Free-electron lasers with electromagnetic standing wave wigglers

A detailed analysis of the electromagnetic standing wave wiggler for free-electron lasers (FEL's) is conducted for both circular and linear wiggler polarizations, following a single-particle approach. After determination of the unperturbed electron orbits in the wiggler field, the single-particle spontaneous emission spectrum and subsequently the gain in the low gain Compton regime (using the Einstein coefficient method) are explicitly calculated. This analysis results in a clear understanding of the resonance conditions and the coupling strength associated with each resonance of this type of FEL. In particular, a striking feature obtained from this investigation is that the electromagnetic standing wave wiggler FEL, under certain circumstances, exhibits a rich harmonic content. This harmonic content is caused by the presence of both the forward and backward wave components of the standing wave wiggler field. In addition, the nonlinear self-consistent equations for this type of FEL are also presented, permitting further investigation of it by the theoretical techniques and numerical codes developed for conventional FEL's.     

CHARACTERISTIC STUDY OF A FREE ELECTRON LASER WITH LINEARLY POLARIZED WIGGLER AND RECTANGULAR WAVEGUIDE

The characteristics of coherent radiation produced by a cylindrical electron beam passing through a rectangular waveguide and linearly polarized wiggler are studied. The instability analysis is based on the linearized Vlasov-Maxwell equations for the perturbations about a self-consistent beam equilibrium. The dispersion equation of TMmn mode is deduced and by making use of numerical calculation the radiation frequency and growth rate as a function of electron beam energy and radius, axial magnetic field, wiggler field and wave length are presented and discussed.     

Three-dimensional theory of free electron lasers with an axial guide field

The collective interaction in a free electron laser with a combined helical wiggler and uniform axial guide field is presented in the low-gain regime. The wiggler model we employ is fully self-consistent and includes all transverse inhomogeneities. The analysis is performed for a free electron laser (FEL) amplifier in which the radial dependence of the radiation is treated using both the TE and TM waveguide modes. Substantial discrepancies are found to exist between the results for the realizable and ideal wigglers, and a selection rule relating the TElnand TMlnmodes with resonant amplification at thelth harmonic of the FEL Doppler upshift.     

Mode analysis of a dielectric-loaded Raman-type free-electron laser

The Raman-type free-electron laser consists of a relativistic electron beam contained in a dielectric-induced parallel plate waveguide and an array of permanent magnets for the wiggler. Under the influence of the periodic magnetostatic field, the coupling between the scattered electromagnetic wave of the TE mode (positive-energy wave) and the electron plasma wave of the TM mode (negative-energy wave) is investigated in detail. The following results are obtained. First, when a dielectric sheet is loaded on the waveguide, the maximum growth rate and the oscillation frequency can be greater than those for the vacuum Raman-type free-electron laser. Second, by choosing proper values for the relative permittivity of the dielectric sheet and the ratio of the beam guide, the beam energy can be greatly lowered without degrading the oscillation characteristics. Third, the growth rate decays exponentially with the oscillation frequency kept almost constant as the beam-dielectric gap increases     

Comparison of dc electric field and tapered wiggler free electron laser efficiency enhancement schemes

A constant parameter wiggler free electron laser using a dc electric field for efficiency enhancement is compared to several tapered wiggler efficiency enhancement schemes. Analytical expressions for the efficiency in the plane wave, infinitesimally small radius electron beam limit are derived and compared. Numerical simulations for a Gaussian radiation field and finite radius electron beam with an output radiation wavelength of 2 μm are presented. For finite radius electron beams, the extraction efficiency using a dc electric field is somewhat greater than or equal to that using proposed tapering schemes. While the dc field offers flexibility in efficient, tunable laser design, the required field strengths for visible radiation are in the megavolt/meter range.     

