High-gain FEL with variable parameter wigglers

A general discussion of FEL with variable parameter wigglers for the production of high power coherent radiation at reasonable efficiency is presented. Some promising modes are identified and analyzed.     

High-gain small-signal modes of the free-electron laser

The small-signal regime of the free-electron laser is analyzed for single-frequency, uniform-wiggler operation, taking diffraction into account. Exponentially growing modes are found with profiles independent of the longitudinal coordinate. Maximum gain occurs for a positive value of the energy detuning, though the gain on resonance is nearly maximal. If the current transverse distribution is uniform and sharp-edged, analytic solutions are obtained in terms of Bessel and Hankel functions with complex arguments. Positive energy detuning broadens the laser modes, while negative detuning concentrates them within the electron beam.     

High-gain lasing and polarization switch with a distributed optical-klystron free-electron laser

This Letter reports the first experimental results from the world's first distributed optical-klystron (DOK) free-electron laser (FEL), the DOK-1 FEL, at Duke University. The DOK-1 FEL is a hybrid system, comprised of four wigglers: two horizontal and two helical. With the DOK-1 FEL, we have obtained the highest FEL gain among all storage ring based FELs at 47.8% (+/-2.7%) per pass. We have also demonstrated that the FEL gain can be enhanced by increasing electron bunching using wigglers with a different polarization. Furthermore, we have realized controlled polarization switches of the FEL beam by a nonoptical means through the manipulation of a buncher magnet.     

High-gain harmonic-generation free-electron laser seeded by harmonics generated in gas

The injection of a seed in a free-electron laser (FEL) amplifier reduces the saturation length and improves the longitudinal coherence. A cascaded FEL, operating in the high-gain harmonic-generation regime, allows us to extend the beneficial effects of the seed to shorter wavelengths. We report on the first operation of a high-gain harmonic-generation free-electron laser, seeded with harmonics generated in gas. The third harmonics of a Ti:sapphire laser, generated in a gas cell, has been amplified and up-converted to its second harmonic (λ(rad)=133 nm) in a FEL cascaded configuration based on a variable number of modulators and radiators. We studied the transition between coherent harmonic generation and superradiant regime, optimizing the laser performances with respect to the number of modulators and radiators.     

Growth rate enhancement of free-electron laser by two consecutive wigglers with axial magnetic field

The operative mechanism for a free-electron laser (FEL) with two consecutive helical wigglers having opposite circular polarization in the presence of an axial magnetic field is proposed and analysed. With the help of fluid theory, a tenth-degree polynomial dispersion equation for electromagnetic and space–charge waves is derived. The results are used to illustrate and discuss the dependence of growth rate on different system parameters. Finally, it is shown that for the same system parameters the growth rate of the proposed structure is more than the growth rate of instability in a conventional FEL.     

Nonlinear harmonic generation in free-electron lasers with helical wigglers

It is widely believed that harmonics are suppressed in helical wigglers. However, linear harmonic generation (LHG) occurs by an azimuthal resonance that excites circularly polarized, off-axis waves, where the hth harmonic varies as exp((ihtheta). Nonlinear harmonic generation (NHG) is driven by bunching at the fundamental and has different properites from LHG. While NHG has been studied in planar wigglers, there has been no analysis of NHG in helical wigglers. The 3D simulation code medusa has been modified for this purpose, and it is shown that NHG is substantial in helical wigglers and that the even and odd harmonics have comparable intensities.     

Partial-coherence method to model experimental free-electron laser pulse statistics

A general numerical approach is described that allows obtaining model sets of temporal pulse shapes of free-electron lasers (FELs) operating in the self-amplified spontaneous emission mode. Based on a random partial-coherence approach, sets of pulse shapes can be calculated that satisfy statistical criteria of FEL light predicted by established FEL theory. Importantly, the numerically retrieved sets of pulses reproduce the experimentally accessible FEL light characteristics as measured at the Free-electron LASer at Hamburg (FLASH), such as the average spectrum, single-shot spectral shape, and pulse duration. The high-precision agreement with the experimental average spectral shape, without further knowledge of FEL machine parameters, makes this approach a convenient tool for the analysis and theoretical modeling of nonlinear optical or pump-probe experiments with FEL light.     

