Inverse Cerenkov acceleration and inverse free-electron laserexperimental results for staged electron laser acceleration

The goal of the staged electron laser acceleration (STELLA) experiment is to demonstrate staging of the laser acceleration process whereby an inverse free electron laser (IFEL) will be used to prebunch the electrons, which are then accelerated in an inverse Cerenkov accelerator (ICA). As preparation for this experiment, a new permanent magnet wiggler for the IFEL was constructed and the ICA system was modified. Both systems have been tested on a new beamline specifically built for STELLA. The improved electron beam (e-beam) with its very low emittance (0.8 mm-mrad normalized) enabled focusing the e-beam to an average radius (1σ) of 65 μm, within the ICA interaction region. This small e-beam focus greatly enhanced the ICA process and resulted in electron energy spectra that have demonstrated the best agreement to date in both overall shape and magnitude with the model predictions. The electron energy spectrum using the new wiggler in the IFEL was also measured. These results will be described as well as future improvements to the STELLA experiment     

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.     

Interaction of a relativistic dense electron beam with a laser wiggler in a vacuum: self‐field effects on the electron orbits and free‐electron laser gain

Employing laser wigglers and accelerators provides the potential to dramatically cut the size and cost of X‐ray light sources. Owing to recent technological developments in the production of high‐brilliance electron beams and high‐power laser pulses, it is now conceivable to make steps toward the practical realisation of laser‐pumped X‐ray free‐electron lasers (FELs). In this regard, here the head‐on collision of a relativistic dense electron beam with a linearly polarized laser pulse as a wiggler is studied, in which the laser wiggler can be realised using a conventional quantum laser. In addition, an external guide magnetic field is employed to confine the electron beam against self‐fields, therefore improving the FEL operation. Conditions allowing such an operating regime are presented and its relevant validity checked using a set of general scaling formulae. Rigorous analytical solutions of the dynamic equations are provided. These solutions are verified by performing calculations using the derived solutions and well known Runge–Kutta procedure to simulate the electron trajectories. The effects of self‐fields on the FEL gain in this configuration are estimated. Numerical calculations indicate that in the presence of self‐fields the sensitivity of the gain increases in the vicinity of resonance regions. Besides, diamagnetic and paramagnetic effects of the wiggler‐induced self‐magnetic field cause gain decrement and enhancement for different electron orbits, while these diamagnetic and paramagnetic effects increase with increasing beam density. The results are compared with findings of planar magnetostatic wiggler FELs.     

Electron acceleration in the inverse free electron laser with a helical wiggler by axial magnetic field and ion-channel guiding

Electron acceleration in the inverse free electron laser (IFEL) with a helical wiggler in the presence of ion-channel guiding and axial magnetic field is investigated in this article. The effects of tapering wiggler amplitude and axial magnetic field are calculated for the electron acceleration. In free electron lasers, electron beams lose energy through radiation while in IFEL electron beams gain energy from the laser. The equation of electron motion and the equation of energy exchange between a single electron and electromagnetic waves are derived and then solved numerically using the fourth order Runge-Kutta method. The tapering effects of a wiggler magnetic field on electron acceleration are investigated and the results show that the electron acceleration increases in the case of a tapered wiggler magnetic field with a proper taper constant.     

Self-field effects on electron dynamics in a three-dimensional helical wiggler free-electron laser with axial magnetic field

An analytic linear theory of the electron dynamics in a three-dimensional helical wiggler free electron laser (FEL) with axial magnetic field is presented. Orbits are obtained by perturbing the steady state-trajectories in order to determine the characteristic frequencies Ω_± of the FEL. The effect of the self-fields on electron dynamics is studied and modified steady-state orbits and their stabilities have been analysed considering variation of electron energy and density. Among the features encountered is that in both group-I and group-II, one of the characteristic frequencies may have either signs affecting then the stability of the motion, while in group-II operation a repulsion of the frequencies at a pseudocrossing leads to highly perturbed trajectories when the wiggler frequency is approximately half the cyclotron frequency. Self-fields effects can significantly impair the stability of the electron orbits. For group-I orbits, they are more important for higher wiggler frequencies and lower beam energies. For group-II orbits, they remain less important for higher wiggler frequencies and lower beam energies before reaching the inversion zone, then they behave as for group-I orbits. It should be remarked that self-fields shift the inversion zone towards higher cyclotron frequencies the thing that is obtained by either decreasing the wiggler frequency or increasing the beam energy. It is shown that the axial velocity-induced self-magnetic field has a diamagnetic effect for both groups orbits, while the wiggler-induced self-magnetic field has a diamagnetic effect for group-I orbits and a paramagnetic effect for group-II orbits. The paramagnetic and diamagnetic effects are more important for higher beam energies and densities.     

