Search

The X-ray free-electron lasers (XFELs) are cutting-edge instruments pivotal in a broad range of fields, providing high-power X-ray pulses with durations spanning from femtoseconds to attoseconds. One of the critical challenges in XFEL facilities is the simultaneous accommodation of diverse requirements for XFEL operation modes and photon properties across different undulator lines. This paper proposes a dipole-kicker combination in the bunch compressors to vary the electron bunch length for the continuous-wave XFEL facilities driven by a superconducting linac. This method enables optimization of the electron bunch length on a per-bunch basis, tailored to each specific needs of each undulator. Through start-to-end simulations based on the parameters of the Shanghai high-repetition-rate XFEL and extreme light facility, we demonstrate the feasibility of this technique. The results show its effectiveness in enabling simultaneous operations of self-amplified spontaneous emission and externally seeded FEL across different undulator lines, ensuring optimal electron bunch compression for each undulator line.

Coupler RF kick due to the asymmetric structure caused by the coupler, is more likely to lead to emittance growth in the SHINE injector with low beam energy. The calculation of coupler RF kick and resulting emittance dilution has been studied in detail in the literature. In this paper, a novel approach is provided that a lossy material is placed on the surface of the superconducting cavity to approximate the Q0 of the TESLA cavity, and a frequency solver of CST is used to simulate the electromagnetic field distribution, which is used to calculate coupler RF kick, and calibrated against the results of CST Particle Tracking Studio with a good agreement. In order to minimize the emittance growth of SHINE injector, a 1.3 GHz symmetric twin-coupler cavity is adoped in the single-cavity cryomodule, and the rotational angle and permutation of the 8 cavities in the 8-cavities cryomodule is optimized. Ultimately, the optimized emittance is lower than the design parameter.

The formation of a double-horn current profile is a challenging issue in the achievement of electron bunch with high peak current, especially for high-repetition-rate X-ray free-electron lasers (XFELs) where more nonlinear and intricate beam dynamics propagation is involved. Here, we propose to correct the nonlinear beam longitudinal phase space distortion in the photoinjector section with a dual-mode buncher. In combination with the evolutionary many-objective beam dynamics optimization, this method is shown to be effective in manipulating the longitudinal phase space at the injector exit. Furthermore, the formation of the current spike is avoided after the multi-stage charge density modulation and electron bunch with a peak current of 1.6 kA is achieved for 100-pC bunch charge.Start-to-end simulations based on the Shanghai high-repetition-rate XFEL and extreme light facility demonstrate that the proposed scheme can increase the FEL pulse energy by more than 3 times in a cascading operation of echo-enabled harmonic generation and high-gain harmonic generation. Moreover, this method can also be used for longitudinal phase space shaping of electron beams operating at a high repetition rate to meet the specific demands of different researches.

The undulator line of the Shanghai soft X-ray Free-electron Laser facility (SXFEL) has very tight tolerances on the straightness of the electron beam trajectory. However, the beam trajectory cannot meet the lasing requirements due to the influence of beam position, launch angle and quadrupole offsets. Traditional mechanical alignment can only control the rms of offsets to about 100 $\mu$m, which is far from reaching the requirement. Further orbit correction can be achieved by beam-based alignment (BBA) method based on electron energy variations. K modulation is used to determine whether the beam passes through the quadrupole magnetic center, and the Dispersion-Free Steering (DFS) method is used to calculate the offsets of quadrupole and BPM. In this paper, a detailed result of simulation is presented which demonstrates that the beam trajectory with rms and standard deviation ($\sigma$) less than 10 $\mu$m can be obtained.

In free electron laser facilities, almost every kind of device will generate wakefield when an electron beam passes through it. Most of the wakefields are undesired and have a negative effect on the electron beam, which means a decrease of FEL performance. As for the SXFEL test facility, the sophisticated layout and the cumulative effect of such a long undulator section lead to an obvious wakefield, which is strong enough that can not be ignored. Based on two deflecting cavities at the entrance and the exit of the undulator section with corresponding profile monitors, we measured the wakefield of the undulator section. In this paper, we give the theoretical and simulation results of resistive wall wakefields which agree well with each other. In addition, the experimental and the simulation results of the overall undulator wakefield are given showing small difference. In order to explore the impact of this wakefield on FEL lasing, we give the simulation results of FEL with and without wakefield for comparison. There is almost no impact on 44 nm FEL in stage-1 of cascaded EEHG-HGHG mode, while the impact on 8.8 nm FEL in stage-2 becomes critical decreasing the pulse energy and peak power by 42% and 27% and broadening the bandwidth.

