Studies of classical radiation emission from plasma wave undulators

The authors examine the characteristics of the classical radiation emitted by a relativistic electron beam that propagates perpendicularly through a large amplitude relativistic plasma wave. Such a study is useful for evaluating the feasibility of using relativistic plasma waves as extremely short wavelength undulators for generating short wavelength radiation. The electron trajectories in a plasma wave undulator are obtained using perturbation techniques and are then compared to numerical simulation results. The frequency spectrum and angular distribution of the spontaneous radiation emitted by a single electron and the stimulated radiation gain are obtained analytically, and are then compared to 3-D numerical simulations. The characteristics of the plasma wave undulator are compared to the AC free-electron laser (FEL) undulator and the conventional FEL     

W波段同轴静磁波荡器自由电子激光优化设计

基于同轴静磁波荡器的自由电子激光主要利用环形电子束与同轴TE模式的相互作用产生电磁辐射,是一种重要的毫米波辐射源。分析了同轴结构作用区内外半径、工作模式、电子束电压、波荡器周期等参数对辐射频率的影响规律,研究了电子束平均半径的选取原则和束波互作用腔的设计方法,设计了辐射频率在W波段的基于同轴静磁波荡器的自由电子激光,并进行了粒子模拟,在电子束电压为720 kV,电子束流为1 kA的情况下,获得了频率107 GHz、辐射功率35 MW的TE01模,束波转换效率为4.9%,束波作用腔总长度小于200 mm,同轴静磁波荡器的磁场幅度0.34 T。     

Preliminary study of EEHG-based superradiant undulator radiation at the HLS-Ⅱ storage ring

We investigate storage ring-based Echo-Enabled Harmonic Generation(EEHG) superradiant undulator radiation as a possible scheme to obtain shorter wavelengths at the HLS-Ⅱ(Hefei Light Source-Ⅱ) storage ring. In this paper we give the designation of the storage ring based EEHG up to the 26 th harmonic, where 31 nm vacuum ultraviolet light is radiated from an 800 nm seeded laser. The novelty of our design is that both the two dispersion sections of EEHG are realized by the storage ring's own magnet structure. In particular, the whole ring is used as the first dispersion section, and two modulators of the traditional EEHG can be done with the same undulator. These two dispersion sections are realized by changing the superperiod of the present lattice structure, and more precisely by changing the focusing strengths of the present structure. Since no additional magnets and chicanes are used,the beam circulates around the storage ring repeatedly, and thus this storage ring-based EEHG can have a higher repetition rate than a linac-based EEHG.     

Generation of Coherent Radiation in the Near X-Ray Band by a Cascade FEL with a Two-Frequency Undulator

Generation of X-ray radiation in a cascade self-amplified spontaneous emission free-electron laser (SASE FEL) using the harmonics of a two-frequency undulator is studied. The advanced phenomenological model of a one-pass FEL that accounts for the main losses in real FELs is presented: the electron energy spread in the beam, the beam divergence, diffraction, and the fact that emission losses are greater at higher harmonics than in the main frequency range. The FEL mathematical model was performed using the Mathematica software and calibrated within the experiment carried out at the operating SPARC facility via complex three-dimensional numerical simulations. The phenomenological model is used to analyze FEL dynamics for generation of a high-energy X-ray emission at a relatively short length. It is proposed to use a two-frequency undulator for the initial electron grouping and subsequent frequency multiplication in a cascade FEL with higher harmonic amplification (HGHG). The advantages of the two-frequency undulator are presented for electron grouping at higher harmonics of the undulator radiation (UR). The operation of several types of FEL is simulated with amplification of the seed laser wave frequency in two and three cascades to generate the soft X-ray radiation. A seed laser with a wavelength of 11.43 nm corresponding to the peak reflectivity of mirror coatings with MoRu/Be is proposed for generating the intensive X-ray laser radiation with λ ~ 1.27–3.37 nm. Here, the intensive radiation power reaches 50 MW at a length of only 35 meters; the radiation shows good temporal coherence corresponding to the performance of a low-power seed laser with a lower frequency.     

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.     