Electron trajectories and gain in free electron laser with ionchannel guiding

A detailed analysis of electron trajectories and gain in a helical wiggler free electron laser (FEL) with ion channel focusing using single particle dynamics is presented. Conditions for stability of electron orbit have been obtained and stable regimes have been identified. Gain has been evaluated in the low-gain-per-pass limit with the help of the self-consistent pendulum equation. It is seen that the presence of ion channel leads to significant gain enhancement under appropriate conditions     

Equations for pulse propagation in a free electron laser

The equations for pulse propagation in a free electron laser are derived. The equations are valid in a reference frame, moving with a relativistic velocity with respect to the laboratory frame, chosen in such a way that the carrier frequency of the pulse equals the pseudoradiation (wiggler) field frequency. In this reference frame the equations assume a simple non relativistic form.     

Electron orbit in a combined plasma wave wiggler and an axial guidemagnetic field free electron laser near magnetoresonance

The nonlinear electron orbit dynamics is presented for a combined electrostatic plasma wave wiggler and an axial guide magnetic field free electron laser near magnetoresonance. The perpendicular orbit equation is derived and simplified identical to the wave-driven nonlinear oscillator equation but with different expressions of parameters. The nonlinear effect induced by the inclusion of β_⊥² in the relativistic factor γ is dominant in governing the perpendicular wiggling velocity and the orbit excursion even in the case of β_⊥²≪β_∥² , where γ=(1-β₁²-β_∥ ²)^-1/2. The dependence of the maximum perpendicular wiggling velocity and the orbit excursion on the wiggler constant, the wiggler frequency, and the initial parameters of motion is given analytically     

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.     

Development of FEL and SASE in the far-infrared region at ISIR, Osaka University

Free electron laser (FEL) and self-amplified spontaneous emission (SASE) are being developed in the far-infrared region using the L-band electron linac at the Institute of Scientific and Industrial Research (ISIR), Osaka University. The L-band linac was recently remodeled extensively not only for higher operational stability and reproducibility but also for high power operation of FEL. After commissioning of the linac, we first began SASE experiment with a newly-developed strong-focusing wiggler. Recently we began FEL experiment and obtained lasing with the high peak power at 70 μm again after a long break.     

短波长自由电子激光器电子运动特性研究

短波长自由电子激光器的电子束在摇摆器中的传输通道长而狭窄, 须得电子具有良好的运动特性, 避免在传输过程中产生横向发散. 本文研究短波长自由电子激光器中超相对论电子在磁场具有横向分布的平面摇摆器中的三维运动特性, 用逐次逼近法推导相对论运动方程的解析表达式, 非线性数值计算模拟传输过程, 采用科尔莫 戈罗夫熵 方法分析运动的稳定性. 结果表明: 摇摆器磁场除使电子做周期性摇摆运动外, 还迭加了偏离轴线的横向漂移运动, 在没有外置的磁场聚焦系统情况下, 电子将偏离轴线横向发散; 但是, 恰当选取电子的横向初始速度, 可有效地防止电子运动的横向发散, 即使没有外置的磁场聚焦系统, 也能在长达10 m 的摇摆器中顺利传输, 横向位移范围不超过0.09 mm, 而且其运动是稳定的. 关键词： 短波长自由电子激光器 平面摇摆器 超相对论电子运动 运动稳定性     

Proposal for an x-ray free electron laser oscillator with intermediate energy electron beam

Harmonic lasing of low-gain free electron laser oscillators has been experimentally demonstrated in the terahertz and infrared regions. Recently, the low-gain oscillator has been reconsidered as a promising candidate for hard x-ray free electron lasers, through the use of high reflectivity, high-resolution x-ray crystals. In this Letter, it is proposed to utilize a crystal-based cavity resonant at a higher harmonic of the undulator radiation, together with phase shifting, to enable harmonic lasing of the x-ray free electron laser oscillator, and hence allow the generation of hard x-ray radiation at a reduced electron beam energy. Results show that fully coherent free electron laser radiation with megawatt peak power, in the spectral region of 10-25 keV, can be generated with a 3.5 GeV electron beam.     

