Laser Shaping By The Influence Of Energetic Electron Flux On The Self-Induced Transparency

In-situ diagnosis of mesoscale and microscale structure is one of the most important challenges for research of dynamic response of materials. The rapid development of high-power laser technology provides feasible diagnostic tools for these researches. In such technology, the laser-driven ultrafast X-ray can probe the ultrafast process of material microstructure. Making use of X rays to penetrate dense or compressed matter with X-ray-scattering technique, we can measure the state of materials under compression, such as the fundamental characteristics of thermo-dynamic properties including phase transitions and new states of matter. The crystallographic structure of materials can be also diagnosed by X-ray diffraction. As well we can capture the image of the shock front due to the compression of the plasma driven by laser through X-ray density image. In short, the use of laser-driven X rays allows us a better understanding of the change in the microscopic structure of materials in the dynamic process, and provides a new experimental tool to get insight into the equation of state, the crystallographic structure of materials, and so on.Intense laser-solid interactions offer a new ultrafast X-ray source for the diagnostic studies on the dynamic micro-mesoscopic properties of materials based on the high-power laser technology. Comparing with the synchrotron radiation source of huge investment, experimental footprint, relatively long pulse structure and polychromaticity, as well as the expensive X-ray free electron laser, the laser-driven ultrafast X-ray source can provide an alternative due to its compactness, subpicosecond pulse duration, and monochromaticity, making a wide range of applications in the laboratory and the medical diagnosis.A large photon flux of X-ray is necessary for ultrafast X-ray probe, which remains the challenge of application of laser-driven X-ray sources. For example, the Ka X-ray conversion efficiency is only about10-5at present. Many methods have been developed to improve the X-ray yield, which can be summarized as laser-pulse optimization and foil optimization in principle. Foil optimization, with special geometries such as cone-shaped target, can reduce the absorption of X-ray and improve the concentration. We can also modify the microstructure of the foil surface, such as nanolayered target, improving laser absorption and increasing the number of hot-electron, thereby enhance the X-ray yield.Laser-pulse optimization is another important way to increase the X-ray yield. In the laser-matter interaction, the plasma is formed by the prepulse of laser ionizing the surface of the matter, the density distribution of the plasma plays a crucial role in the laser-plasma interaction, which is determined by the prepulse and the rise front of laser. Therefore, to improve the conversion efficiency of laser-driven ultrafast X-ray source, it is necessary to achieve the control of the plasma, and hence the control of the prepulse and the rise front of laser.Self-induced transparency (SIT) of overdense plasmas makes it possible to control the pulse shape. Our research showed that the influence of energetic electron flux on the SIT can be used for pulse shaping by the control of the prepulse and the rise front of laser, and thus it is able to achieve accurate control of the gradient of plasma density, increasing the energy absorption efficiency of hot electron, and thus improving the conversion efficiency of laser-driven ultrafast X-ray source.This paper first introduced the physical principle of laser-driven X-ray source and its yield optimization, and proposed a scheme for increasing the X-ray yield by pulse shaping through the influence of energetic electron flux on the SIT. We then presented the self-induced transparency in detail. And then the influence of energetic electron refluxing on the SIT was carefully discussed. Finally, we studied the influence of energetic electron flux on the SIT and with which to shape the laser pulse, getting the control of the prepulse and the rise front of laser. Our main research results and innovations are listed as following:(1) We first found that energetic electron refluxing can influence the SIT, making the laser with intensity lower than the threshold intensity of SIT could penetrate through the overdense plasma. This phenomenon can be interpreted as that. the electron density peak due to the compression by laser prevents the penetration of the laser into the plasmas, when the energetic electron refluxing arrives the front surface, it will generate the Coulomb force which causes the electrons to dissipate, then the laser can penetrate through the plasma.(2) We also found that the energetic electron flux has the same influence on the SIT as the energetic electron refluxing.(3) We shaped the laser pulse with the influence of energetic electron flux on the SIT, achieving the control of the prepulse and the rise front of laser, and got a laser pulse with a high contrast and a steep rise front.