Observation of an optical vortex beam from a helical undulator in the XUV region

The observation of an optical vortex beam at 60 nm wavelength, produced as the second‐harmonic radiation from a helical undulator, is reported. The helical wavefront of the optical vortex beam was verified by measuring the interference pattern between the vortex beam from a helical undulator and a normal beam from another undulator. Although the interference patterns were slightly blurred owing to the relatively large electron beam emittance, it was possible to observe the interference features thanks to the helical wavefront of the vortex beam. The experimental results were well reproduced by simulation.     

相对论速调管放大器中微波放大机制

解析地研究了电子束在相对论速调管放大器（ＲＫＡ）的调制腔和漂移管中的预群聚；第一次用粒子－波相互作用的场方法导出了在辐射腔中预群聚电子束产生辐射的自洽方程；并且计算了线性区的增益．     

用于THz源的混合型波荡器数值模拟

波荡器是基于自由电子激光的小型THz源关键器件, 其可调节的周期性磁场结构与两端的光腔配合, 使得穿越的电子束产生带增益的相干辐射, 最终达到THz源所需要的功率. 同纯永磁结构相比, 混合型波荡器通过软铁材料调节由永磁块磁化方向性差异导致的磁场分布误差, 同时可提供更高的场强. 本文针对小型THz源需求, 对混合型波荡器进行了相关物理设计. 在解析方法分析的基础上, 采用OPERA3D/TOSCA有限元分析软件, 对波荡器进行了三维磁场数值模拟和积分场优化. 通过对波荡器端部结构的调整, 优化后模型的一次场积分(导向误差)小于0.01Gs﹒m, 电子轨迹偏移小于0.02mm.     

Experimental observation of superradiance in the stimulated scattering of an intense microwave pump wave by a counterpropagating subnanosecond high-current relativistic electron bunch

The generation of ultrashort superradiant pulses in the stimulated scattering of an intense microwave (38 GHz) pump wave by a counterpropagating high-current relativistic electron bunch has been observed. Scattered radiation is a single ～200-ps pulse with a peak power of about 1 MW. Owing to the Doppler shift of the radiation frequency, frequencies up to 150 GHz are present in the spectrum of the scattered pulse.     

波荡器谐波场增强效应对电子自发辐射谱的影响

 有效利用电子束在波荡器中运动时产生的谐波辐射,是获得更短波长辐射最直接的方法之一。提高波荡器磁场的谐波分量可以提高电子束的谐波辐射光场强度。分析了一种改进Halbach型波荡器结构,计算了其磁场的构成,分析表明这种结构可以使磁场的三次谐波分量提高至基波分量的25%左右。以合肥光源的电子束参数为例,计算了波荡器磁场谐波分量增强后对电子束自发辐射谱的影响。计算结果表明,波荡器磁场谐波分量增强至25%时,可以使电子束自发辐射的三次谐波辐射增强至基波辐射光子通量的67%左右。     

Harmonic operation of high gain harmonic generation free electron laser

In high gain harmonic generation (HGHG) free electron laser (FEL), with the right choice of parameters of the modulator undulator, the dispersive section and the seed laser, one may make the spatial bunching of the electron beam density distribution correspond to one of the harmonic frequencies of the radiator radiation, instead of the fundamental frequency of the radiator radiation in conventional HGHG, thus the radiator undulator is in harmonic operation (HO) mode. In this paper, we investigate HO of HGHG FEL.Theoretical analyses with universal method are derived and numerical simulations in ultraviolet and deep ultraviolet spectral regions are given. It shows that the power of the 3rd harmonic radiation in the HO of HGHG may be as high as 18.5% of the fundamental power level. Thus HO of HGHG FEL may obtain short wavelength by using lower beam energy.     

Harmonic operation of high gain harmonic generation free electron laser

In high gain harmonic generation (HGHG) free electron laser (FEL), with the right choice of parameters of the modulator undulator, the dispersive section and the seed laser, one may make the spatial bunching of the electron beam density distribution correspond to one of the harmonic frequencies of the radiator radiation, instead of the fundamental frequency of the radiator radiation in conventional HGHG, thus the radiator undulator is in harmonic operation (HO) mode. In this paper, we investigate HO of HGHG FEL. Theoretical analyses with universal method are derived and numerical simulations in ultraviolet and deep ultraviolet spectral regions are given. It shows that the power of the 3rd harmonic radiation in the HO of HGHG may be as high as 18.5% of the fundamental power level. Thus HO of HGHG FEL may obtain short wavelength by using lower beam energy.     

