The spectroscopic methods for the ultrafast electronic and structural dynamics of materials require fully coherent extreme ultraviolet and soft X-ray radiation with high-average brightness. Seeded free-electron lasers (FELs) are ideal sources for delivering fully coherent soft X-ray pulses. However, due to state-of-the-art laser system limitations, it is challenging to meet the ultraviolet seed laser’s requirements of sufficient energy modulation and high repetition rates simultaneously. The self-modulation scheme has been proposed and recently demonstrated in a seeded FEL to relax the seed laser requirements. Using numerical simulations, we show that the required seed laser intensity in the self-modulation is ~3 orders of magnitude lower than that in the standard high-gain harmonic generation (HGHG). The harmonic self-modulation can launch a single-stage HGHG FEL lasing at the 30th harmonic of the seed laser. Moreover, the proof-of-principle experimental results confirm that the harmonic self-modulation can still amplify the laser-induced energy modulation. These achievements reveal that the self-modulation can not only remarkably reduce the requirements of the seed laser but also improve the harmonic upconversion efficiency, which paves the way for realizing high-repetition-rate and fully coherent soft X-ray FELs.
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 μm, 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 (σ) less than 10 μm can be obtained.
The wakefield is always an important issue in a free electron laser (FEL) facility. A longitudinal wakefield could lead to an energy spread of an electron bunch, and a transverse wakefield could lead to a deterioration in quality of an electron bunch. As a result, the performance of the FEL will be decreased under these impacts. Due to the discontinuous structures and the resistive wall of the chambers in the undulator section, the wakefield is ineluctable. The Shanghai high-repetition-rate XFEL and extreme light facility (SHINE) is under construction with three undulator lines as planned. These three lines have the similar framework, thus we consider the first undulator line (FEL-I) as a representation. FEL-I contains 40 undulators and many other devices in the inner segments between undulators which might generate a strong wakefield. In this paper, we give the theoretical and simulation results of the resistive wall wakefield in the vacuum chambers. In addition, the simulation result of geometry wakefields and the overall undulator section wakefield are given, which is as strong as predicted. Lasing simulations with and without wakefield are also given to show the impact of the wakefield. In order to counteract the damage to the FEL performance caused by the wakefield, we employ an undulator tapering scheme and the simulation result shows a great performance.
As a new-generation light source, free-electron lasers (FELs) provide high-brightness x-ray pulses at the angstrom-femtosecond space and time scales. The fundamental physics behind the FEL is the interaction between an electromagnetic wave and a relativistic electron beam in an undulator, which consists of hundreds or thousands of dipole magnets with an alternating magnetic field. We report the first observation of the laser–beam interaction in a pure dipole magnet in which the electron beam energy modulation with a 40-keV amplitude and a 266-nm period is measured. We demonstrate that such an energy modulation can be used to launch a seeded FEL, that is, lasing at the sixth harmonic of the seed laser in a high-gain harmonic generation scheme. The results reveal the most basic process of the FEL lasing and open up a new direction for the study and exploitation of laser–beam interactions.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.