Development of readout electronics for bunch arrival-time monitor system at SXFEL

A bunch arrival-time monitor(BAM) system,based on electro-optical intensity modulation scheme, is under study at Shanghai Soft X-ray Free Electron Laser.The aim of the study is to achieve high-precision time measurement for minimizing bunch fluctuations. A readout electronics is developed to fulfill the requirements of the BAM system. The readout electronics is mainly composed of a signal conditioning circuit, field-programmable gate array(FPGA), mezzanine card(FMC150), and powerful FPGA carrier board. The signal conditioning circuit converts the laser pulses into electrical pulse signals using a photodiode. Thereafter, it performs splitting and low-noise amplification to achieve the best voltage sampling performance of the dual-channel analog-to-digital converter(ADC) in FMC150. The FMC150 ADC daughter card includes a 14-bit 250 Msps dual-channel high-speed ADC,a clock configuration, and a management module. The powerful FPGA carrier board is a commercial high-performance Xilinx Kintex-7 FPGA evaluation board. To achieve clock and data alignment for ADC data capture at a high sampling rate, we used ISERDES, IDELAY, and dedicated carry-in resources in the Kintex-7 FPGA. This paper presents a detailed development of the readout electronics in the BAM system and its performance.

Analysis of electro-optical intensity modulator for bunch arrival-time monitor at SXFEL

A bunch arrival-time monitor(BAM) based on an electro-optical intensity modulation scheme is currently under development at Shanghai Soft X-ray Free-Electron Laser to meet the high-resolution requirements for bunch stability. The BAM uses a radio frequency signal generated by a pickup cavity to modulate the reference laser pulses in an electro-optical intensity modulator(EOM), and the bunch arrival-time information is derived from the amplitude change of the laser pulse after laser pulse modulation.EOM is a key optical component in the BAM system.Through the basic principle analysis of BAM, many parameters of the EOM are observed to affect the measurement resolution of the BAM system. Therefore, a systematic analysis of the EOM is crucial. In this paper, we present two schemes to compare and analyze an EOM and provide a reference for selecting a new version of the EOM.

Dispersion effects on performance of free-electron laser based on laser wakefield accelerator

In this study, we investigate a new simple scheme using a planar undulator(PU) together with a properly dispersed electron beam(e beam) with a large energy spread(～1%) to enhance the free-electron laser(FEL) gain. For a dispersed e beam in a PU, the resonant condition is satisfied for the center electrons, while the frequency detuning increases for the off-center electrons, inhibiting the growth of the radiation. The PU can act as a filter for selecting the electrons near the beam center to achieve the radiation. Although only the center electrons contribute, the radiation can be enhanced significantly owing to the high-peak current of the beam. Theoretical analysis and simulation results indicate that this method can be used for the improvement of the radiation performance, which has great significance for short-wavelength FEL applications.