Three-dimensional simulation of long-wavelength free-electron lasers with helical wiggler and ion-channel guiding

A three-dimensional simulation of a steady-state amplifier model of a long-wavelength free-electron laser (FEL) with realizable helical wiggler and ion-channel guiding is presented. The set of coupled nonlinear differential equations for electron orbits and fields of TE 11 mode in a cylindrical waveguide are solved numerically by the Runge-Kutta algorithm with averages calculated by the Gaussian quadrature technique. Self-fields and space-charge effects are neglected, and the electron beam is assumed to be cold and slippage is ignored. The parameters correspond to the Compton regime. Evolution of the radiation power and growth rate along the wiggler is studied. Ion-channel density is chosen to obtain optimum efficiency. Simulations are preformed for the FEL operating in the neighborhood of 35 GHz and 16.5 GHz for the electron beam energies of 250 keV and 400 keV, respectively. The result of the saturated efficiency was found to be in good agreement with the simple estimation based on the phase-trapping model.     

Three-dimensional Wigner-function description of the quantum free-electron laser

A free-electron laser (FEL) operating in the quantum regime can provide a compact and monochromatic x-ray source. Here we present the complete quantum model for a FEL with a laser wiggler in three spatial dimensions, based on a discrete Wigner-function formalism taking into account the longitudinal momentum quantization. The model describes the complete spatial and temporal evolution of the electron and radiation beams, including diffraction, propagation, laser wiggler profile and emittance effects. The transverse motion is described in a suitable classical limit, since the typical beam emittance values are much larger than the Compton wavelength quantum limit. In this approximation we derive an equation for the Wigner function which reduces to the three-dimensional Vlasov equation in the complete classical limit. Preliminary numerical results are presented together with parameters for a possible experiment.     

Three-dimensional simulation analysis of a 3 cm wavelengthfree-electron laser afterburner

We have simulated a 3 cm wavelength free-electron laser afterburner (FEL Afterburner) using two sets of parameters: one is for a 3-cm period wiggler and the other is for a 5.4 cm period wiggler. For the 3 cm period wiggler, the input beam energy is 112.5 keV, and for the 5.3 cm period wiggler the beam energy is increased to 290 keV to make the FEL Afterburner operate at the same frequency. It is found, from the simulations, that the FEL Afterburner with a longer period wiggler has a higher power conversion efficiency: larger than 16% $ for the 5.4 cm wiggler while only about 9% for the 3 cm wiggler. It is also shown that to enhance the interaction efficiency in the slow wave cavity, the slow wave number should be a little larger than the sum of the fast wave number and the wiggler wave number     

Chaotic electron trajectories in electromagnetic wiggler free-electron laser with a guide magnetic field

Summary The Hamiltonian for an electron travelling through a large-amplitude backward electromagnetic wave, an axial guide magnetic field and radiation field is formulated. Poincaré surface-of-section plots show that this Hamiltonian is non-integrable, and leads to chaotic trajectories. Equilibrium conditions are derived in the limit where the radiation field approaches zero. Compared to conventional FEL, the total energy of the system at pondermotive resonanceE _c is large, while the electron's critical energy γ_c is low for electromagnetic wiggler FEL. Moreover, the threshold wave amplitude (A _r=A _c) of beam chaoticity is found at lower values of the radiation field amplitude compared to magnetostatic wiggler FEL. Previous features confirmed that electromagnetic wiggler FEL can operate more coherently and more efficiently at moderated particle's energy compared to magnetostatic wiggler FEL.     

CAEP远红外100 μm FEL自发辐射谱的实验研究

在FEL实验中,电子束通过摇摆器,一方面由于周期性磁场作用,电子束轨迹要周期性的摆动,另一方面还要辐射同FEL辐射波长一致的自发辐射,该辐射谱反映电子束、摇摆器集成后的参数。在CAEP(Institute of China Academic Engineering Physics)远红外100 μm FEL实验中,自发辐射谱通过Ge∶Ga低温探头和远红外100 μm光栅谱仪测量。文章侧重从实际摇摆器磁场分析了远红外100 μm FEL的自发辐射谱。     

