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.     

高品质激光尾波场电子加速器

激光尾波场电子加速的加速梯度相比于传统直线加速器高了3—4个量级,对于小型化粒子加速器与辐射源的研制具有重要的意义,成为当今国内外的研究热点.台式化辐射源应用需求的提高,特别是自由电子激光装置的快速发展,对电子束流品质提出了更高的要求,激光尾波场电子加速的束流品质和稳定性是目前实现新型辐射源的首要障碍.本文归纳整理了中国科学院上海光学精密机械研究所电子加速研究团队十年来在研制台式化激光尾波场电子加速器过程中采取的方案和取得的进展.例如率先提出了注入级和加速级分离的级联加速方案,通过实验获得了GeV量级的电子束能量;基于级联加速方式利用能量啁啾控制,实验获得世界最高品质的电子束流;通过优化激光系统稳定性和特殊的气体喷流结构,获得稳定的高品质电子束流输出等.这一系列实验结果有利于进一步推进激光尾波场电子加速器的应用.     

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.     

基于激光等离子体加速的自由电子激光研究新进展

激光等离子体加速器能够在cm尺度内产生GeV量级的高品质电子束,为研制台式化自由电子激光提供驱动源。但是受限于激光等离子体加速中的难点和现有技术发展,电子束的品质难以达到自由电子激光的需求,尤其在稳定性、发散角和能散等方面,阻碍了台式化自由电子激光的研制。介绍了基于激光等离子体加速器的自由电子激光的最新进展,整理了目前高增益自由电子激光实验过程中存在的主要挑战和对应的解决方案与实验进展,并展望未来的发展方向。最近的研究结果证明,通过控制和优化激光等离子体加速器的注入和加速过程产生的高品质电子束可以在指数增益区域实现自发辐射放大,产生高增益的辐射,这也推动基于激光等离子体加速器的自由电子激光研究进入了一个新的阶段。     

Attosecond electron bunches

Electron bunches of attosecond duration may coherently interact with laser beams. We show how p-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches. Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer. In the plasma, due to the self-intersection of electron trajectories, electron concentration is abruptly peaked. A group of counterstream electrons is pushed away from the plasma through nulls in the electromagnetic field, having inherited a peaked electron density distribution and forming relativistic ultrashort bunches in vacuum.     

Laser-driven relativistic electron dynamics in a cylindrical plasma channel

The energy and trajectory of the electron, which is irradiated by a high-power laser pulse in a cylindrical plasma channel with a uniform positive charge and a uniform negative current, have been analyzed in terms of a single-electron model of direct laser acceleration. We find that the energy and trajectory of the electron strongly depend on the positive charge density, the negative current density, and the intensity of the laser pulse. The electron can be accelerated significantly only when the positive charge density, the negative current density, and the intensity of the laser pulse are in suitable ranges due to the dephasing rate between the wave and electron motion. Particularly, when their values satisfy a critical condition,the electron can stay in phase with the laser and gain the largest energy from the laser. With the enhancement of the electron energy, strong modulations of the relativistic factor cause a considerable enhancement of the electron transverse oscillations across the channel, which makes the electron trajectory become essentially three-dimensional, even if it is flat at the early stage of the acceleration.     

A miniature electron beam pumped laser

The 1.73 μm XeI laser has been operated in a fully continuous mode using a table-top electron beam pumped laser setup. A 12 keV electron beam sent through a 300 nm thick silicon nitride membrane into a laser gas mixture of typically 600 mbar gas pressure was used for pumping. A low loss cavity was installed, resulting in a very low pumping power of 37 mW to reach laser threshold. The geometrical conditions for the laser setup such as the shape of the beam pumped volume and its overlap with the optical mode volume are discussed. The laser scheme has been clearly identified as a recombination laser scheme by operating the laser in pulsed mode and observing the time structure of the laser pulse.     

Formation of ultrashort electron pulses on scattering of an electron beam by a standing laser wave of ultrashort duration

The scattering of a well collimated electron beam by a strong standing laser wave of ultrashort duration, giving rise to a great number of scattered photons, is considered. This type of scattering is found to cause an electron beam to divide effectively into two parts. Ultrashort laser pulses are shown to be capable of forming ultrashort electron bunches whose length is governed by the laser beam diameter.     

Free electron laser spectral dynamics with a modulation of electron beam energy

Recent experimental and theoretical works on free electron laser spectral dynamics have pointed out the difficulty to obtain a narrow and stable spectrum operation. This goal can only be achieved by avoiding the sideband generation leading to a broadband and unstable spectrum. Tapered wiggler and two-frequency wiggler are well suited for combining sharp spectrum and high efficiency but are not really compatible with a wide tunability of laser light. Filtering sidebands is a good way for lower power experiments but it seems to be difficult to conceive wideband filters, specially in the far-infrared region. Modulation of electron energy is a new potential soft way for controlling the spectral dynamics of longpulse free electron laser. Spectral dynamics under the modulation is investigated in the linear and non-linear regimes in the far-infrared region. Simulations show that a pulsed and sharp spectrum behavior can be obtained by optimizing the modulation parameters. The interest of such a method for the far-infrared experiments is discussed.     

Stepwise electron and laser excitation of atoms

A review is presented of the application of stepwise electron/laser excitation techniques to the study of electron-atom collision processes. The theory associated with such techniques is reviewed and includes a number of typical case studies. A full discussion of laser excitation processes is included. A detailed review of stepwise electron/laser excitation experiments is also presented.     

