Search

C-band RF acceleration is a crucial technology for the compact Free Electron Laser (FEL) facility at the Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences. A project focusing on C-band RF acceleration technology was launched in 2008, based on high-gradient accelerating structures powered by klystron and pulse compressor units. The target accelerating gradient is 40 MV/m or higher. Recently one prototype of C-band RF unit, consisting of a 1.8 m accelerating structure and a klystron with a TE0115 mode pulse compressor, has been tested with high-power and electron beam. Stable operation at 40 MV/m was demonstrated and, 50 MV/m approached by the end of the test. This paper introduces the C-band R&D program at SINAP and presents the experiment results of high-power and beam tests. [ABSTRACT FROM AUTHOR]

A new C-band (5712 MHz) compact spherical radio frequency (RF) pulse compressor was designed for the Soft X-ray Free Electron Laser facility (SXFEL) at the Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences (CAS). Using only one high Q 0 spherical RF resonant cavity which works on two TE 113 modes and a dual-mode polarized coupler, this pulse compressor could achieve an average power gain of 3.8. Associated with the C-band accelerator, an energy gain of 1.85 with the coupling coefficient of 4.9 could be achieved. Particularly it could make the output power stable. This paper presents the scheme of the C-band spherical pulse compressor, as well as the RF design and details of the frequency sensitivities and machining considerations. [ABSTRACT FROM AUTHOR]

An X-band high gradient accelerating structure is a challenging technology for implementation in advanced electron linear accelerator facilities. The present work discusses the design of an X-band accelerating structure for dedicated application to a compact hard X-ray free electron laser facility at the Shanghai Institute of Applied Physics, and numerous design optimizations are conducted with consideration for radio frequency (RF) breakdown, RF efficiency, short-range wakefields, and dipole/quadrupole field modes, to ensure good beam quality and a high accelerating gradient. The designed X-band accelerating structure is a constant gradient structure with a 4π/5 operating mode and input and output dual-feed couplers in a racetrack shape. The design process employs a newly developed effective optimization procedure for optimization of the X-band accelerating structure. In addition, the specific design of couplers providing high beam quality by eliminating dipole field components and reducing quadrupole field components is discussed in detail. [ABSTRACT FROM AUTHOR]

A tuning method augmented by the bead-pull technique based on nonresonant perturbation field distribution measurements has been widely applied for traveling-wave (TW) accelerating structures. The method is also suitable for deflecting structures, but some key considerations of the field components of the HEM 11 mode and the selection of bead merit discussion. A “cage”-type perturbing object has been designed, fabricated and applied in nonresonant perturbation measurements. Measurements on an S-band TW deflecting structure are carried out, and the measurement and tuning method will be used on the newly developed X-band deflecting structure at Shanghai Institute of Applied Physics. [ABSTRACT FROM AUTHOR]

Cascading stages of high-gain harmonic generation (HGHG) have been demonstrated to be a promising candidate for producing fully coherent soft X-ray radiation directly from UV seed sources. However, the large shot-to-shot output pulse energy fluctuation may still be a serious problem for its user applications. In this paper, we study the effects of various electron beam parameters jitters on the output pulse energy fluctuations of a two-stage HGHG. Theoretical calculations and intensive simulations have been performed and the results demonstrate that the relative timing jitter between the electron bunch and the seed laser pulse is mainly responsible for the large output pulse energy fluctuation. Several methods that may be helpful to improve the FEL stability have also been discussed. [ABSTRACT FROM AUTHOR]

Abstract An X -band transverse-deflecting structure is a crucial RF unit for bunch-length measurement conducted at the Shanghai Soft-X-ray Free Electron Laser Facility (SXFEL) at the Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences. The target of this prototype deflector is to operate at 30 MW to have a deflection of 10 MV/structure to realize 20-fs resolution in a full-size structure (0.6 meters long, used on user facility of SXFEL), which satisfies the requirement of the linac beam size measurement at 840 MeV. The program was started in 2010 focusing on X -band deflectors, including design, fabrication, and RF measurement. The electrical tuning of the deflector was successfully done using a unique "cage"-type bead. Recently, a high-power test was conducted at KEK on the first prototype deflector. The high-gradient features at the nominal gradient was proven, but the further increase of power and pulse width was limited at 45 MW with a 173-ns pulse width due to approaching the feasible limit of the modified pointing vector value 5 MW/mm2. In this final stage, frequent breakdowns near the badly brazed point occurred and further conditioning had to be suspended owing to limited available time. [ABSTRACT FROM AUTHOR]

The surface plasmon polaritons make the emittance of the electron beam emitted from the photocathode complicated. In this article, numerical investigations are carried out to study the influence of the plasmonic near field (PNF) excited by a square hole array on the beam emittance. The characteristics of the emittance induced by PNF are studied. The coupling between the emittance caused by PNF and the original intrinsic emittance due to the excess energy of electrons and the surface roughness of the cathode is discussed. The results provide insights into the suppression of the emittance caused by PNF and new possibilities to break the limits of the nature of the materials, improving the performance of a photocathode. [ABSTRACT FROM AUTHOR]

The European XFEL is a free-electron laser facility based on superconducting linac with high repetition rate of up to 4.5 MHz. A corrugated structure is planned to be installed in front of the SASE beam line at the European XFEL, which can be used as a kicker for fresh-slice schemes or a dechirper to control the bandwidth of SASE radiation. When the beam passes through the corrugated structure module, strong longitudinal and transverse wakefields can be excited to introduce a correlated energy chirp and/or a kick along the bunch. However, due to the relatively small gap of the corrugated structure, beam halo particles experience large betatron oscillations after hitting the corrugated structure, which can lead to energy deposition in the undulator beam line. This may cause demagnetization of the undulators especially at high repetition rate. For this reason, we have performed beam loss simulations using BDSIM for two different configurations: double-plate corrugated structure with nominal FODO optics and single-plate corrugated structure with small beta optics. Our simulation results showed that the latter configuration can help us not only to relax the tolerance on beam halo extension, but also to reduce the peak radiation doses by more than factor 2. Besides, the possibility of adding a collimator downstream of the corrugated structure has also been studied, which can further help us to reduce the loss in the downstream cells by factor 100. [ABSTRACT FROM AUTHOR]

Abstract: The development of the micro-pulse electron gun (MPG) mainly relies on the mechanism of the evolutionary process for the multipacting effect. On the other hand, mitigating the limitations of multipacting is what we needed. Here, a new kind of cavity shape is proposed to confine multipacting effect within the spatial range of electrodes, which is of interest to avoiding the occurrence of multipacting in the copper cavity. Based on the theoretical analysis of the saturation process, it is found that increasing the number of impacts for electrons until to saturation is important to attain high quality bunches, while at the expense of losing out-of-phase electrons. In this paper, a novel electrode structure is studied in detail using both theoretical analysis and numerical simulations in order to improve the performance of the gun. After optimizing the cavity shape and electrode structure, the beam quality and saturation currents are maximized and the simulation results are presented. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

An ultrafast electron diffraction (UED) facility is located at Shanghai Jiao Tong University. The layout of the UED is divided into two modules. The accelerating module has one S-band photocathode RF gun, one C-band harmonic buncher, and one C-band deflector. The power-generating module has one S-band klystron and one C-band klystron. To meet the high-precision requirements, a MicroTCA.4-based low level radio frequency (LLRF) system is used to provide a stable RF field in the cavities. In this paper, we introduce the RF distributions, LLRF system, firmware and software of the feedback controller, and field stability of the RF gun; and present an analysis of the measurement results for UED. [ABSTRACT FROM AUTHOR]