Laser-accelerated electrons are promising in producing gamma-photon beams of high peak flux for the study of nuclear photonics, obtaining copious positrons and exploring photon–photon interaction in vacuum. We report on the experimental generation of brilliant gamma-ray beams with not only high photon yield but also low divergence, based on picosecond laser-accelerated electrons. The 120 J 1 ps laser pulse drives self-modulated wakefield acceleration in a high-density gas jet and generates tens-of-MeV electrons with 26 nC and divergence as small as $1.51{}^{\circ}$ . These collimated electrons produce gamma-ray photons through bremsstrahlung radiation when transversing a high-Z solid target. We design a high-energy-resolution Compton-scattering spectrometer and find that a total photon number of $2.2\times {10}^9$ is captured within an acceptance angle of $1.1{}^{\circ}$ for photon energies up to $16\;\mathrm{MeV}$ . Comparison between the experimental results and Monte Carlo simulations illustrates that the photon beam inherits the small divergence from electrons, corresponding to a total photon number of $2.2\times {10}^{11}$ and a divergence of $7.73{}^{\circ}$ .

A linearly polarized Laguerre–Gaussian (LP-LG) laser beam with a twist index $l = -1$ has field structure that fundamentally differs from the field structure of a conventional linearly polarized Gaussian beam. Close to the axis of the LP-LG beam, the longitudinal electric and magnetic fields dominate over the transverse components. This structure offers an attractive opportunity to accelerate electrons in vacuum. It is shown, using three-dimensional particle-in-cell simulations, that this scenario can be realized by reflecting an LP-LG laser off a plasma with a sharp density gradient. The simulations indicate that a 600 TW LP-LG laser beam effectively injects electrons into the beam during the reflection. The electrons that are injected close to the laser axis experience a prolonged longitudinal acceleration by the longitudinal laser electric field. The electrons form distinct monoenergetic bunches with a small divergence angle. The energy in the most energetic bunch is 0.29 GeV. The bunch charge is 6 pC and its duration is approximately $270$ as. The divergence angle is just ${0.57}^{\circ }$ (10 mrad). By using a linearly polarized rather than a circularly polarized Laguerre–Gaussian beam, our scheme makes it easier to demonstrate the electron acceleration experimentally at a high-power laser facility.

采用紧聚焦的超强短脉冲激光与固体通道靶相互作用是获得大电量、高准直相对论电子束的一种有效方式。实验中由于激光预脉冲烧蚀靶壁产生预等离子体会膨胀、填充到真空通道中，从而导致电子束品质发生变化。采用二维PIC粒子模拟程序研究了通道靶中填充预等离子体的电子加速过程。模拟结果显示，在功率密度为5.0 $ \times {10}^{20}\;{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2} $的超强短脉冲激光条件下，通道中填充一定密度的等离子体时激光场优先与低密度等离子体相互作用，激光脉冲与通道壁的相互作用减弱，电子加速机制由纵向场主导的真空电子加速转变为横向电场主导的等离子体电子加速，产生电子束具有更大的电荷量，但能量降低，发散角增大。

针对空泡机制中的双束等离子体尾波电子加速设计，给出了能够快速得到被加速束流在最大加速距离下的相对能散的预测公式。通过加速初始时刻束流纵向分布以及束流所处位置的纵向尾波场可得到束流最终相对能散。该预测公式不仅可应用于驱动束流与被加速束流初始能量相同的情况，还可应用于两个束流初始能量不相同的情况。由该预测公式得到的束流相对能散与被加速束流和驱动束流的初始能量的比值有关，而与两个束流初始能量的数值无关。利用准静态近似的粒子网格模拟程序QuickPIC对理论进行了模拟验证，模拟结果与理论预期结果一致。

The improved laser-to-pedestal contrast ratio enabled by current high-power laser pulse cleaning techniques allows the fine features of the target survive before the main laser pulse arrives. We propose to introduce the nano-fabrication technologies into laser–plasma interaction to explore the novel effects of micro-structures. We found out that not only laser-driven particle sources but also the laser pulse itself can be manipulated by specifically designed micro-cylinder and -tube targets, respectively. The proposal was supported by full-3D particle-in-cell simulations and successful proof-of-principle experiments for the first time. We believe this would open a way to manipulate relativistic laser–plasma interaction at the micro-size level.