自由电子激光振荡器中电子束能量跳跃对增益的影响

 微脉冲中电子束能量、电流的跳跃会影响电子束的质量, 从而也影响激光的增益。在CAEP FIR FEL 理论设计模型参数的基础上,利用一维定态程序对电子束能量跳跃对增益的影响进行了数值研究,并与非定态程序计算结果进行比较,给出可容许的能量跳跃范围。     

激光电子散射中的最大可能增益

借鉴自由电子激光(FEL)发展之初Madey对自由电子激光器中受激辐射引起的增益的讨论,通过在激光场中的量子电动力学(QED)的模型中引入激光电子系统初态态密度以及由不确定的系统初态到确定光子末态的跃迁速率,推导了激光电子正碰过程中受激辐射至单一电磁模式产生的最大可能增益。采用成功得到X射线或γ射线光子的三个激光电子Compton背散射实验的实验参数计算了激光电子散射过程中的最大可能增益,与第一台X射线自由电子激光(XFEL)中的最大可能增益作比较,进而对激光电子散射作为激光光源的可行性进行评估。计算结果表明,现有的能够得到X射线光子或γ射线光子的激光电子散射实验中的最大可能增益远低于第一台XFEL中的。本工作未能找到合适的激光电子参数以获得比第一台XFEL中更高的最大可能增益,但是在入射电磁波位于射频波段范围内找到了能够实现较高增益的参数组合。     

Conditions for generation of X-ray cherenkov radiation during motion of charges in realistic media

The influence of the spatial distribution of electrons, atoms, and nuclei in condensed media on a medium’s susceptibility and on the conditions for the formation of short-wavelength Cherenkov radiation are considered. It is shown that taking into account the inhomogeneous (atomic and electronic) structure of material media in which fast charged particles travel leads to a change in the effective susceptibility and permittivity in the X-ray range, as compared with cases of model homogeneous media with the same average concentration of electrons. The influence of the distribution functions of electrons and nuclei in a target on the conditions for Cherenkov radiation generation and its parameters and on the threshold energy of fast charged particles required to generate such radiation is studied. It is shown that the function of the spatial distribution of electrons and nuclei in a target affects the conditions for generating laser radiation in the X- and λ-ray ranges (on the problem of X- and λ-ray lasers). The obtained results show that using the Fresnel approximation in the X-ray range is insufficiently justified and can lead to significant errors.     

曙光一号自由电子激光器的理论计算

系统总结曙光一号自由电子激光器理论计算的主要结果:包括曙光一号装置主要参数的选取和理解;磁场失谐曲线的计算;常参数摇摆器和变参数摇摆器的主要结果;高阶波导模的贡献;电子束参数扰动对激光性能的影响;空间电荷效应等。计算结果表明,常参数摇摆器激光输出功率可达80MW,效率约50％;变参数摇摆器激光输出功率可达250MW左右,效率约16％。     

Remaining noise-induced spatiotemporal dynamics in a free electron laser stabilized by feedback control

We investigate the efficiency of the pulsed regime stabilization by feedback control in a free electron laser (FEL) in oscillator configuration. We show that a hidden dynamics remains in the controlled FEL. Though not visible in the evolution of the pulse envelope, this dynamics affects the internal pulse shape evolution, and thus the FEL spectrum and coherence properties. The experimental results obtained on the UVSOR FEL are interpreted in the framework of a field master equation model. The stochastic nature of spontaneous emission is taken into account, and appears as an essential ingredient since the dynamics of the controlled structures is governed by noise like the other stable regimes of the FEL.     

