma thickness) and magnetic field amplitudes on THz wave attenuation characteristic are investigated. The transmission ratios at several typical THz signal frequencies in both unmagnetized and magnetized plasma are presented as well. These simulation results are meaningful for the flight communication.

Design of a 0.13THz folded-waveguide TWTA

We present a preliminary design of a 0.13THz folded-waveguide (FW) traveling-wave-tube amplifier (TWTA). The beam-wave interaction is simulated using a 1-D large signal code, BWISFW-1D, which uses a three-port network representation of the unit cell of the slow wave structures. The simulated output power is in excess of 55W from 128-138GHz, and the maximum efficiency is 4%.

Effects of magnetized plasma on the propagation properties of obliquely incident THz waves

In this paper, the propagation of obliquely incident terahertz (THz) wave in a non-uniform magnetized plasma slab is investigated. The electron density and the collision frequency across the plasma are assumed to have a Gaussian profile. To deal with the non-uniform profile, the plasma slab is divided into a series of subslabs. For more accuracy, twice reflection between the interfaces of each subslab is considered, and the corresponding transmitted and reflected power are derived. Effects of collision frequency, magnetic field amplitude and incident angle on THz wave propagation characteristics are investigated. Specifically, the refraction angles in each subslab are presented. These simulation results are meaningful for the hypersonic flight communication.

Investigation of ionization speed in field ionization with laser–plasma interaction

The effects of target density and laser intensity on ionization speed are studied in this paper by 1D3V particle-in-cell simulations, where the field ionization of single atom is involved basing Ammosov-Delone-Krainov model in the form of Penetrante and Bardsley. To consider the ionization speed, the evolution of plasma density for the helium target, particularly, the ion density change rate near the target front surface, are discussed. The results show that not only the laser intensity, but also the target density will affect field ionization and further affect the plasma formation. This work will be helpful for further understanding of plasma formation in intense laser pulse. Also, it may be benefit for the setup of initial parameters before the simulation of laser–plasma interaction.

Effects of the plasma profiles on photon and pair production in ultrahigh intensity laser solid interaction

The model of photon and pair production in strong field quantum electrodynamics is implemented into our 1D3V particle-in-cell code with Monte Carlo algorithm. Using this code, the evolution of the particles in ultrahigh intensity laser (∼1023 W/cm2) interaction with aluminum foil target is observed. Four different initial plasma profiles are considered in the simulations. The effects of initial plasma profiles on photon and pair production, energy spectra, and energy evolution are analyzed. The results imply that one can set an optimal initial plasma profile to obtain the desired photon distributions.