Injector For Free Electron Laser Research Articles

Generation of two-color x-ray free-electron-laser pulses from a beam with a large energy chirp and a slotted foil

We present a novel and flexible method to generate two-color x-ray free-electron-laser (FEL) pulses with variable color separation and time delay using a double-slotted foil on a beam with a large energy chirp. The color separation and time delay between pulses can be set independently by tuning the separation between the slots and the longitudinal dispersion of a magnetic chicane. This method presents some advantages in comparison to previous proposals; it is compact, no large hardware component is needed, and no modifications of the machine settings in the FEL injector are required. Start-to-end numerical simulations for the SwissFEL case show the validity and tunability of our method and its application range: $\ensuremath{-}60$ to $+80\text{ }\text{ }\mathrm{fs}$ time delay and 0 to 3% color separation between the two pulses for a central wavelength of 0.1 nm.

Development of a novel thermionic RF electron gun applied on a compact THz-FEL facility

The current requirements from civil and commercial applications lead to the development of compact free-electron laser (FEL)-based terahertz (THz) radiation sources. A picosecond electron gun plays an important role in an FEL-THz facility and attracts significant attention, as machine performance is very sensitive to initial conditions. A novel thermionic gun with an external cathode (EC) and two independently tunable cavities (ITCs) has been found to be a promising alternative to conventional electron sources due to its remarkable characteristics, and correspondingly an FEL injector can achieve a balance between a compact layout and high brightness benefitting from the velocity bunching properties and RF focusing effects in the EC-ITC gun. Nevertheless, the EC-ITC gun has not been extensively examined as part of the FEL injector in the past years. In this regard, to fill this gap, a development focusing on the experimental setup of an FEL injector based on an EC-ITC gun is described in detail. Before assembly, dynamic beam simulations were performed to investigate the optimal mounting position for the Linac associated with the focusing coils, and a suitable radio-frequency (RF) system was established based on a power coupling design and allocation. The testing bench proved to be fully functional through basic experiments using typical diagnostic approaches for estimating primary parameters. Associated with dynamic beam calculations, a performance evaluation for an EC-ITC gun was established while providing indirect testing results for an FEL injector.

Synthesis and x-ray characterization of sputtered bi-alkali antimonide photocathodes

Advanced photoinjectors, which are critical to many next generation accelerators, open the door to new ways of material probing, both as injectors for free electron lasers and for ultra-fast electron diffraction. For these applications, the nonuniformity of the electric field near the cathode caused by surface roughness can be the dominant source of beam emittance. Therefore, improving the photocathode roughness while maintaining quantum efficiency is essential to the improvement of beam brightness. In this paper, we report the demonstration of a bi-alkali antimonide photocathode with an order of magnitude improved roughness by sputter deposition from a K2CsSb sputter target, using in situ and operando X-ray characterizations. We found that a surface roughness of 0.5 nm for a sputtered photocathode with a final thickness of 42 nm can be achieved while still yielding a quantum efficiency of 3.3% at 530 nm wavelength.

Design of RF chopper system for improving beam quality in FEL injector with thermionic gun

For a linac-based Free Electron Laser (FEL), good beam quality largely contributes to the success of the final radiation. An imperfection confronted with the HUST THz-FEL facility is the long beam tail that emerges in the electron gun and exists through the whole beam line. This paper proposes to deploy a chopper system after the electron gun to truncate the beam tails before they enter into the linac. Physical dimensions of the chopper cavity are discussed in detail and we have developed and derived new analytical expressions applying to all frequencies for the optimal design. Also, technical issues of the cavity are considered. Beam dynamic simulation is performed to examine the truncation effect and the results show that more than 78% of the beam tail can be removed effectively, while preserving the emittance and energy spread in acceptable level.

Beam tail effect of a performance-enhanced EC-ITC RF gun

The beam tail effect of multi-bunches will influence the electron beam performance in a high intensity thermionic RF gun. Beam dynamic calculations that illustrate the working states of single beam tail and multi-pulse feed-in of a performance-enhanced EC-ITC (external cathode independent tunable cavity) RF gun for an FEL (free electron laser) injector are performed to estimate the extracted bunch properties. By using both Parmela and homemade MATLAB codes, the effects of a single beam tail as well as interactions of multi-pulses are analyzed, where a ring-based electron algorithm is adopted to calculated RF fields and the space-charge field. Furthermore, the procedure of unexpected deviated-energy particles mixed with an effective bunch head is described by the MATLAB code as well. As a result, the performance-enhanced EC-ITC RF gun is proved to have the capability to extract continual stable bunches suitable for a high requirement THz-FEL.

Initial source of microbunching instability studies in a free electron laser injector

We present the first experimental studies of the initial source of electron beam microbunching instability in a free electron laser (FEL) injector. By utilizing for the studies a transform-limited laser pulse at the photocathode, we eliminated laser-induced microbunching at the National Synchrotron Light Source Source Development Laboratory (SDL). The detailed measurements of the resulting electron beam led us to conclude that, at SDL, microbunching arising from shot noise is not amplified to any significant level, thereby allowing us to set an upper limit on the initial modulation depth of microbunching arising from shot noise. Our analysis demonstrated that the only significant source of microbunching instability under normal operational conditions at SDL is the longitudinal modulation of the photocathode laser pulse. Our work shows that assuring a longitudinally smoothed photocathode laser pulse allows mitigating microbunching instability at a typical FEL injector with a moderate microbunching gain.

Research on DC-RF superconducting photocathode injector for high average power FELs

To obtain high average current electron beams for a high average power Free Electron Laser (FEL), a DC-RF superconducting injector is designed. It consists of a DC extraction gap, a 1+ 1 2 superconducting cavity and a coaxial input system. The DC gap, which takes the form of a Pierce configuration, is connected to the 1+ 1 2 superconducting cavity. The photocathode is attached to the negative electrode of the DC gap. The anode forms the bottom of the 1 2 cavity. Simulations are made to model the beam dynamics of the electron beams extracted by the DC gap and accelerated by the superconducting cavity. High quality electron beams with emittance lower than 3 π-mm-mrad can be obtained. The optimization of experiments with the DC gap, as well as the design of experiments with the coaxial coupler have all been completed. An optimized 1+ 1 2 superconducting cavity is in the process of being studied and manufactured.

Progress of the high current FEL injector at CIAE

Abstract A high current injector for the CIAE free electron laser (FEL) will soon be completed. At present, all the devices have been finished and installed in the accelerator hall. It is expected that the beam will be delivered to the outlet of the buncher before October 1991. A travelling wave accelerator section with a coved cavity structure has been designed.

Rf guns: bright injectors for FEL

Free electron lasers (FELs) require very bright electron beams. During the past few years, rf guns have demonstrated their potentiality to produce such bright beams. A review of the on-going worldwide effort to develop this new technology is given. Present limitations and possible improvements are also presented. General design considerations are introduced.