Echo-enabled谐波放大自由电子激光参数

通过数值模拟,结合解析计算分析,研究了Echo-enabled谐波放大自由电子激光对装置参数和主要束流参数变化的独立敏感性,给出了谐波聚束因子和饱和功率随参数变化的数值模拟结果;对色散段由相干同步辐射效应引起的能散和发射度增长进行了估算和数值模拟。结果表明:相干同步辐射效应引起的切片能散和切片发射度增长在5%以内,对最后辐射影响不大。     

高平均功率FEL的标度律

 讨论了设计高平均功率康普顿型自由电子激光（FEL）装置的基本思路与相关的标度律。强调了除电子束平均流强外，能量抽取率是装置实现高平均功率的决定性因素。这个量的标度律基本上由摇摆器参数（周期数）单独确定，与摇摆器周期长度和强度无关，与激光波长无关。指出如无能量回收系统，按目前光阴极电子枪+超导射频加速结构装置可预期的电子束团100 pC/bunch，平均电流为mA情况下，15MeV电子束能给出的激光平均输出功率为百瓦量级。     

稳定波长反馈引起的失谐对自由电子激光的影响

 利用修改的一维非定态程序,基于美国杰弗逊实验室（JLab）Demo自由电子激光装置的参数,对高功率自由电子激光振荡器稳定波长的反馈系统进行了数值模拟。电子微脉冲为高斯型分布,每个纵向网格中取16个模拟宏电子,不考虑电子束的能散度。结果表明：当无反馈时,腔内光功率和波长都可以在一定范围内稳定;加入反馈后,由于电子束能量的变化所引起的等效失谐对整个系统有着重要的影响,甚至可能导致电子和光场失去相互作用,从而使得装置不能工作。提出应该在光场达到饱和以后再启动稳定波长反馈系统。模拟结果证明,该实施方案是合理有效的,可以避免其对FEL运行的严重影响。     

自由电子激光中电子与辐射波相互作用有效时间的分析

利用已获得的大信号自由电子激光增益公式,导出了激光平均增长率表达式,分析了自由电子激光中电子与辐射波相互作用的有效时间,讨论了其主要物理参量对相互作用有效时间的影响 关键词：     

Unified Analysis for Calculating the Incoherent Spontaneous Emission of Cooperative Radiations

A unified analysis is presented to calculate the incoherent spontaneous power of cooperative radiations based on self-amplified spontaneous emission. Using quantum mechanical tools, we derive analytical expressions for the incoherent spontaneous power of undulator and Cherenkov free-electron lasers(FELs). The undulator and Cherenkov FELs are considered as two different examples for the radiation that accumulate cooperatively. In the case of the undulator FEL, we show an excellent agreement between an expression for the incoherent radiation power derived in the present work and that obtained using a completely different approach [Phys. Rev. E65(2002) 026501] For the Cherenkov radiation,we demonstrate a satisfactory agreement between the incoierent power predicted in our analysis and previous experimental results.     

基于氟化铝基玻璃光纤的瓦级~3 μm激光

研究了基于Ho3+/Pr3+共掺AlF₃基玻璃单包层光纤的瓦级～3 μm激光。采用单模1150 nm光纤激光器泵浦上述增益光纤,得到了波长2.87 μm的激光输出,其最大输出功率为1.02 W,激光斜率效率为10.7%,输出激光的光束质量因子M2≈1.2。研究结果表明,AlF₃基玻璃光纤是一种潜在的可获得高功率中红外激光输出的增益介质。     

Moments of the intensity of a single-mode laser driven by coloured pump noises with a cross-correlation between the real and imaginary parts of the quantum noise

