【摘要】 百MHz重复频率电子加速器的工作物质是百MHz重复频率待加速电子束脉冲串。该电子束脉冲串是由百MHz重复频率驱动激光脉冲串辐照加速器起始处的光阴极产生的，其是获取百MHz重复频率自由电子激光的先决条件。目前，百MHz重复频率紫外激光器尚无成熟产品，因此需要开展百MHz重复频率激光产生技术的探索。利用偏振分束器件的空间光分光延时技术产生了百MHz重复频率驱动激光，将钛宝石激光器输出的波长为266 nm、脉冲长度为1.2 ps的单脉冲激光，改造成包含4个脉冲、重复频率为433 MHz、脉冲长度为20 ps的驱动激光脉冲串。开展了基于该驱动激光产生电子束串的相关实验研究。搭建了空间光分光光路，分光后的激光脉冲能量一致性优于3%，指向一致性优于50μm。更多还原

【Abstract】 Objective The X-ray Free Electron Laser(XFEL) facility serves as a cornerstone of modern scientific infrastructure, capable of delivering intense X-ray pulses that penetrate samples to extract internal information, such as static atomic-scale structures and dynamic evolutions on tens of femtosecond(fs) timescales. In XFEL facility, a single electron beam produces only one X-ray pulse at a time. The quality of the electron beam, with brightness, coherence, and lateral distribution as its core parameters, plays a critical role in X-ray laser pulse generation. Currently, one of the major research areas in XFEL development is the advancement of hard Xray FEL systems with repetition rates in the hundreds of MHz to GHz range. The hundreds of MHz to GHz repetition rate of the electron beams requires the driver laser to operate at the same repetition rate. However, to date, ultraviolet(UV) laser facilities cannot operate at hundreds of MHz repetition rate or higher. To support the construction of XFEL facility with a repetition rate in the hundreds of MHz to GHz range, it is necessary to conduct research on hundreds of MHz repetition rate driving laser techniques based on the currently more mature Hz-level high-energy laser platform.Based on the spatial light splitting technique, the high-energy single UV laser pulse, with a single-pulse energy exceeding 700 μJ generated in the Ti∶Sapphire laser facility, is transformed into four driving laser pulses with a 433 MHz repetition rate, which travel in different optical paths. The original pulse duration of ～1 ps is stacked into a flat-top distribution of 20 ps using several α-barium borate(α-BBO) crystals, and the generated 4 laser pulses with a 433 MHz repetition rate are delivered to the photocathode electron gun to generate a low-emittance electron beam pulse train by electro-optical effect. In this paper, it is reported for the first time that a highrepetition rate driving laser pulse train is generated using the spatial light splitting technique, and the emittance of the 433 MHz repetition rate electron beam pulses is lower than 0.5 μm.Methods On the Ti∶Sapphire laser experimental platform, two light splitting arms, in which the key optical element is the thin film polarizer(TFP) used to split one laser pulse into two equal parts, are employed to generate four 1 ps laser pulses from a single 1 ps laser pulse. By controlling the lengths of the different arms in which the laser pulses travel, the repetition rate of the four laser pulses is around 433.33 MHz. To control the energy of each pulse, a 1/2 wave plate is inserted in front of the TFP to rotate the polarization state of the laser pulse before the polarizer and thus to adjust the energy consistency of the laser pulses. The pulse interval is about 2.307 ns(1/433 MHz), and the corresponding optical path difference is about 738 mm. Therefore, the optical path differences of the two arms are 769 mm and 1538 mm, respectively. The fine-tuning of the pulse interval is achieved by adjusting the one-dimensional translation stage, on which the reflectors are located.The longitudinal shaping of the pulses uses the birefringence effect to achieve ps-level pulse-interval fine-tuning, and the pulses are stacked to form a 20 ps longitudinal platform. In this article, 4 α-BBO crystals are used to achieve 20 ps pulses. The thicknesses of the crystals are 2.49, 4.98, 9.96, and 19.92 mm, and the delays achieved are 1.25, 2.5, 5, and 10 ps, respectively. After passing through 4 α-BBO crystals, a single micro-pulse is shaped into a flat-top pulse composed of 16 micro-pulses.Results and Discussions After experimental debugging, a 433 MHz repetition rate four-pulse driving laser is realized. After passing through the α-BBO crystal, its pulse duration is adjusted from 1.2 ps to 20 ps with a longitudinal flat-top distribution. The total energy of the four-pulse driving laser at the photocathode is 6 μJ, and the energy of each micro-pulse is about 1.5 μJ. A charge of not less than 200 pC is generated on the Cs2 Te cathode. The generated electron beam emittance can be less than 0.5 μm.Conclusions The single-pulse laser is split into a driving laser pulse train containing four micro-pulses with a repetition rate of 433 MHz by using the spatial light splitting method. These four micro-pulses are longitudinally shaped into 20 ps long pulses in sequence using four BBO crystals. After beam splitting and longitudinal shaping, the driving laser pulse train is transmitted into the electron gun to generate an electron beam pulse train with a repetition rate of 433 MHz and duration of 20 ps. The generated electron beam emittance can be less than 0.5 μm.更多还原