Generating Intense Ultrashort Radiation By Laser-plasma Interaction And The Expansion Of Electron-positron Plasma

With the arrival of short pulse light sources, properties of matter on very short time scales as well as new research areas such as high energy density physics are being explored. Since the short pulse light sources have important applications in many areas, new radiation sources are being developed, providing radiation with higher intensity, shorter wavelength, and shorter duration. Driven by such advances in the laser technology, research on laser-plasma interaction based on compact radiation sources have been rapidly advancing. One topic of this thesis is on the generation of ultra-short intense radiation from the interaction of a short pulse laser and a thin solid-density target.On the other hand, advances in short pulse technology have led to significant increases in the laser intensity as well as new physical phenomena and processes, such as in high energy density physics. For light intensity at～1029W/cm2, electron-positron pair creation in vacuum can be expected. Moreover, electron-positron pair plasmas are believed to be ubiquitous in the very early universe. They can also be produced in laboratory by intense-laser interaction with heavy atoms. It is therefore of interest to investigate the behavior of electron-positron plasmas. The second topic of this thesis is on the nonlinear behavior of expanding electron-positron plasmas.The main content of this thesis is as follows:1. Particle-in-Cell simulation and analytical modeling demonstrate that the reflection of a single-cycle circularly polarized light pulse from a thin target can produce an ultra-short ultra-intense electromagnetic field. In simulations, upon impact of the intense laser pulse the target electrons are greatly compressed and accelerated forward, creating an intense space-charge field. The latter tends to pull back the high-density compressed electron layer, so that most of the incident laser field is effectively reflected by a fast-moving mirror. Furthermore, there is a large difference in relativistic Doppler shifts for different part of the incident pulse which is also distorted by the highly nonlinear laser-plasma interaction. As a result, the reflected pulse is highly deformed, and an isolated propagating spike of high-intensity ultra-short electromagnetic field is obtained. We also find a model for the evolution of the interaction structure in the nonlinear reflection process. The characteristics of the reflected pulse agree well with that from the analytical model that takes into account the relativistic Doppler effect induced by the moving laser compressed electron layer.2. Three dimensional nonlinear motion of cold fluid electron-positron plasma is investigated. By first introducing basis electrons and positron flows, the time and the three spatial degrees of freedom of the flow fields are separated and the resulting set of ordinary differential equations governing cold plasma are solved non-perturbatively. Exact solutions describing the expanding plasma are then obtained. It is found that in general the energy in the irrotational flow components is transferred to the rotational flow component, but not in the reversed direction. Furthermore, depending on the initial conditions, there can appear purely electrostatic or electromagnetic, as well as hybrid oscillations, and the oscillations can also be limited to certain degrees of freedom.3. Large amplitude oscillations in the expansion of a cylindrical electron-positron plasma layer are investigated. The cold fluid equations and the Poisson’s equation are solved non-perturbatively in order to allow for very large amplitude oscillations. It is found that oscillations are self-excited during the expansion and can grow to very large amplitudes as the expansion slows down with time and the plasma density decreases, if there is slight imbalance of the initial density or flow of the electrons and positrons. Kinetic energy of the expansion is thereby converted into that of the oscillations. Regardless of the initial imbalance among the electron and positron flow components, the oscillations always appear mainly in the space-independent velocity components. However, when the plasma layer is initially more non-neutral, significant oscillations can appear in all the flow components, as well as the layer boundaries. The oscillation frequency is red-shifted from the linear value of the plasma frequency.