A novel X-ray generation mechanism by relativistic flying mirror

利用强激光与等离子体的非线性相互作用，我们研究了一种新的X射线产生机制(相对论飞镜)。在低密度等离子体中，激光激发尾波，在高度非线性区域形成尖点密度分布。由于双多普勒效应，尖点部分反射X射线形式的反向传播激光脉冲。我们实现了两个反向传输的激光脉冲与氦气靶相互作用的实验装置。观察到的反射光子波长为12~20 nm，入射激光波长为820 nm。以光子数表示的反射率为2×10~(-5)~(-5)。这与理论估计值接近，证明了飞镜方案的可行性和有效性。此外，在单个激光脉冲聚焦到具有相对论强度的氦气靶的情况下，我们在正向观察到了具有独特性质的高次谐波辐射。谐波延伸到“水窗”光谱区，包含300多个分辨级数。线偏振激光脉冲和圆偏振激光脉冲都产生了相同强度的奇数次和偶数次谐波。我们提出了一种新的高次谐波产生机制，解释了我们的实验结果。这一机制是基于突变理论，该理论预测了激光脉冲内部结构稳定的电子密度奇异性的形成。计算机模拟显示了奇点的形成和相关的谐波发射，与实验结果一致。

We investigated a novel x-ray generation mechanism (relativistic flying mirror) using the nonlinear interaction of a high-intensity laser with plasma. In an underdense plasma, the laser excites a wake wave and form cusp density profile in the highly nonlinear regime. The cusp partially reflects a counter-propagating laser pulse in the form of X-rays due to the double Doppler effect. We implemented an experimental setup with two counter-propagating laser pulses interacting with the helium gas target. The observed reflected photon wavelength ranged from 12 nm to 20 nm, while the incident laser wavelength was 820 nm. The reflectivity in terms of photon number was 2×10^<-5>. This is close to the theoretical estimate, which proves the feasibility and efficiency of the flying mirror scheme. In addition, with a single laser pulse focused onto a helium gas target with a relativistic intensity, we observed in the forward direction a high-order harmonic radiation with unique properties. The harmonic extend to the 'water window' spectral region and contain more than 300 resolved orders. The harmonics with both even and odd orders of the same intensities are generated by both linearly and circularly polarized laser pulses. We propose a new mechanism of high-order harmonics generation that explains our experimental results. This mechanism is based on catastrophe theory, which predicts structurally stable electron density singularity formation inside the laser pulse. Computer simulations show the singularity formation and associated harmonic emission, in agreement with the experimental results.