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     

自由电子激光器中轴向磁场作用的实验研究

从约束强流电子束及回旋共振增大激光器增益两方面考虑,确定了喇曼自由电子激光器轴向引导磁场的工作参数,并进行了实验测试。     

Finite-temperature effects in free-electron lasers

The effect of energy spread on the gain of a dense-beam free-electron laser amplifier is analyzed. The formalism includes collective effects and allows for the use of an arbitrary distribution function. The dispersion relation is solved numerically. Besides broadening of the spectrum and decrease in gain, lowering of the central emission frequency with energy spread is observed. An initial-value problem is formulated and numerically solved, with power computed as a function of distance. The relative contributions of the different modes when temperature changes are analyzed. In particular, the competing effects of a decrease in coupling loss with temperature together with Landau damping are investigated.     

Nonlinear harmonic generation in free-electron lasers

A three-dimensional nonlinear simulation code to treat multiple frequencies simultaneously is described and used to study nonlinear harmonic generation in free-electron lasers (FELs). Strong nonlinear harmonic gain is found where the gain length varies inversely with the harmonic number. Substantial power levels are found in the harmonics. The odd harmonics are favored with generally higher power levels since a planar wiggler geometry is employed; however, the second harmonic exhibits substantial power as well. The analysis is relevant to the emission expected from self-amplified spontaneous emission (SASE) free-electron laser schemes     

Intensity saturation mechanism in free-electron lasers

It is shown that the free-electron laser (FEL) exhibits a scaling of the gain saturation versus the intensity analogous to that of conventional lasers. The concept of FEL intensity saturation is introduced and its link to the maximum output intensity is discussed     

自由电子激光器的工作条件

分析了光辐射与相对论电子相互作用过程中,光辐射被相对论电子放大的条件。结果表明,只有在满足位相关系和频率关系的条件下,入射辐射才有可能获得较高的净增益。     

Suppression of feedback oscillations in free-electron lasers

The suppression of feedback oscillations in a free-electron laser (FEL) oscillator has been investigated. It is found, through numerical simulation that a narrow attenuating strip inserted in a waveguide will not reduce FEL performance at the desired frequency and will attenuate feedback oscillations. Simulations are presented for a 5-mm-wavelength FEL with a 40-dB attenuator inserted in the interaction region     

Wavelength and emittance requirements in free-electron lasers

Improvements in electron beam quality are required for operation of free-electron lasers (FELs) at short wavelengths and low voltages. The authors investigated this scaling for a FEL amplifier in the exponential regime of operation wherein the optical beam is tied to the electron beam as a result of gain focusing, subject to the requirements of (1) good geometrical overlap between the radiation beam and the matched electron beam and (2) cold-electron-beam interaction. The scaling is given by simple analytical formulas     

Thermal effects on the linear gain in free-electron lasers

The authors develop a self-consistent formulation of the linear gain in both helical and planar wiggler configurations in the presence of an axial energy spread derived from a beam pitch-angle spread. Such a beam may be thought of as monoenergetic but with a non-vanishing emittance. The analysis includes collective Raman effects for both the helical and planar wiggler systems, and describes the gain at harmonics in the case of a planar wiggler. General dispersion equations are derived, and solved numerically, for each wiggler configuration which includes a general thermal function which describes the effect of the pitch-angle spread     

High-brightness injectors for RF-driven free-electron lasers

Free-electron laser (FEL) oscillators require pulse trains of high peak brightness and, in some applications, high average power. Recent developments in the technology of photoemissive and thermionic electron sources in RF cavities for electron linac-injector applications offer promising advances over conventional electron injectors. Reduced emittance growth in high peak current electron injectors may he achieved by using high field strengths and by linearizing the radial component of the cavity electric field at the expense of lower shunt impedance. Harmonically excited cavities have been proposed for enlarging the phase acceptance of linac cavities and thereby reducing the energy spread produced in the acceleration process. Operation of injector linacs at a subharmonic of the main linac frequency is also proposed for enlarging the phase acceptance.     

