A staggered-array wiggler for far-infrared, free-electron laseroperation

A staggered-array wiggler for a far-infrared free-electron laser (FEL) has been built at Stanford, and its magnetic properties have been tested. This type of wiggler has several desirable features: high wiggler field at short wiggler periods, wavelength tuning by a solenoid current, electron beam confinement by a solenoid field, and looser machining tolerances. A 10.8-kilogauss peak wiggler field has been measured at a 7.0-kilogauss solenoid field for a 1.0-cm wiggler period and a 2.0-mm gap. The small-signal gain has been calculated analytically and by computer simulation for a 0.5-m long wiggler. For an 8-A, 9-ps current pulse and a 3.3-MeV electron beam, 5-dB gain is predicted. Twenty- to thirty-percent wavelength tuning can be achieved by adjusting the solenoid field and still maintain reasonable small-signal gain. The pulsed-wire technique was employed to test the field uniformity of this novel wiggler, and the measured field variation was about 1%     

Hybrid Planar Wiggler with a Period of 2 cm for Higher Harmonic FEL Lasing

A hybrid planar wiggler with a period of 20 mm has been studied as the simplest one which gives the strong field including some higher harmonic components by selecting proper sizes of the ferromagnetic and the permanent magnet. Small gap length of the wiggler and small width of permendur satisfy these conditions to a certain degree. Gain analysis of FEL suggests that for high wiggler field of K>1 ～ 1.6, higher harmonic gains are improved primarily due to its strong field, and for low wiggler field of K< 1 ～ 1.6, they are mainly due to the modification of the wiggler field distribution.     

Analysis and nonlinear simulation of a quadrupole wiggler FEL atmillimeter wavelengths

The use of a helical quadrupole wiggler field in a circular guide for amplification of millimeter waves is discussed. The effects of space charge and axial field are considered for beam stability for near-axis orbits. When space charge is neglected, the orbit equations and properties and characteristic betatron oscillation of frequencies are examined by a linearization of the orbit equations. A nonlinear simulation code for dipole wigglers is modified to examine optimum gain TE₂₁ waveguide modes for a quadrupole wiggler in the 30-300 GHz range     

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     

在环型自由电子激光器中电子运动的混沌特性

本文分析了相对论性电子在环型摆动器磁场,轴向磁场和由非中性电子束产生的平衡自电场和自磁场中的运动。通过数值计算画出Poincare截面映射图,表明当自场足够强时,这种运动变成混沌的。虽然现实的环型摆动器场和自场一样使电子运动方程成为不可积的,但自场使运动产生混沌的作用要比摆动器场强。轴向磁场有抑制混沌发生的作用。     

无纵向电场分量离子波纹摆动器自由电子激光

本文在离子波纹摆动器中,提出改变电子束的入射方向,保证离子波纹场的纵向分量为零,从而消除了纵向电场对离子波纹摆动器的影响,在小振幅条件下,给出了摆动器的工作方程,导出了束电子的运动轨迹和自发辐射谱分布。     

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     

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.     

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.     

变参量Wiggler FEL的三维非线性理论

本文建立了变参量Wiggler自由电子激光放大器的三维自洽非线性理论。考虑电磁波、电子注和Wiggler场三者均在三维空间中变化,忽略电子注的自身场和空间电荷效应,导出了带有变参量双绕螺旋Wiggler场和轴向导引磁场的矩形波导,轴对称电子注自由电子激光器的非线性互作用微分方程组。数值模拟表明:在适当位置逐渐减小Wiggler场的幅值或周期均能有效地提高电子注的能量转换效率。     

Nonlinear Analysis of Relativistic Motion of Electrons in Self-Amplified Spontaneous Emission Free-Electron Laser in Ultraviolet and X-Ray Spectral Regions

The free-electron laser (FEL) based on the self-amplified spontaneous emission (SASE) is an effective candidate of the coherent optical sources at wavelengths in ultraviolet and x-rays. It requires a relativistic electron beam with extremely high quality and extremely stable transmission. In this paper we analyze the dynamic behavior of the relativistic electron in that device by calculating the entropy-like quantity. Results show that if there is no adiabatic field of the wiggler, the electron beam may have great fluctuation in velocity space and diverge in configuration space.     

The Linear Analysis of Coaxial Helical-Groove Slow-Wave Structure

The coaxial helical-groove slow-wave structure is a new-type high power circuit. The dispersion equation of this circuit with an annular electron beam is obtained according to self-consistent field theory. The computation results of the hot dispersion equation show the relations between electron beam parameters and the small signal gain. The presented analysis will be useful for the design of the TWT with coaxial helical-groove circuit.     

Design of a compact, optically guided, pinched, megawatt class free-electron laser

A conceptual design for a compact megawatt class free-electron laser (FEL) operating at 1 /spl mu/m is presented. The proposed FEL consists of an optically guided, pinched amplifier configuration driven by a RF linac. The gain length, efficiency, electron pulse slippage, and the distance between the wiggler and first relay mirror are determined for a megawatt class design. Of particular concern in the design is the overall length of the optical system, i.e., the wiggler length and distance to the first relay mirror. In the present design, the wiggler length is /spl sim/1 m and the distance between the first relay mirror and the wiggler is determined by the average intensity damage threshold on the mirror. By focusing the electron beam, the optical beam can be pinched upon exiting the wiggler. The pinched optical beam has a reduced Rayleigh length which permits the first relay mirror to be relatively close to the wiggler. By pinching the optical beam and employing grazing incidence the first relay mirror can be located within /spl sim/3 m of the wiggler. It is shown that frequency detuning can more than double the FEL efficiency. In the present design with frequency detuning and uniform wiggler the efficiency is /spl sim/1.8%. In addition, electron-pulse slippage is shown to be substantially reduced in a high-gain amplifier.     

A solenoid-derived wiggler

A novel wiggler design for use in free-electron lasers (FELs) is proposed, consisting of a staggered array of magnetic poles situated inside the bore of a solenoid. The resultant field pattern consists of a periodic transverse magnetic field on axis, as well as a longitudinal guide field. Such a wiggler has several advantages: the longitudinal field acts to confine the electrons near the FEL axis, high fields can be attained at short wiggler periods, the field strength is easily varied, and fabrication and testing of the wiggler are relatively easy. It is planned to use this wiggler design in a far infrared FEL to be built at Stanford University     

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.     

一种SASE-FEL用摇摆器机制探讨

利用带电粒子在均匀等离子体中传播时,在其后激发的尾波场作为自发辐射自放大自由电子激光(SASE-FEL)的摇摆器,称为尾波摇摆器。分析了此摇摆器在工作机制及其相应的周期、场强度,根据SASE-FEL理论计算了其增益长度Lg,并和APS的SASE实验参数作了比较。     

Effect of electron beam temperature on the gain of a collective free-electron laser

At sufficiently low beam currents, electron beam temperature effects cause the gains of collective (Raman) regime free-electron lasers to be lower than the predictions of cold beam theory. This gain degradation has been measured as a function of the beam current, the wiggler magnetic field, and the interaction frequency. The measurements are used to estimate the electron beam temperature, and the estimated temperature is close to the temperature predicted by numeric simulations.     

自由电子激光器虚火花束源的480 kV实验

介绍了虚火花放电实验 .改变马克斯发生器开关中的放电气体 ,提高了放电的稳定性 .改善虚火花放电室外绝缘油的性能 ,获得了能量为 4 80 ke V,束流为 13k A的电子束 ,电子束的传输距离达到 75cm.证明了摇摆器磁场能提高电子束聚焦性能 .     

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.     

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.