Principles of gyrotron powered electromagnetic wigglers for free-electron lasers

The operation of free-electron lasers (FEL's) with axial electron beams and high-power electromagnetic wiggler fields such as those produced by high-power gyrotrons is discussed. The use of short wavelength electromagnetic wigglers in waveguides and resonant cavities can significantly reduce required electron beam voltages, resulting in compact FEL's. Gain calculations in the low- and high-gain Compton regime are presented, including the effects of emittance, transverse wiggler gradient, and electron temperature. Optimized scaling laws for the FEL gain and the required electromagnetic wiggler field power are discussed. Several possible configurations for FEL's with electro-magnetic wigglers powered by millimeter wavelength gyrotrons are presented. Gyrotron powered wigglers appear promising for operation of compact FEL's in the infrared regime using moderate energy (<10 MeV) electron beams.     

Plasma Induced Wiggler Magnetic Field in a Cavity

The transformation of an elliptically polarized standing wave in a cavity by a suddenly and uniformly created plasma is discussed. Theoretical expressions for the plasma induced wiggler magnetic field as well as the frequency-upshifted standing wave are derived. By choosing appropriate values of the source wave parameters and plasma parameters, one can get wiggler magnetic field of desired magnitude, direction and wiggler wavelength. A few representative results are discussed.     

The influence of equilibrium self-fields on the spontaneous emission and stimulated emission of fel with a linearly polarized wiggler

The orbits of an electron in a linear wiggler and axial field under the influence of self-fields are calculated. Then the spontaneous emission coefficient and the growth rate of the stimulated scattering are obtained. The numerical results show under some condition the self-fields can enhance the radiation.     

行波场Wiggler自由电子激光器

分析了辐射行波场Wiggler自由电子激光器的工作状态,计算了激光器的增益,分析了获得正增益的条件。     

平衡自场对有线性摆动器的自由电子激光器的自发辐射和受激辐射的影响

本文计算了在平衡自场、线性摆动器场和轴向磁场作用下的电子轨道,并由此求得有线性摆动器的自由电子激光器的自发辐射和受激辐射。结果表明自场有增强辐射的作用。     

A novel small-period wiggler and simulations of its free-electronlaser

A small-period wiggler constructed of edgy-wound bifilar-helical conducting sheets with ferromagnetic cores, which is intended for free-electron lasers, is presented. The performance characteristics of the wiggler fields with a 100 mm period are measured. A field as high as 1500 G has been obtained. Free-electron lasers with this small-period wiggler have been investigated numerically with a three-dimensional nonlinear theory. Simulation results estimated that a radiation power of 20.2 MW and a frequency of 170 GHz with an efficiency of 5.1% can be obtained. It is feasible to make Raman free-electron lasers with this type of wiggler operating in the millimeter and submillimeter wavelength range with a relatively low electron energy (<500 keV) beam     

Phase variation in free-electron laser amplifiers

The evolution of the phase of the output radiation of free-electron laser amplifiers is investigated by means of a three-dimensional simulation code. The configuration employed consists of the propagation of a relativistic electron beam through a loss-free cylindrical waveguide in the presence of a helically symmetric wiggler and a uniform axial guide magnetic field. The analysis is fully three dimensional, and a set of model equations is discussed which describes the coupling between an ensemble of electrons and the radiation field of either the TE or TM modes. The model equations are solved numerically, and the output phase is studied with respect to variations in either frequency or electron beam energy. The output phase is found to depend sensitively on the wave frequency within the unstable bandwidth. In addition, the phase stability of the output radiation is discussed as a function of beam energy. Finally, the evolution of the phase for tapered wiggler free-electron lasers is studied.     

Novel approaches to FEL operation: The gas-loaded FEL and a high efficiency FEL design

Two novel methods for improving free-electron laser (FEL) oscillator performance are discussed: (a) The gas-loaded FEL (GFEL) allows operation at snorter wavelengths for a given accelerator energy and wiggler. Experimental results of laser operation with a gas retention foil in the electron beam line and with the introduction of gas to the wiggler cavity are presented, (b) An FEL design utilizing a time-ramped microwave field to accelerate electrons as they lose energy to radiation allows for high conversion efficiencies. Parameter constraints for such an FEL are discussed, leading to a structure that integrates a wiggler with a linac. It is shown that conversion efficiencies of 50% at λ = 10 μm with a 2m wiggler length can be achieved for typical FEL parameter values without sacrificing small-signal gain     