A slow wave free-electron laser

A simple calculation of a free-electron laser in the Compton regime that uses a dielectric-lined waveguide is presented. The introduction of a dielectric lining in a free-electron laser considerably reduces the requirements on beam voltage for generating a given frequency ω₁=k₀c/(1 - v_b η/c), where k₀ is the wiggler wave period η is the effective index of refraction (1<η<√ϵ) and ϵ is the permittivity. The system supports electromagnetic waves whose Poynting flux is largely concentrated in the dielectric; hence the electron beam is required to propagate close to the dielectric lining. The mode structure and dispersion behavior of the guiding system without the beam are discussed. a thin annular beam is introduced, and a perturbation theory is used to obtain the frequency and growth rate of radiation     

Three dimensional gain analysis of free electron laser with focusing permanent magnet wiggler

A three dimensional simulation software system developed to estimate a free electron laser (FEL) gain has been applied to FEL using a standard plane polarized wiggler and an alternately shifted magnet wiggler. It is seen for the latter wiggler that a large filling factor could be selected and each maximum gain corresponding to each orbit of electron beam concentrates at a certain frequency region of FEL radiation. It is, therefore, implied that a proper shift between the adjacent magnets in the wiggler produces the improvement of the FEL gain.     

Effect of normalized plasma frequency on electron phase-space orbits in a free-electron laser

Irregular phase-space orbits of the electrons are harmful to the electron-beam transport quality and hence deteriorate the performance of a free-electron laser （FEL）. In previous literature, it was demonstrated that the irregularity of the electron phase-space orbits could be caused in several ways, such as varying the wiggler amplitude and inducing sidebands. Based on a Hamiltonian model with a set of self-consistent differential equations, it is shown in this paper that the electron- beam normalized plasma frequency functions not only couple the electron motion with the FEL wave, which results in the evolution of the FEL wave field and a possible power saturation at a large beam current, but also cause the irregularity of the electron phase-space orbits when the normalized plasma frequency has a sufficiently large value, even if the initial energy of the electron is equal to the synchronous energy or the FEL wave does not reach power saturation.     

Quantum regime of a plasma‐wave‐pumped free‐electron laser in the presence of an axial magnetic field

The quantum regime of a plasma‐whistler‐wave‐pumped free‐electron laser (FEL) in the presence of an axial‐guide magnetic field is presented. By quantizing both the plasma whistler field and axial magnetic field, an N‐particle three‐dimensional Hamiltonian of quantum‐FEL (QFEL) has been derived. Employing Heisenberg evolution equations and introducing a new collective operator which controls the vertical motion of electrons, a quantum dispersion relation of the plasma whistler wiggler has been obtained analytically. Numerical results indicate that, by increasing the intrinsic quantum momentum spread and/or increasing the axial magnetic field strength, the bunching and the radiation fields grow exponentially. In addition, a spiking behavior of the spectrum was observed with increasing cyclotron frequency which provides an enormous improvement in the coherence of QFEL radiation even in a limit close‐to‐classical regime, where an overlapping of these spikes is observed. Also, an upper limit of the intrinsic quantum momentum spread which depends on the value of the cyclotron frequency was found.     

Amplification and superradiant emission from a 33.3 GHz freeelectron laser with a reversed axial guide magnetic field

A new regime of free-electron laser (FEL) operation using a helical wiggler field and a reversed axial guide magnetic field is reported. The orientation of the axial field is such as to oppose the electron rotation imparted by the helical field. The 33.3-GHz FEL amplifier is driven by a mildly relativistic electron beam (750 kV 300 A, 30 ns) and generates 61 MW of radiation with a 27% conversion efficiency. The results are compared with those obtained when the axial guide field is in its conventional orientation, where considerable loss of power and efficiency is observed     

带Wiggler场平板型部分填充介质CFEL的切仑柯夫自由电子激光的动力学耦合模理论

用二维波动力学耦合模理论研究了带Wiggler场平板型部分填充介质的FEL。研究结果表明,该系统可用低能电子束产生对介质厚度无特殊要求的短波长FEL。适当选择参数,可使其增长率和效率高于普通自由电子激光和切仑柯夫自由电子激光,并证明本系统超辐射放大的工作主模应是纵向截面电场模。     

Linear Theory of Free Electron Laser with a Plasma-Loaded Cylindrical Waveguide

The dispersion characteristics of plasma–loaded free-electron laser has been analyzed using linear fluid model. The device under consideration consists of the cylindrical metallic waveguide, completely filled with background plasma and a relativistic electron beam which passes through a helical wiggler magnetic field. The result predicts that reasonable plasma density tends to improve the growth rate of the low-frequency optical wave of FEL and causes an shiftup in the operating frequency, However it has little effect on the growth rate of the high-frequency wave. In the plasma–loaded FEL, for the FEL oscillator, it may be tuned by varying the plasma density; and for the FEL amplifier, the wider frequency bandwidth is gained. A critical density n ^c _p for the background plasma density is found.     