By means of the unified coloured noise approximation and phase lock, we study in this paper the stationary intensity distribution of the single-mode laser cubic model driven by coloured pump noise with cross-correlation between the real and imaginary parts of the quantum noise. We present a thorough discussion of how the cross-correlation λ_q between the real and imaginary parts of the quantum noise and the self-correlation time τ of the pump noise determine the behaviours of the mean intensity 〈I〉 and variance λ_2 (0) for both below and above the threshold operation, and many new phenomena are discovered. When the laser is operated above the threshold, increasing the cross-correlation intensity λ_q makes the 〈I 〉－τ curves exhibit a "re-entrant phase transition". Whether the laser is above or below the threshold, the whole 〈I 〉－τ curve moves down as λ_q increases; however, when λ_q=1 (perfect cross-correlation), the curve abruptly runs up. A nonzero value of cross-correlation time τ (τ≠0) leads to the entire λ_2(0)－p′ curve being suppressed (here p′ is the pump noise intensity). This indicates the increasing precision of the laser beam due to the existence of the self-correlation time τ. The behaviour of the λ_2 (0)－p′ curve as a function of λ_q is similar to that of the 〈I 〉－τ curve against λ_q: that is, whether the laser is above or below the threshold, the λ_2 (0)－p′ curve moves up as λ_q increases; however, when λ_q=1, the curve suddenly moves down. Through the study in this paper, we can find a way to optimize for output laser intensity.τ     

Quantum regime for dielectric Cherenkov radiation and graphene surface plasmons

In this article, we propose a quantum regime for Cherenkov free-electron laser (CFEL) and surface plasmon polaritons (SPPs) excited in dielectric and multilayer graphene waveguides, respectively. This quantum regime is realized when the momentum spread induced in the interaction is smaller than the photon recoil. The discrete momentum exchange characterizing this interaction yields a significantly narrow single emission line. To determine the condition of the quantum regime, we derive an expression for the gain in the Cherenkov effect using a quantum mechanical treatment. It is assumed that the effective spread in momentum is due to the finite interaction length L (or the propagation length in the case of SPPs). For both cases, CFEL and SPPs, the effects of electron beam and waveguide parameters on the possibility of the quantum regime are studied. We conclude that the quantum regime can be basically verified at low electron beam energy (<40 keV) and at emission wavelengths in the near infrared range (<5 μm) when L is in the order of millimeters. In the case of SPPs, we also show that the feasibility to realize quantum SPPs is enhanced by increasing the chemical potential and number of graphene layers.     

自由电子激光放大啁啾脉冲的数值模拟

 利用1维非定态程序对单程自由电子激光(FEL)放大器放大线性啁啾脉冲的过程进行了数值模拟,得到了不同啁啾参数的脉冲通过FEL放大器的增益曲线,计算了不同啁啾参数的脉冲被单程FEL放大器放大后的腔外压缩情况。通过对数值模拟结果的定性分析得出：影响输出脉冲宽度和峰值功率的主要因素是FEL放大器的增益线宽和啁啾参数,并以此说明,通过先使用FEL放大器放大啁啾脉冲、然后再将其在腔外压缩的方法产生超短脉冲FEL是可行的。     

A proposed method to characterize high harmonic microbunching in EEHG operation of SDUV-FEL

The echo-enabled harmonic generation(EEHG)scheme offers remarkable efficiency for generatinghigh harmonic microbunching with a relatively small energy modulation.A proof of principle experiment of the EEHG scheme has been proposed at the Shanghai deep ultraviolet(SDUV)free electron laser(FEL)facility,where the 4th harmonic of the seed laser is amplified in the 9 m long radiator.To explore the advantages of the EEHG scheme,in this paper,a method of measuring the coherent high harmonic radiation of the radiator is proposed to investigate the electron beam microbunching corresponding to the 10th-20th harmonics of the seed laser.The principle of the proposed method,comparisons with existing methods and the simulation results are presented and discussed.     

Intense Cherenkov-type terahertz electromagnetic radiation from ultrafast laser-plasma interaction

A Cherenkov-type terahertz electromagnetic radiation is revealed, which results efficiently from the collective effects in the time-domain of ultrafast pulsed electron current produced by ultrafast intense laser--plasma interaction. The emitted pulse waveform and spectrum, and the dependence of laser pulse parameters on the structure of the radiation field are investigated numerically. The condition of THz radiation generation in this regime and Cherenkov geometry of the radiation field are studied analytically.