Coaxial Wiggler Research Articles

Transverse magnetic electromagnetic mode analysis with electron beam effect in overmoded waveguide of coaxial magnetic wiggler

The coaxial wiggler with periodic permanent magnet is one of the ways to realize portable and practical high power microwave, which has a relatively small volume and low weight. However, the electromagnetic mode competition will significantly affect the output power at a high electron current. Based on the dispersion curve considering the effect of the electron beam, we propose a method of judging whether the mode can oscillate as the main mode in the coaxial magnetic wiggler. We analyze the wave–beam interaction by the first-order perturbation and predict some modes that seem impossible in traditional methods. It is verified by 3D particle-in-cell simulation that these electromagnetic modes will be the operating modes.

Coaxial electrostatic wiggler with corrugated inner and outer walls

In this paper, a coaxial electrostatic wiggler with corrugated inner and outer walls is investigated theoretically. The field distributions in this wiggler correspond to the special solution of a Laplace equation in a cylindrical coordinate system with two sinusoidal ripple boundaries. Through numerical analysis and comparison to the coaxial electrostatic wiggler with a corrugated outer wall, the results indicate that in a coaxial electrostatic wiggler with corrugated inner and outer walls, electric field undulation can be enhanced. The particle-in-cell simulation further demonstrates that the electrons can be modulated deeply.

The analysis of Raman scattering in a free-electron laser with a rectangular hybrid wiggler

A one dimensional theory of the stimulated Raman backscattering process in a free electron laser with rectangular hybrid wiggler (RHW) is analyzed. The dispersion relation in the rest frame of the electron beam and also a formula for the lab-frame spatial growth rate are derived. A numerical computation of the growth rate for RHW is conducted and a comparison with that for coaxial hybrid wiggler is made away from the resonance. The growth rate is found larger for the rectangular wiggler than for the coaxial wiggler. A much narrower magnetoresonance associated with the third spatial harmonic is also obtained compared to the first one.

Analysis of electron dynamics and two mechanisms in a coaxial magnetic wiggler

The electron dynamics in a coaxial magnetic wiggler are analyzed numerically and studied in particle-in-cell (PIC) simulation. Electrons wiggle in angular direction to coherently generate a TE01 mode, and the results are consistent with each other. The trajectory of the electron in 3-D space and the effects on the trajectories of the initial phases of wiggler magnetic fields are researched, which helps to choose appropriate initial phases to control the trajectory of the electron. An oscillator of FEL with a coaxial magnetic wiggler based on a quasi cavity is designed for efficiently generating the TE01 mode with the high saturation power and high efficiency. The advantages of the quasi cavity structure are analyzed and the parameter-selection rules are investigated. Based on the electron dynamics, the process of electron bunching in the self amplified spontaneous emission (SASE) and the Seeded mechanisms are studied. The Seeded mechanism could accelerate electron bunching greatly and reduce the saturation time than the SASE by half.

Behavior of a free-electron laser oscillator with a hybrid coaxial wiggler

We have analyzed the behaviors of a free-electron laser (FEL) with a hybrid coaxial wiggler and a normal wiggler for a free-electron laser oscillator. The radiation field is decomposed into fundamental and higher-order-Gauss Laguerre modes. A comparison of the behaviors of the radiation amplitudes of these modes and their efficiencies in the hybrid and the normal wigglers is made by using a 3D FEL code that we developed.

Field distribution in a coaxial electrostatic wiggler

The field distribution in a coaxial electrostatic wiggler corresponds to the special solution of a Laplace equation in a cylindrical coordinate system with a boundary value problem of sinusoidal ripples. This paper is devoted to the physical and mathematical treatment for an analytical solution of the field distribution in the coaxial electrostatic wiggler. The explicit expression of the solution indicates that the field distribution in the coaxial electrostatic wiggler varies according to a periodic function in the longitudinal direction, and is related to the first and second kinds of modified Bessel functions in the radial direction, respectively. Comparison shows excellent agreement between the analytical formula and the computer simulation technology (CST) results. The physical application of the considered system and its analytical solution are discussed.