Identification of the Amplification Mechanism in the First Free-Electron Laser as Net Stimulated Free-Electron Two-Quantum Stark Emission

We find that the electron phase with respect to the incident laser radiation must be random in the first freeelectron laser （FEL） and, hence, the incident laser radiation works as a relaxation force to keep a Maxwellian distribution. We formulate the threshold laser intensity for amplification which agrees with the measured value in the order of magnitude in the first FEL. The magnetic wiggler must produce an electric wiggler whose period is the same as that of the magnetic wiggler. We find that net stimulated free-electron two-quantum Stark （FETQS） emission driven by this electric wiggler is the mechanism responsible for the measured gain and the measured laser intensity at the plateau in the first FEL.     

High-gain free-electron-laser amplifier with warm plasmabackground: linear analysis

The linear free-electron laser (FEL) theory with plasma background is considered using the hybrid model in contrast with the fluid model to describe the FEL interaction with plasma proposed by Weng-Bing and Ya-Shen (1988) and Tripathi and Liu (1990). The basic dynamical equations for the FEL with warm plasma background are derived for all ranges of plasma and beam densities with unspecified wiggler period number and strength. The linear behavior of the FEL is analyzed     

High-efficiency operation of free-electron-laser amplifiers

A high-efficiency regime was studied for free-electron-laser (FEL) amplifier configurations that employ a helically symmetric wiggler and a uniform-axial-guide magnetic field. Efficiencies on the order of 50% are found to be possible in the presence of axial fields sufficiently strong that the Larmor period is shorter than the wiggler period. This operating regime is accessible for electron-beam energies greater than approximately 400 keV     

Mode-selective amplification in a waveguide free electron laserwith a two-sectioned wiggler

One important issue in waveguide free electron lasers (FELs) involves an interaction of the electron beam with one waveguide mode at two different resonant frequencies. Since the low-frequency mode often has a higher pain, the usually preferred high-frequency mode is suppressed as a result of mode competition. In this paper, possible control of this mode competition is considered using a nonstandard wiggler magnet consisting of two cascaded wiggler sections with different periods and field strengths. It is demonstrated that with an appropriate differentiation between the two wiggler sections the high-frequency mode may be amplified preferentially. This mode-selective amplification may be used to suppress the low-frequency mode. A small signal gain formulation is developed for a waveguide FEL with such a two-sectioned wiggler arrangement and numerical examples are used to demonstrate its applicability to mode control in waveguide FELs. Effects of wiggler field errors and electron energy spread are also considered. It is shown that the requirement for wiggler field errors and electron energy spread in the two-sectioned wiggler arrangement is similar to that in the usual straight wiggler configuration     

Non-relativistic quantum theory of stimulated Cherenkov radiation and Compton scattering in plasma

The stimulated processes in electron plasma, i.e., Cherenkov radiation by a nonrelativistic electron beam of longitudinal oscillations and Compton scattering of a transverse electromagnetic wave in plasma with quantum mode excitation (de Broglie wave), are considered in the three-wave approximation. The possibility of the occurrence of quantum oscillations is discussed.     

Studies of classical radiation emission from plasma wave undulators

The authors examine the characteristics of the classical radiation emitted by a relativistic electron beam that propagates perpendicularly through a large amplitude relativistic plasma wave. Such a study is useful for evaluating the feasibility of using relativistic plasma waves as extremely short wavelength undulators for generating short wavelength radiation. The electron trajectories in a plasma wave undulator are obtained using perturbation techniques and are then compared to numerical simulation results. The frequency spectrum and angular distribution of the spontaneous radiation emitted by a single electron and the stimulated radiation gain are obtained analytically, and are then compared to 3-D numerical simulations. The characteristics of the plasma wave undulator are compared to the AC free-electron laser (FEL) undulator and the conventional FEL