SHINE, as the first hard x-ray free-electron-laser (FEL) facility in China, is designed to provide high-brightness FEL lasing under high-repetition-rate operation mode. In order to drive x-ray FEL pulses with high qualities, the photoinjector section is deployed to provide the specified electron beam with low transverse emittance and high brightness. Normally the multi-objective optimization algorithm is employed in the injector beam dynamics design. In this paper, the many-objective optimization algorithm NSGA-III is introduced to the injector physical design for optimizing the 4 beam quality properties for the first time. The results of the optimization are presented and analyzed. The application of NSGA-III can provide guidance for further physical research as well as improve the beam dynamics optimization efficiency.

The spectroscopic techniques for time-resolved fine analysis of matter require coherent X-ray radiation with femtosecond duration and high average brightness. Seeded free-electron lasers (FELs), which use the frequency up-conversion of an external seed laser to improve temporal coherence, are ideal for providing fully coherent soft X-ray pulses. However, it is difficult to operate seeded FELs at a high repetition rate due to the limitations of present state-of-the-art laser systems. Here, we report the novel self-modulation method for enhancing laser-induced energy modulation, thereby significantly reducing the requirement of an external laser system. Driven by this scheme, we experimentally realize high harmonic generation in a seeded FEL using an unprecedentedly small energy modulation. An electron beam with a laser-induced energy modulation as small as 1.8 times the slice energy spread is used for lasing at the 7th harmonic of a 266-nm seed laser in a single-stage high-gain harmonic generation (HGHG) setup and the 30th harmonic of the seed laser in a two-stage HGHG setup. The results mark a major step towards a high-repetition-rate, fully coherent X-ray FEL.

A superconducting linear accelerator operating in continuous-wave mode could produce X-ray free electron lasers (XFEL) at megahertz repetition rate, with the capability that delivering wide spectral range coherent radiation to multi end stations. In this Letter, the energy recovery Linac (ERL) mode is proposed to flexibly control the electron beam energy for a continuous-wave superconducting Linac. Theoretical investigations and multi-dimensional numerical simulations are applied to the Linac case of Shanghai high-repetition-rate XFEL and extreme light facility. The results show that, with ERL operation in the last 25 cryo-modules, the strict requirements on RF power system could be significantly relaxed. And if one exhaust the RF power, the maximum electron beam energy can be enhanced from 8.74 GeV to 11.41GeV in ERL mode. The optimization of the ERL operation, the multi-energy electron beam transport and the XFEL performance improvements are presented., Comment: 9 pages, 5 figures

In this article, a feasible switchyard design is proposed for the Shanghai soft x-ray free electron laser facility. In the proposed scheme, a switchyard is used to transport the electron beam to different undulator lines. Three-dimensional start-to-end simulations have been carried out to research the beam dynamic during transportation. The results show that the emittance of the electron beam increases less than 10%, at meanwhile, the peak current, the energy spread and the bunch length are not spoiled as the beam passes through the switchyard. The microbunching instability of the beam and the jitter of the linear accelerator (linac) are analyzed as well., Comment: 8 pages, 8 figures 2 tables

The introduction of the surface plasmon polarizations makes the emittance of the photocathode complicated. In this paper, the emittance of plasmon-enhanced photocathode is analyzed. It is first demonstrated that the plasmonic near field can increase the emittance of the plasmon-enhanced photocathode. A simulation method has been used to estimate the emittance caused by plasmonic near field, and the suppression method also has been discussed, both of which are significant for the design of high performance plasmon-enhanced photocathode.

For the proper operation of Dalian Compact Light Source (DCLS) linac, measurement and control of the electron bunch is critical. In order to test control algorithms and high level physical applications, a virtual accelerator environment is constructed based on ELEGANT and SDDS toolkit. The required beam parameter measurement and orbit control tools are verified under this virtual environment. The design concept and current development status are presented., Comment: 7 pages, 7 figures

Dalian Coherent Light Source will use a 300MeV LINAC to produce fully coherent photon pulses in the wavelength range between 150-50nm by high gain harmonic generation free electron laser (FEL) scheme. To generate stable FEL pulses, stringent tolerance budget is required for the LINAC output parameters, such as the mean beam energy stability, electron bunch arrival time jitter, peak current variation and the transverse beam position offset. In order to provide guidance for the design of the Dalian Coherent Light Source, in this paper, the sensitivity of FEL pulse energy fluctuation to various error sources of the electron bunch was performed using intensive start-to-end FEL simulations., Comment: 4 pages, 7 figures, 2 tables, to be published in Chinese Physics C