能量啁啾电子束自由电子激光放大啁啾脉冲

 提出采用具有能量啁啾电子束的自由电子激光放大器放大啁啾脉冲,分析说明了它的工作机理和滑移现象对其的影响。采用研制的1维非定态程序GOFEL-P,对能量啁啾自由电子激光放大器放大啁啾脉冲的过程进行了数值模拟和分析,计算了不同啁啾参数的脉冲被放大后的腔外压缩情况。结果表明：与单能电子束时相比,能量啁啾自由电子激光可以放大具有更大啁啾参数的啁啾脉冲,使压缩后的脉冲峰值功率增大至568 GW,脉冲宽度缩短至2.29 fs,大幅增强自由电子激光放大啁啾脉冲以及腔外压缩脉冲的效果。     

具有导向磁场的可变Wisggler自由电子激光器的物理机制

本文在忽略电子间相互作用的条件下,通过求解相对论洛伦兹方程,给出了在附加有导向磁场的可变Wiggler场中电子与电磁场相互作用的物理机制。同时讨论了几种通过改变导向磁场或Wiggler场参数的方法以提高能量转换速率的方法。     

具有导向磁场的可变Wiggler自由电子激光器的物理机制

本文在忽略电子间相互作用的条件下,通过求解相对论洛伦兹方程,给出了在附加有导向磁场的可变Wiggler场中电子与电磁场相互作用的物理机制。同时讨论了几种通过改变导向磁场或Wiggler场参数的方法以提高能量转换速率的方法。     

脉宽易变的高功率Nd:YAG激光器

本文描述一台由两级Nd:YAG激光器组成的紧凑高功率激光系统的结构及其工作性能。激光系统分成三个部分:对撞脉冲式的主-被动锁模振荡器,单脉冲选择器和四程放大器。振荡器输出锁模脉冲列的包络幅度起伏小于±4％,其脉冲宽度可以在宽范围内(20ps～2.5ns)方便地切换。对于单个0.1mJ、200ps的激光脉冲,四程放大器能提供7×10~2的放大倍数。     

An automatically controlled Nd:YAG laser with variable pulse width

The development of a high-energy, pulsed Nd:YAG laser system for materials-processing and medical applications is reported here. A variable pulse width in the range of 0.3–10 ms and a variable pulse repetition rate up to 50 pps are provided. An automatic operation system using a microprocessor-based driver/ controller enables safe operation of the laser system and automatic material processing when integrated in a laser/robot system.     

Subpicosecond pulse laser absorption by an overdense plasma with variable ionization

Transient ionization of an overdense plasma produced by a subpicosecond, p-polarized obliquely incident pulse laser of moderate intensity (10(16)-10(18) W/cm(2)) changes the plasma heat transfer via processes dominated by the return current and the absorption rate via ion acceleration. To explore the effect of variable ionization, a hybrid one-dimensional electro-magnetic particle-in-cell method that conforms to a direct solution of the Fokker-Planck-Landau equation is applied. A method that includes the Langevin equation to account for Coulomb collisions and the average ion model to calculate the nonlocal thermodynamic equilibrium ionization balance provides good agreement between the computed absorption and the measured results.     

频率啁啾光脉冲在行波半导体激光放大器中的传播特性

本文就行波半导体激光放大器(TW-SLA)中载流子引起的群速度色散对激光脉冲传播特性的影响作了理论研究.由于载流子引起的群速度色散效应,使远离饱和区工作的TW-SLA会对有频率啁啾的光脉冲产生展宽或压缩.本文就TW-SLA参数、频率啁啾参数跟脉冲增宽因子的关系作了理论研究.结果表明,线性的TW-SLA具有对低能量光脉冲压缩的可能性.因此,在光学通信系统中,它可用作对光纤损耗及色散的补偿元件.     

The multi-cavity free-electron laser

Analysis is made of a free-electron laser with many optical cavities where the cavities communicate with each other, not optically, but through the electron beam. Analytic analysis is made in a one-dimensional, linear approximation. A general expression is given for the growth rate in the exponential (high current) regime. In the regime where lethargy is important expressions are given in the two opposite limits of small and large numbers of cavities and bunches. Numerical simulation results, still in the one-dimensional approximation, but including nonlinearities, are presented. Three examples of the multi-cavity free-electron laser (MC/FEL) are presented.     

Quantum effects with an x-ray free-electron laser

A quantum kinetic equation coupled with Maxwell's equation is used to estimate the laser power required at an x-ray free-electron laser (XFEL) facility to expose intrinsically quantum effects in the process of QED vacuum decay via spontaneous pair production. A 9 -TW-peak XFEL laser with photon energy of 8.3 keV could be sufficient to initiate particle accumulation and the consequent formation of a plasma of spontaneously produced pairs. The evolution of the particle number in the plasma will exhibit non-Markovian aspects of the strong-field pair production process, and the plasma's internal currents will generate an electric field whose interference with that of the laser leads to plasma oscillations.