Laser wakefield acceleration relies on the excitation of a plasma wave due to the ponderomotive force of an intense laser pulse. However, plasma wave trains in the wake of the laser have scarcely been studied directly in experiments. Here we use few-cycle shadowgraphy in conjunction with interferometry to quantify plasma waves excited by the laser within the density range of GeV-scale accelerators, i.e., a few ${10}^{18}{\mathrm{cm}}^{-3}$. While analytical models suggest a clear dependency between the nonlinear plasma wavelength and the peak potential $a_0$, our study shows that the analytical models are only accurate for driver strength $a_0\lesssim 1$. Experimental data and systematic particle-in-cell simulations reveal that nonlinear lengthening of the plasma wave train depends not solely on the laser peak intensity but also on the waist of the focal spot.

中文翻译：

---

激光尾波加速器中的非线性等离子体波长缩放

由于强激光脉冲的动能，激光尾波加速依赖于等离子体波的激发。但是，几乎没有在实验中直接研究过激光唤醒后的等离子体波列。在这里，我们结合干涉法使用了少周期的阴影摄影术，以量化在GeV尺度加速器的密度范围内由激光激发的等离子体波，即，${10}^{\mathrm{18岁}}{\mathrm{厘米}}^{-3}$。分析模型表明非线性等离子体波长和峰值电势之间存在明显的相关性${\mathrm{一种}}_0$，我们的研究表明，分析模型仅对驾驶员的力量准确 ${\mathrm{一种}}_0\lesssim \mathrm{1个}$。实验数据和系统的单元内粒子模拟显示，等离子波列的非线性延长不仅取决于激光峰值强度，还取决于焦点的腰部。