Electron-beam quality requirements for tapered-wiggler free-electron lasers

A general approach to optimization of the free-electron laser interaction is used to develop the scaling of emittance and energy spread requirements for tapered wigglers optimized for highest optical gain at fixede-beam energy extraction. The requirede-beam properties for a high extraction oscillator are found to be quite stringent at visible wavelengths, but state-of-the-art accelerators should be sufficient. The applicability of various methods of emittance acceptance enhancement is examined. One very promising option is a magnet canting scheme for providing two-plane focusing in planar wigglers. Two-plane focusing relaxes the severe emittance requirement resulting from the need to maintain spatial overlap between the optical beam and the free-expandinge-beam. In addition, options for adjusting various system parameters for enhanced emittance acceptance, at reduced gain per unit current, are explored.     

Smith-Purcell型自由电子激光器的增益特性

本文应用耦合模近似以及电子Bloch方程相结合的方法,从量子力学的角度计算了受激Smith-Purcell型自由电子激光器的增益,并探讨了通过改变光栅空间频率以提高增益的可能性。     

Low-voltage infrared free-electron lasers based on gyrotron-poweredRF wigglers

Designs are presented for infrared free-electron lasers (FEL's) with gyrotron-powered electromagnetic wigglers and relatively low voltage electron beams. The use of an electromagnetic wiggler formed by a quasi-optical gyrotron, where the radiation is coupled into a corrugated waveguide, has the potential for substantial increases of FEL gain compared to an open mirror system. Designs for proof-of-principle low-voltage infrared FEL experiments based on both radio-frequency (RF) linear accelerator and electrostatic accelerator technology are presented together with point designs for portable systems covering the infrared windows in the atmosphere     

Relation between the cavity detuning and the output opticalintensity in free-electron lasers

The authors use a simple model of saturation in free-electron lasers operating with pulsed beams to study the intensity growth as a function of the cavity mismatch and of the coupling parameter. In spite of the simplicity of the model, the dependence of the intracavity power, as a function of the cavity detuning, is reproduced remarkably well. It is also pointed out that an enhancement of the optical power is obtained by adjusting the cavity length while the optical signal grows. The problem is analyzed qualitatively and realization schemes are proposed     

Correction to the adiabatic approximation for variable parameter free-electron lasers

The limits of validity of the adiabatic approximation commonly used in one-dimensional theories of variable parameter free-electron lasers (FEL) are obtained by formulating a Lie-transform perturbation theory in which the zeroth-order problem is the adiabatic approximation, modified to include any secular effects due to the nonadiabatic part of the Hamiltonian. It is shown that secular effects are absent in first order, and that the second-order secular terms can be neglected in practical FEL calculations. The lowest order nonsecular effects are evaluated by perturbation theory and are shown to yield small terms which can be easily included in Maxwell's equations for the signal field.     

面电流摆动器与线电流摆动器自由电子激光理论

从电子轨迹的相似性出发，用类比的方法得到了面电流摆动器和线电流摆动器自由电子激光的自发辐射谱；然后用单粒子理论推导出了自发辐射与受激光辐射的关系，根据这种关系求出了两种电流摆动器自由电子激光的小信号增益。     

Microtemporal and spectral structure of storage ring free-electron lasers

The microtemporal and spectral structure of free-electron lasers has been analyzed in the low gain, low field, and small slippage regime. Analytical expressions of the longitudinal eigenmode (super-modes) is derived for various configurations including the undulator, the optical klystron, and an intracavity dispersive element. Longitudinal and spectral narrowing of the laser pulse is predicted down to a limit set by the Fourier transform. On storage ring, it is found that early saturation prevents the laser from reaching this limit, in good agreement with results from the Orsay experiment. The insertion of a Fabry-Perot etalon in the cavity should make possible a relative linewidth on the order of 10^-6.     

The nonlinear wave equation for higher harmonics in free-electron lasers

The nonlinear wave equation and self-consistent pendulum equation are generalized to describe free-electron laser operation in higher harmonics; this can significantly extend their tunable range to shorter wavelengths. The dynamics of the laser field's amplitude and phase are explored for a wide range of parameters using families of normalized gain curves applicable to both the fundamental and harmonics. The electron phase-space displays the fundamental physics driving the wave, and we use this picture to distinguish between the effects of high gain and Coulomb forces.