自由电子激光器谐振腔内电子束和激光脉冲的时间同步

研究自由电子激光器谐振腔内电子束和辐射的时间同步问题.这是激光器谐振腔设计中的一个重要问题.本文讨论了腔内布儒斯特窗片对光路的影响.研制出由微机控制谐振腔的精密微调装置,扫描范围为±3cm,微调精度达1μm.     

Parametric amplification of laser-driven electron acceleration in underdense plasma

A new mechanism is reported that increases electron energy gain from a laser beam of ultrarelativistic intensity in underdense plasma. The increase occurs when the laser produces an ion channel that confines accelerated electrons. The frequency of electron oscillations across the channel is strongly modulated by the laser beam, which causes parametric amplification of the oscillations and enhances the electron energy gain. This mechanism has a threshold determined by a product of beam intensity and ion density.     

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.     

Interferences of ultrashort free electron wave packets

Interferences of free electron wave packets generated by a pair of identical, time-delayed, femtosecond laser pulses which ionize excited atomic potassium have been observed. Two different schemes are investigated: threshold electrons produced by one-photon ionization with parallel laser polarization and above threshold ionization electrons produced by a two-photon transition with crossed laser polarization. Our results show that the temporal coherence of light pulses is transferred to free electron wave packets, thus opening the door to a whole variety of exciting experiments.     

Simplified small signal gain calculations in free electron lasers

A method proposed recently is used for the evaluation of the small signal gain in various free electron laser configurations. The theory is applied to (a) the wigglerfree free electron laser with a uniform axial guide magnetic field and arbitrary direction of propagation of the amplified radiation and (b) the free electron laser with the axially modulated guide field (lowbitron). It is demonstrated that the new approach simplifies the gain calculation significantly in comparison with the traditional method.     

Electron dynamics and harmonics radiation due to electron oscillation driven by laser standing wave

The electron dynamics and harmonics radiation spectra due to electron oscillation driven by laser standing wave (SW) have been investigated considering a single electron mode. It is found that electron can pass through or be reflected from laser SW as electron is shot into laser SW from antinodes plane with initial velocity parallel to electric field of SW, and for electron incidence at arbitrary plane the attraction plane of electron trajectories will transfer from nodes plane to antinodes plane if initial electron kinetic energy exceeds a critical value. Further, the harmonics emission due to electron oscillation during passing through laser SW is considered in two cases of electron incidence plane at nodes and antinodes. In case of electron incidence at nodes, the spectrum of radiation is redshifted for backward radiation and blueshifted for forward radiation, and broadening of spectrum is noticed. These effects are much more pronounced at higher laser intensities. However, for case with electron incidence at antinodes, these phenomena vanish.     

Coherent electron scattering captured by an attosecond quantum stroboscope

We demonstrate a quantum stroboscope based on a sequence of identical attosecond pulses that are used to release electrons into a strong infrared (IR) laser field exactly once per laser cycle. The resulting electron momentum distributions are recorded as a function of time delay between the IR laser and the attosecond pulse train using a velocity map imaging spectrometer. Because our train of attosecond pulses creates a train of identical electron wave packets, a single ionization event can be studied stroboscopically. This technique has enabled us to image the coherent electron scattering that takes place when the IR field is sufficiently strong to reverse the initial direction of the electron motion causing it to rescatter from its parent ion.     

超强圆偏振激光直接加速产生超高能量电子束

研究表明, 峰值强度为10²²–10²⁵ W/cm²量级的圆偏振激光脉冲的有质动力场可以直接加速并产生GeV–TeV的单能电子束, 其中被加速电子的能量与激光脉冲的峰值强度成线性定标关系. 为了获得更高能量的电子束, 通过对一维解析模型的分析得到: 如果电子束在激光传播的方向上具一个初始能量E₀, 那么这种线性的定标关系可以被打破, 被加速电子束最终的能量可以被放大E₀倍. 这是由于具有一定初始能量的电子束不容易被激光脉冲抛在后面, 进而获得更高的加速距离. 二维粒子模拟结果显示: 当电子束的初始能量E₀为MeV量级时这个方法是有效的, 而当E₀过大时这个方法失效. 这是因为当电子的加速距离远大于激光脉冲的瑞利长度时, 激光强度的衰减使得电子束的加速错过了最佳加速场.     

On a promising way of determining effective electron–electron collision frequencies in metals

It is shown that measurements of coefficients of femtosecond laser pulse absorption or reflection by metals provide a unique possibility to determine effective frequencies of electron–electron collisions including umklapp processes.     

Ultra-high order harmonic generation via a free electron laser mechanism

In this paper,by using the"frequency up-conversion"principle with a high gain harmonic generation free electron laser and an external seed laser,we consider the possibility of modulating a relativistic electron beam on the attosecond scale,so that it can produce coherent spontaneous radiation from the deep ultraviolet to the hard X-ray spectral region with a very short radiator.Analytical estimation and three-dimensional numerical modeling show the great potential to reach ultra-high harmonics up to several thousand.For an electron bunch with the typical quality as in the free electron laser scheme and a seed laser with 800 nm wavelength,0.8 nm attosecond trains with alterable duration and GW scale peak power are modeled.The output radiation exhibits good shot-to-shot stability,full coherence and perfect tuning ability between the discrete harmonics of the seed frequency.