Amplification of a fast wave by extracting both the kinetic energy and electrostatic potential energy of a large-orbit relativistic electron beam in a coaxial electrostatic wiggler

Nonlinear model and simulation technique of the interaction and energy transfer between a fast wave and a large-orbit relativistic electron beam in a coaxial electrostatic wiggler are presented. Unlike the situations in a magnetostatic-wiggler free-electron laser (MWFEL) and in an electron cyclotron maser (ECM), the electrostatic potential of the electrons plays an important role and participates in the energy exchange between the wave and the electron beam. Compared to MWFEL and ECM, the coaxial electrostatic-wiggler configuration has a distinguishing peculiarity that besides the electron-beam’s kinetic energy, its electrostatic potential energy can be effectively transferred to the fast wave. Simulation shows that wave could be amplified with ultrahigh gain by extracting both the kinetic energy and electrostatic potential energy of the electron beam.

Boundary effects on Raman free-electron laser with coaxial wiggler

A three-dimensional theory of the stimulated Raman scattering mechanism for a coaxial free-electron laser (FEL) is developed. The relativistic mass correction is included in the analysis and the boundary effects through the radial dependencies of the wave field quantities are taken into account. The configuration consists of a coaxial cylindrical waveguide with annular electron beam. Parametric decay of the wiggler pump wave, in the beam frame, into a space-charge wave and an electric-magnetic waveguide mode is analyzed in three dimensions. A nonlinear wave equation for the three-wave interaction is used to obtain a formula for the spatial growth rate of the excited eigenmodes. Numerical analysis is conducted to study the growth rate and required relativistic factor. Growth rate is found to be substantially larger than in the case of one-dimensional coaxial wiggler. Significantly higher growth rate is also obtained compared to a similar FEL with a helical wiggler instead of the coaxial wiggler.

On the beam transport of a helical coaxial wiggler for a free-electron laser

Abstract Dynamics of an electron beam in a helical coaxial wiggler is studied. It is observed that the electrons perform betatron oscillation in a potential well that reflects the effective radiation pressure of the wiggler field and a centrifugal force due to an average azimuthal drift.

Raman free-electron laser with a coaxial wiggler

A one-dimensional theory of the stimulated Raman scattering mechanism for a coaxial free-electron laser (FEL) is developed. The beam-frame FEL dispersion relation and a formula for the lab-frame spatial growth rate are derived. A numerical study of the growth rate for the coaxial wiggler is made and compared with that for the helical wiggler. Except for a part of the group II orbits, the growth rate is found to be less than the helical wiggler. Relativistic effects due to the transverse oscillation of electrons in the wiggler field prevent the FEL operation from approaching magnetoresonance. In the absence of these relativistic mass effects, the calculations show a magnetoresonance associated with the first spatial harmonic and a much narrower resonance at the third spatial harmonic.

Equilibrium orbit analysis in a free-electron laser with a coaxial wiggler

An analysis of single-electron orbits in combined coaxial wiggler and axial guide magnetic fields is presented. Solutions of the equations of motion are developed in a form convenient for computing orbital velocity components and trajectories in the radially dependent wiggler. Simple analytical solutions are obtained in the radially-uniform-wiggler approximation and a formula for the derivative of the axial velocity with respect to the Lorentz factor gamma is derived. Results of numerical computations are presented and the characteristics of the equilibrium orbits are discussed. The third spatial harmonic of the coaxial wiggler field gives rise to group III orbits which are characterized by a strong negative mass regime.

The Magnetic Field Distributions and Beam Propagation Properties of the Hybrid Coaxial Wiggler

In this paper a novel coaxial wiggler, the hybrid coaxial wiggler, is proposed. The analytical formula for magnetic field of the wiggler is derived, and the beam propagation properties are investigated numerically. The results show that the hybrid coaxial wiggler is scalable to small periods with high field amplitude, high beam current acceptance, and excellent transverse focusing properties.

