A novel X-ray generation mechanism by relativistic flying mirror

点击翻译按钮获取中文摘要

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.