Analysis of the uniform magnetic field free-electron laser with scalarized photons in the microwave-spectral region

The recently developed concept of scalarized photons (formally photons of any polarization) is used to analyze the spontaneous and stimulated emission in the uniform magnetic field free-electron laser in the microwave spectral region. In fact, this free-electron laser is the simplest of many other, wiggler and wiggler-free free-electron lasers whose analyses could be done with scalarized photons in the small signal regime and whose physical parameters can be conveniently chosen for radiation to be generated in the microwave spectral region. As to the uniform magnetic field free-electron laser, which is treated here in some detail, with the electron beam energy of up to 10 MeV and the uniform magnetic field of up to 4 Tesla, the radiation (occurring with the fundamental and higher harmonic frequencies) can cover easily a 10 to 10,000 GHz spectral region.     

Net acceleration of high-energy electrons by net inverse Bremsstrahlung of a laser wave in a uniform magnetic field and a transverse undulating magnetic field

The nonzero net dc force acting on relativistic beam electrons traveling in a uniform magnetic field, a laser wave, and transverse undulating magnetic field (magnetic wiggler) is calculated by using quantum-kinetics in accordance with the correspondence principle. It is found that the average of this force can be as strong as the Lorentz force of the laser wave in an electron energy region beyong energies for free electron lasing, and decreases linearly with the inverse of the electron energy far beyond this energy region.     

磁场预增强的锥型波荡器

研究了波荡器磁场增强对提高自由电子激光器效率的影响。模拟计算发现采用磁场增强波荡器能使自由电子激光器的效率提高到１７．６％，采用磁场预先增强而后又增弱的锥型波荡器则能获得高达４３．３￥的输出效率，自由电子激光器的功率得到进一步的提高。     

远红外自由电子激光器的带宽扩展

本文研究了波导尺寸、电子束能量、摇摆器周期及磁场强度对波导型自由电子激光带宽的扩展作用，文中给了带宽约４００μｍ的远红外（工作波长６００－１０００μｍ）自由电子激光振荡器的设计参数，并对有关工程问题进行了讨论。     

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     

Exact one-particle theory of a free electron laser

From a semiclassical discussion of the one-particle quantized model we demonstrate that the free electron dynamics exhibits a phase transition in correspondence to a threshold value of the wiggler magnetic field. Below threshold the FEL can work as a small gain amplifier and can be described by the well-known pendulum analogy and gain-spread expressions. Above threshold the analogy is broken and the FEL works as a large gain amplifier which goes from spontaneous to stimulated emission regime.     

激光驻波中运动电子的谐波自发辐射

 采用单电子模型分析了电子在线极化激光驻波中的动力学及谐波自发辐射谱,数值计算了电子在驻波中的运动情况及辐射谱。结果表明:电子在波节和波腹处入射后,其辐射谱出现不同的特征;电子在波节处垂直磁场入射后,在洛伦兹力作用下快速振动并向前运动,其向后辐射的光谱发生红移,向前辐射的光谱发生蓝移,谱线出现展宽;当激光强度或者电子初始能量增大时,这些效应更加突出,以至于产生更高阶谐波,形成连续谱;而电子在波腹处以平行电场的方向入射后,仅在电场作用下作直线运动,其自发辐射谱线没有发生移动和展宽。     

Tailored terahertz pulses from a laser-modulated electron beam

We present a new method to generate steady and tunable, coherent, broadband terahertz radiation from a relativistic electron beam modulated by a femtosecond laser. We have demonstrated this in the electron storage ring at the Advanced Light Source. Interaction of an electron beam with a femtosecond laser pulse copropagating through a wiggler modulates the electron energies within a short slice of the electron bunch with about the same duration of the laser pulse. The bunch develops a longitudinal density perturbation due to the dispersion of electron trajectories, and the resulting hole emits short pulses of temporally and spatially coherent terahertz pulses synchronized to the laser. We present measurements of the intensity and spectra of these pulses. This technique allows tremendous flexibility in shaping the terahertz pulse by appropriate modulation of the laser pulse.