The effects of field errors on low-gain free-electron lasers

The effects of random wiggler magnetic field errors on low-gain free-electron lasers (FELs) are examined analytically and numerically through the use of ensemble averaging techniques. Wiggler field errors perturb the electron beam as it propagates and lead to a random walk of the beam centroid, variations in the axial beam energy, and deviations in the relative phase of the electrons in the ponderomotive wave. The random walk of the beam centroid and the consequent variations in the axial beam energy are discussed. The deviations in the relative phase resulting from the field errors are examined. The effect of the field errors on the FEL gain in the low-gain regime is determined. The benefits of beam steering are analyzed, and addition methods for reducing the detrimental effects of field errors are discussed     

电子在多层膜摇摆器中的辐射

提出了利用多层膜作自由电子激光器的摇摆器,利用虚光子和康普顿散射讨论了运动电子在多层膜摇摆器中的自发辐射。(Fe/Cr)N等铁磁-非磁层状材料的铁磁层之间存在反铁磁耦合,因此可以利用磁性多层膜制作自由电子激光器的摇摆器。考虑到静磁场的空间周期只有7.8nm,要精确计算电子在多层膜摇摆器中运动的自发辐射的波长和辐射功率就必须考虑电子的反冲。给出了运动电子在多层膜摇摆器中的自发辐射波长和辐射功率的精确计算表达式。结果发现,自发辐射波长和辐射功率表达式中都含有康普顿波长。     

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.     

Field Distribution of a Planar Electrostatic Wiggler and Modulation Effect on the Motion of Relativistic Electrons

A planar electrostatic wiggler is formed by two parallel metallic plates, where the upper-plate is corrugated with sinusoidal ripples and connected to a negative voltage and the lower-plate is smooth and grounded. The field distribution is mathematically derived in detail. It is demonstrated that this planar electrostatic wiggler can efficiently modulate the motion of relativistic electrons just as a magneto-static wiggler does in a free-electron laser. Results obtained here will provide basis to analyze the amplification mechanism of a fast wave by a relativistic electron beam in a planar electrostatic wiggler.     

Frequency and Polarization Transformer: Transverse Modes: II Finite Rise Time

A frequency transformer that converts a linearly polarized standing wave into frequency shifted extraordinary (X) standing waves is considered. The transformer is a one-dimensional cavity in which a magnetoplasma, that supports ldquotransverse modes,rdquo is created. The plasma is assumed to be created in finite rise time. The broad conclusions, arrived in a preceding paper based on zero rise time, that the switching would result in transformation of the original source wave into 3 new waves including a wiggler electric as well as a wiggler magnetic field mode are shown to be still valid. However, it is shown that the amplitudes of the various modes are affected by the rise time. Finite-difference time-domain technique is used to study the effect of the finite rise time.     

变周期wiggler自由电子激光器

分析了由于相对论电子在wiggler内传播过程中,相对论因子丁不断地减小,引起辐射频率移动,致使激光器的增益下降。提出采用空间周期随电子传播方向变化的wiegler,可维持辐射频率不变,从而提高激光增益。文中给出空间周期可采用的变化规律。     

Resotron: A Novel Tunable Millimeter and Submillimeter Wave Source of Radiation

The need for tunable radiation sources in the millimeter and submillimeter range for spectroscopic purposes is still a research area of great interest. The tunable radiation source, proposed in this paper, is a special free electron laser device with the prerequisite of low electron energy. The output power density in the millimeter (GHz)-range is of the order of MW/cm² and in the submillimeter (THz)-range of the order of kW/cm². The device consists of an electron source with electron optics, wiggler/microwiggler, and a longitudinal magnetic field. The wiggler for the THz-regime has a very short wiggler period of approximately 400 μm and could be manufactured with laser micromachining techniques. The free electron laser operates in magnetoresonance and shows surprisingly stable electron orbits and therefore narrow output frequencies. Computational results of the temporal behavior of the output power done with a multi-frequency code are reported.