Collective and single-electron interactions of electron beams with electromagnetic waves,and free-electron lasers

The field of radiation emission from electron beams is reviewed with special reference to work related to free-electron lasers. Different schemes of interaction in periodic structures, electromagnetic slow-wave structures, and in transverse confining force are distinguished. Various effects and devices such as traveling wave amplifiers, Smith-Purcell radiators, Cerenkov and bremsstrahlung-free electron lasers, cyclotron resonance masers, coherent bremsstrahlung and channeling radiation are discussed and the differences and relations among them are explained. A simple comprehensive model is developed to describe electron-beam interaction with an electromagnetic wave in periodic electromagnetic structures. The model is general enough to describe both collective and single-electron modes of interaction and quantum mechanical, classical and Fermi degenerate regimes. Simplified expressions are developed for the gain by stimulated emission of radiation and for gain conditions of the Smith-Purcell-Cerenkov type free-electron lasers under conditions of very thin electron beams and infinite interaction length. This research is supported by the Air Force Office of Scientific Research under contract AFOSR-76-2933     

Contribution to the linear theory of the channeling of radiation by thin electron beams in free-electron lasers

Institute of Applied Physics, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 33, No. 7, pp. 854–867, July, 1990.     

SASE自由电子激光

SASE自由电子激光可以产生短至0.1nm的高亮度（峰值亮度比当前的第三代同步辐射高10个量级;平均亮度高3~5个量级）、短脉冲（脉冲长度小于2个量级、达到亚皮秒水平）硬X射线相干光。因而被称为是继第三代同步辐射之后的第四代光源。SASE依据的是高增益自由电子激光原理,利用了光阴极微波电子枪技术和电子直线加速器技术。综述了SASE的历史发展、基本原理、基本结构、主要物理特征和对电子束的要求。     

Realizable free-electron lasers

We analyze the general class of constant period free-electron lasers (FELs) based on single-pass linear accelerator technology. The emittance and energy spread of the electron beam used to drive an FEL must be chosen to match the acceptance of the FEL wiggler. This acceptance determines the attainable current, and the current determines the gain and power output. For an optimized system in which the optical mode size in the interaction region is minimized, the gain is found to be independent of the laser length, while the efficiency and power output scale as the inverse and inverse cube of the length. Very high power output and good efficiencies are predicted.Work supported by the Office of Naval Research     

SASE自由电子激光

 SASE自由电子激光可以产生短至0.1nm的高亮度（峰值亮度比当前的第三代同步辐射高10个量级;平均亮度高3~5个量级）、短脉冲（脉冲长度小于2个量级、达到亚皮秒水平）硬X射线相干光。因而被称为是继第三代同步辐射之后的第四代光源。SASE依据的是高增益自由电子激光原理,利用了光阴极微波电子枪技术和电子直线加速器技术。综述了SASE的历史发展、基本原理、基本结构、主要物理特征和对电子束的要求。     

Single-shot spectrometry for x-ray free-electron lasers

An experimental scheme to realize single-shot spectrometry for the diagnostics of x-ray free-electron lasers (XFELs) is presented. The combination of an ultraprecisely figured mirror and a perfect crystal form a simple, high-precision spectrometer that can cover an energy range from a few eV to a hundred eV with high resolution. The application of the spectrometer to determine XFEL pulse widths was investigated theoretically and experimentally. It has been shown that the present system can determine pulse widths from sub-fs to ps in a single shot even for spontaneous radiation. The system can be easily extended to even shorter pulses.     

Superheterodyne electron-wave free-electron lasers

A new class of free electron lasers (electron wave superheterodyne FEL) has been described. They differed from traditional ones by construction based on utilization of one of electron wave (plasma-beam, two-stream and others) instability mechanisms. In this case a parametrical interaction mechanism was used only for transformation of the longitudinal beam waves into the transverse electromagnetic ones. Nonlinear superposition the electron wave and parametric instabilities was interpreted as an effect of superheterodyne amplification.     

Two-color operation in high-gain free-electron lasers

Two-color operation in free-electron laser (FEL) amplifiers is studied using a 3D nonlinear polychromatic simulation. We assume the FEL is seeded at two closely spaced wavelengths within the gain band, and study the growth of the seeds and a discrete spectrum of beat waves that are outside the gain band. The beat waves grow parasitically due to electron bunching in the seeded waves with growth rates higher than the seeded waves. Injection of narrow-band seeds ensures a discrete spectrum. An example is discussed corresponding to an x-ray FEL; however, the physics is applicable to all spectral ranges.     