Effects of energy spread on brightness and coherence of undulator sources

The (spectral) brightness for partially transverse coherent sources such as synchrotron radiation and free‐electron laser sources can be defined as the maximum of the Wigner distribution. Then, the brightness includes information on both coherence and wavefront characteristics of the radiation field. For undulator sources, it is customary to approximate the single‐electron electric field at resonance with a Gaussian beam, leading to great simplifications. Attempts to account for the modified spatial and angular profile of the undulator radiation in the presence of detuning due to energy spread, currently build on the simplified brightness expression derived under the assumption of Gaussian beams. The influence of energy spread on undulator radiation properties is becoming important in view of diffraction‐limited rings with ultralow emittance coming on‐line. Here the effects of energy spread on the brightness of undulator radiation at resonance are discussed, as well as relevant relations with coherence properties.     

Focusing effects in nonlinear resonant four-wave mixing (RFWM) MIR and FIR generation

Summary The influence of pump beam geometry in a NH₃ superradiant Raman FIR laser with RFWM generation of MIR emission has been analysed. We have observed that, in general, MIR radiation is emitted in a stimulated configuration, so pump and MIR beam overlap is basically important to exhibit an efficient RFWM. In particular, strong focusing, also in the presence of self-focusing, shows a lowering of the threshold of Raman FIR emission, as expected, plus a reduction of RFWM MIR emission. COMB-Arrich., CRE Frascati.     

Momentum spread in a relativistic electron beam in an undulator

The motion of the relativistic electron beam in the spatially periodic magnetic field of an undulator has been considered taking into account the effect of the incoherent field of the spontaneous undulator radiation on the motion of the electrons. An expression for the rms momentum of the electrons has been obtained. It has been shown that the momentum spread in the ultrarelativistic electron beam increases in the spontaneous incoherent emission mode. Conditions for the self-amplification of the spontaneous undulator radiation in ultrashort-wavelength free-electron lasers have been discussed.     

CAEPXFEL波荡器束线初步物理设计

中国工程物理研究院计划开展XFEL光源的研制,主要通过电子束在波荡器中的辐射特性产生硬X射线。根据理论分析和数值模拟,在电子束最高能量可达12GeV的实际条件下,初步设计了一条间隙可调的真空外波荡器束线,波荡器分为25段,每段长3.86m,间隙7~10mm,周期长度25.4mm,磁场0.636~1.03T,束线总长120m左右,辐射光子能量为3~25keV。     

Efficiency and sideband observations of a Raman FEL oscillator witha tapered undulator

Power and spectral measurements are reported from the Columbia Raman free-electron laser (FEL) oscillator experiment. High-power radiation pulses (~12 MW, 100 ns) are generated at a wavelength of ~2.5 mm, using a 750-kV electron beam injected into a helical undulator. The undulator is made up of a 40-cm long constant-period (1.45 cm) section followed by an equal length of tapered undulator. The period is decreased by 7.6% in such a way that the on-axis field remains constant. It is reported that the taper allows an increase in total power efficiency from ~4 to ~12%. Most noteworthy is that the tapered undulator reduces the sideband radiation compared with a constant-period undulator FEL which is studied in the same configuration. The power was measured calorimetrically and compared with the results of a 1-D Raman code. The reduction of sideband power observed in the experiment was consistent with computational results obtained with a 2-D sideband code     

Optimal energy-phase correlation for superradiance of intense electron bunches

Investigated is the optimal energy-phase correlation for superradiance of an intense electron bunch in an undulator field. It is noted that the superradiant emission saturates due to the bunch self-field at an energy proportional to the total charge in the bunch.     

Equilibrium and stability of free-running, phase-locked, andmode-locked quasi-optical gyrotrons

Equilibria and stability (both single-mode and sideband) are calculated and contrasted for free-running, phase-locked, and mode-locked oscillator configurations in a quasi-optical gyrotron. The oscillator can be phase locked by direct injection of radiation into the oscillator cavity. The equilibrium and stability properties are not greatly affected at low injection power levels. Alternatively, the oscillator could be phase locked by prebunching the beam. If the beam is prebunched, there are dramatic effects on both equilibrium and stability. The transverse efficiency can be considerably enhanced by prebunching the beam. This prebunching can be done on either a phase-locked (using an external RF source) or mode-locked (using the oscillator output) configuration. The stable locking bandwidth turns out to be about half the ω/Q linewidth of the mode