The gain and efficiency improvement of a free-electron laser by anoptical klystron configuration

The feasibility of a high-grain amplifier in an optical klystron configuration at wavelengths of 1-2 mm was investigated using a computer simulation. A one-dimensional single-particle model for the free-electron laser (FEL) interaction was used in the numerical studies. It was found that for a three-cavity optical klystron gain of at least 30 dB and relatively high efficiency is possible for relatively modest electron-beam parameters (V⩽500 kV, J≈100 A/cm²)     

北京自由电子激光器的光学速调管结构

采用KMR方程,结合即将运转的北京自由电子激光器总体实验参数,对其光学速调管结构研究.详细分析计算输入功率、电子束能散度、色散磁场、漂移空间长度及位置、以及电子束流等参数对光学速调管增益的影响.基于北京自由电子激光器的振荡器结构,提出一组对电子束能散度要求适中的实用化光学速调管参数.并对其饱和功率、功率谱以及渐变摆动器进行分析.     

The simulation of a free-electron laser amplifier with a ribbon relativistic electron beam

An amplifier based on a high-power free-electron planar laser operating at a wavelength of 4 mm is simulated. The system is built around the U-3 accelerator, which forms a ribbon relativistic electron beam with an energy of up to 1 MeV and a total operating current of up to 2 kA. The simulation uses various approaches, including the direct numerical simulation of the Maxwell equations and particle motion equations by means of the PIC-code KARAT. The approaches are shown to give close results.     

Degradation in gain for a free electron laser amplifier due to electron momentum spread

A finite spread in axial momentum for the electron beam in a free electron laser amplifier is shown to decrease the small-signal gain. For millimeter and sub-millimeter wave amplifiers, where exponential growth dominates the gain, it is shown that the gain is approximately 3 db below that for a cold beam if the relative momentum spread (u/u)_1/2 = (G_o/248)^1/2 (_o/L), where G_o1 is the gain in db for the cold-beam case, _o is the magnetic wiggler period, and L is the amplifier length. Exact numerical examples are given for representative FEL amplifiers at 35 and 550 GHz.This research was sponsored by the U.S. Office of Naval Research, and by the U.S.-Israel Binational Science Foundation.     

Self-amplified spontaneous emission free-electron laser with an energy-chirped electron beam and undulator tapering

We report the first experimental implementation of a method based on simultaneous use of an energy chirp in the electron beam and a tapered undulator, for the generation of ultrashort pulses in a self-amplified spontaneous emission mode free-electron laser (SASE FEL). The experiment, performed at the SPARC FEL test facility, demonstrates the possibility of compensating the nominally detrimental effect of the chirp by a proper taper of the undulator gaps. An increase of more than 1 order of magnitude in the pulse energy is observed in comparison to the untapered case, accompanied by FEL spectra where the typical SASE spiking is suppressed.     

Analytical formula of free electron laser exponential gain for a non-resonant electron beam

The free electron laser(FEL) gain formulas for a non-resonant case are studied, and some new rigorous analytical formulas are given explicitly. For the mono-energetic and non-resonant electron beam, the exact expression of the solution of the FEL characteristic cubic equation is obtained with a form much more simple than that in the literatures, and the gain length as the function of the detuning parameter is explicitly given. Then the gain for different detuning parameters and from low to high can be easily calculated. A simplified approximation formula is also given for the exponential gain calculation in the non-resonant case. For the case of the electron beam with an energy spread, the solution of the characteristic cubic equation is given explicitly for rectangular energy distribution and Lorentz distribution, respectively. Moreover the explicit expression also can be used for the solution of the characteristic cubic equation including the impact of the space charge. The transition from the low gain to the high gain is analyzed. The variations of the gain bandwidth and of the detuning parameter for the maximum gain are demonstrated. The applicable ranges of the small signal gain formula and the exponential gain formula are analyzed.     

Measurement of beam quality degradation due to spherical aberration in a simple lens

We have measured the effect on laser beam quality factorM ² of spherical aberration in a simple planoconvex lens. Theory predicts that the laser beam quality should remain essentially unchanged until the incident beam widthw exceeds a critical widthw _q above which the beam quality will degrade rapidly, with theM ² value increasing as (w/w _q ⁴). We find good agreement between this theory and our experimental data.     

Start current and gain measurements for a Smith-Purcell free-electron laser

Coherent Smith-Purcell radiation is a promising source of coherent emission in the THz domain. Although it has been observed in several experiments, some physical quantities related to the bunching of an initially continuous beam had not yet been studied experimentally. Among them, the gain as function of beam current, together with the value of the start current, needed to be addressed. We report here their measurements in a microwave experiment using a sheet beam. A start current of about 20 A/m was found. Two-dimensional simulations with a very thin beam agree well with our results.     

Effect of beam energy spread on cascade optical klystron undulator radiation

In this paper we analyze the effect of beam energy spread on cascade optical klystron undulator radiation. The analysis suggests that stronger dispersion strength needs a better beam quality in both single stage and cascade optical klystron undulator radiation.     

A free-electron laser generated gamma-ray beam for nuclear physics

The High Intensity Gamma Ray Source (HIγS), a collaborative project between TUNL and the Duke Free Electron Laser Laboratory, is described. An initial experiment and plans for a future research program are discussed briefly.     

On the theory of electron motion due to superposed em waves and its application to the free-electron laser with linear polarization

The relativistic motion of a charged particle in an electromagnetic field depending only on one coordinate is shown to be exactly equivalent to the unidimensional motion of a particle having a time and space-varying mass. By means of the averaged Lagrangian method an approximate solution is obtained for the relativistic electron motion in two oppositely moving waves of any polarization, one of them being weaker than the other one, thus generalizing the results previously obtained for circular polarization. A comparison with numerical calculations shows a good agreement. A pendulum-like motion in an effective periodic potential is shown to exist not only for two waves of the same frequency, but more generally when the frequency of the weaker wave is an odd harmonic of the frequency of the stronger one.This research was supported in part by CNR and the Italian Ministry of Education     

An optical device to homogenize a laser beam

Based on segmentation-recombination principle, a specific optical device is designed to homogenize a highpower CO2 laser beam which is used as a heating source.     

Non-linear propagation of laser beam and focusing due to self-action in optical fiber: Non-paraxial approach

A somewhat more general analysis for solving spatial propagation characteristics of intense Gaussian beam is presented and applied to the laser beam propagation in step-index profile as well as parabolic profile dielectric fibers with Kerr non-linearity. Considering self-action due to saturating and non-saturating non-linearity in the refractive index, a general theory has been developed without any kind of power series expansion for the dielectric constant as is usually done in other theories that make use of paraxial approximation. Result of the steady state self-focusing analysis indicates that the Kerr non-linearity acts as a perturbation on the radial inhomogeneity due to fiber geometry. Analysis indicates that the paraxial rays and peripheral rays focus at different points, indicating aberration effect. Calculated critical power matches with the experimentally reported result.     

Analysis of the effects of plasmas and beam slippage in optical klystron FEL devices

We examine here the effects of plasmas and beam slippage on the gain of an optical klystron (OK) free electron laser (FEL) device under the influence of a weak guiding magnetic field and background plasma. The beam energy spread decreases with background plasma density nop and increases with beam plasma density nob. The maximum gain (Gmax) scales proportional to . The beam slippage phase Δψ scales directly to five power of relativistic factor γ₀ and one half power to nob.