超快X射线自由电子激光研究进展北大核心CSCD

现代光源的发展不断推动着人们从更深层次上理解物质的基本结构和动力学行为。X射线自由电子激光作为最先进的光源,其超高的峰值功率、超短的脉冲长度和优良的相干性,为人们以原子级时空分辨率探测和操控物质中的超快过程提供了可能。目前全世界已有多个X射线自由电子激光装置建成并投入使用,在原子分子物理、化学、生命科学、材料科学等各学科应用中都显示出了重要价值。同时大量的研究工作也集中于继续提高X射线自由电子激光的性能,包括把脉冲持续时间从fs量级进一步缩短至as量级,这将为超快科学的发展带来新突破。以超快脉冲产生为主线,综述了近年来超快X射线自由电子激光产生方案的研究进展,从产生原理、方案特性、最新成果等方面介绍了各类产生方案,总结对比了各方案的优缺点,最后对超快X射线自由电子激光的未来发展方向进行了展望。     

Energy transfer in constant period free-electron lasers

A simple single particle model of a free-electron laser (FEL) amplifier has been used in a computer simulation to determine the maximum fractional conversion of electron kinetic energy to laser energy. The simulation results can be represented by a single universal curve. A simple scaling relationship for the length of the optimized constant period helix together with the universal curve permit one to predict maximum fractional energy conversion for any set of values of initial electron energy, initial laser intensity, magnetic field amplitude, and magnet period.     

Theory of group synchronism in free-electron waveguide lasers fed a sequence of short electron pulses

A theory of free-electron lasers fed a sequence of short electron pulses is developed. It is assumed that the group velocity of the electromagnetic pulse that develops in the cavity is the same as the translational velocity of the particles, and the repetition period of the electron pulses equals the transit time of the electromagnetic radiation in the cavity. Under these conditions of group synchronism, the principal factors governing the feasibility of establishing a stationary pulsed lasing regime are found to be the dispersive spread of the electromagnetic pulse and the channeling properties of an electron bunch. The conditions for self-excitation are found, and the characteristics of the stationary lasing regimes are determined assuming that the cavity has a high Q and using a parabolic equation for the evolution of the electromagnetic pulse shape. Zh. Tekh. Fiz. 69, 78–83 (February 1999)     

Far-infrared free-electron lasers and their applications

Free-electron lasers based on radio-frequency linear accelerators provide an important source of far-infrared radiation which allow exciting experiments that cannot be performed in any other way. Facilities such as FELIX (Nieuwegein, The Netherlands), JFEL (Newport News VA, USA), FELBE (Dresden, Germany), CLIO (Paris, France) and others provide mid- and far-infrared output in picosecond pulses with micro-joules of energy. They give continuous, wide tuning in far-IR for resonant pumping of discrete transitions (with simultaneous coverage of mid-IR) from around 3 to 250 μm wavelength. This enables time-resolved spectroscopy, non-linear optics and spectroscopy of weak absorptions. They have been applied to a wide variety of problems in condensed matter physics, physical chemistry and biophysics. We review the physics applications of these sources.     

Effect of dispersion on the operation of free-electron lasers driven by short electron bunches

The effect of dispersion on the lasing of waveguide free-electron lasers driven by short periodic electron bunches is considered. In the high-Q cavity approximation, the generation of electromagnetic pulses is described in terms of a parabolic equation. For the linear stage of interaction, starting lasing conditions are found analytically and the spatio-temporal structure of supermodes, which represent a set of phase-locked eigenmodes of the cold cavity, is determined. Dispersion is shown to allow the free-electron laser to operate at both positive and negative mismatches between the period of electron bunch injection and the time taken for the electromagnetic pulse to circulate over the cavity. The simulation of the nonlinear lasing conditions with allowance for dispersion spread makes it possible to find the stationary profile of radiated pulses and the optimum values of the group and time detunings that provide a maximum efficiency of the waveguide free-electron laser. It is also shown that, when the length of interaction is much longer than the starting value, the laser operates under the conditions of periodic or chaotic pulse profile self-modulation.     

Numerical experiments on free-electron lasers without inversion

The inversionless free-electron laser having a drift region consisting of two magnets is analyzed. Performing numerical simulations of electron motion inside wigglers and the drift region, we have shown that this system has a positive mean gain over the entire energy distribution of the electron beam. We study the influence of emittance and the spread of electron energies on the gain.