Electrostatic beam modes in a free-electron laser with a coaxial wiggler

An analysis of the propagation of an electrostatic beam space-charge wave through a coaxial wiggler magnetic field and an axial-guide magnetic field is presented. Equations for the electron orbital velocity, rate of change of axial velocity with Lorentz factor, and the dispersion relation are derived. The effects of the combined wiggler and axial-guide fields on the rate of change of the axial velocity with electron energy and on the effective electron density in the dispersion relation are studied numerically. The present theory predicts that electrostatic negative mass instabilities do not exist in a coaxial wiggler.

Design of a low-voltage annular beam Ku-band free-electron laser

A nonlinear analysis of an annular beam free-electron laser (FEL) with a helical wiggler and axial guide field is presented. An annular beam has the advantage over a solid beam of reduced dc self-fields, facilitating beam transport in short period wigglers. The annular beam geometry also permits use of a central structure to enhance the wiggler field, which tends to decrease as the period is decreased. Two configurations of the waveguide and wiggler were theoretically investigated. First, an annular beam interacting with a TE/sub 11/ cylindrical waveguide mode in the presence of a helical wiggler was studied. Next, the interaction with a coaxial TE/sub 11/ mode in the presence a novel coaxial wiggler with both inner and outer bifilar helical current windings was examined. The beam under consideration is nominally 55 kV and 5 A and the wiggler period is 0.9 cm. An axial guide field up to 4 kG was used. The beam and wiggler parameters correspond to grazing incidence with the TE/sub 13/ mode in Ku-band. Nonlinear slow-time-scale simulations show that for an ideal beam, efficiencies greater than 10% can be achieved with instantaneous bandwidths in excess of 20%. The effect of axial energy spread on interaction efficiency is examined.

Theory of the free-electron laser based upon a coaxial hybrid wiggler

A 3D nonlinear FEL formulation based upon a Coaxial Hybrid Iron (CHI) wiggler is described. The CHI wiggler is created by insertion of a central rod and an outer ring (composed of alternating ferrite and nonferrite spacers) along the axis of a solenoidal. Relatively high field strengths can be achieved with the CHI wiggler at shorter wiggler periods than is possible in many other conventional wiggler designs. The formulation is a slow-time-scale analysis of the interaction of an annular electron beam with the CHI wiggler in a coaxial waveguide. Simulations are presented for W-band operation. The results indicate that operation over a wide bandwidth is practical, and that the uniform- and tapered-wiggler bandwidths are comparable.

Nonlinear theory of the free-electron laser based upon a coaxial hybrid wiggler

A three-dimensional nonlinear formulation of a free-electron laser based upon a coaxial hybrid iron (CHI) wiggler is described. The CHI wiggler is created by insertion of a central rod and an outer ring [composed of alternating ferrite and dielectric spacers in which the ferrite (dielectric) spacer on the central rod is opposite to the dielectric (ferrite) spacer on the outer ring] along the axis of a solenoidal. An analytic model of the CHI wiggler is developed which is in good agreement with the Poisson/Superfish group of codes. The free-electron laser (FEL) formulation is a slow-time-scale analysis of the interaction of an annular electron beam with the CHI wiggler in a coaxial waveguide. The electromagnetic field is represented as the superposition of the vacuum transverse electric (TE), transverse magnetic (TM), and transverse electromagnetic (TEM) modes of the waveguide, and a set of nonlinear second-order differential equations is derived for the amplitudes and phases of these modes. These equations are solved simultaneously with the three-dimensional Lorentz force equations for the combined magnetostatic and electromagnetic fields. An adiabatic taper is used to model the injection of the beam, and an amplitude taper is included for efficiency enhancement. Simulations are presented for Ka-, Ku- and W-band operation. Multimode operation is also studied. The results indicate that operation over a wide bandwidth is practical with the CHI wiggler, and that the bandwidth in the tapered-wiggler cases is comparable to that for a uniform wiggler. Therefore, relatively high field strengths can be achieved with the CHI wiggler at shorter wiggler periods than is possible in many other conventional wiggler designs.