Laser–plasma acceleration at kilohertz repetition rates has recently been shown to work in two different regimes with pulse lengths of either 30 fs or 3.5 fs. We now report on a systematic study in which a large range of pulse durations and plasma densities were investigated through continuous tuning of the laser spectral bandwidth. Indeed, two laser–plasma accelerator (LPA) processes can be distinguished, where beams of the highest quality, with a charge of 5.4 pC and a spectrum peaked at 2–2.5 MeV, are obtained with short pulses propagating at moderate plasma densities. Through particle-in-cell (PIC) simulations, the two different acceleration processes are thoroughly explained. Finally, we proceed to show the results of a 5-h continuous and stable run of our LPA accelerator accumulating more than $18\times {10}^6$ consecutive shots, with a charge of 2.6 pC and a peaked 2.5 MeV spectrum. A parametric study of the influence of the laser driver energy through PIC simulations underlines that this unprecedented stability was obtained thanks to micro-scale density gradient injection. Together, these results represent an important step toward stable laser–plasma accelerated electron beams at kilohertz repetition rates.

中文翻译：

---

千赫激光等离子体加速器的优化和稳定性

最近已经证明，以千赫兹重复率的激光等离子体加速可以在两种不同的情况下工作，脉冲长度为30 fs或3.5 fs。现在，我们报告了一项系统研究，其中通过连续调整激光光谱带宽来研究大范围的脉冲持续时间和等离子体密度。确实，可以区分两种激光等离子体加速器（LPA）工艺，其中在中等等离子体密度下传播的短脉冲可获得最高质量的光束，其电荷为5.4 pC，峰峰值为2–2.5 MeV。通过细胞内颗粒（PIC）仿真，对两种不同的加速过程进行了详尽的解释。最后，我们继续显示LPA加速器连续5个小时稳定运行的结果，该加速器积累了超过$\mathrm{18岁}\times {10}^6$连续拍摄，电荷为2.6 pC，峰值为2.5 MeV光谱。通过PIC仿真对激光驱动器能量的影响进行了参数研究，结果表明，由于采用了微米级密度梯度注入技术，因此获得了前所未有的稳定性。总之，这些结果代表了以千赫兹重复频率实现稳定的激光等离子体加速电子束的重要一步。