Theoretical Study of Topics in the Interaction of Laser and Under-dense Plasma

Abstract

Laser-plasma interaction is an interesting field with numerous practical and scientific applications. In this thesis, I will present three topics in the system of laser and underdense plasma: terahertz (THz) generation by a current driving near cutoff of plasma, plasma diagnostics by Raman scattering, and particle injection in laser wake field acceleration. For the first issue, I will present a new mechanism of generating THz using colliding laser pulses in a magnetized plasma. In this scheme, a THz radiation with a monochromatic feature and high power of 0.1MW was observed. Every aspect of the phenomena was analyzed successfully by a new theory. From this study, I and my collaborators found a new physics of growing electromagnetic field in the range of cut-off frequency. As a 2nd topic, a new diagnosing method of magnetized plasmas is presented. A simple theory of X-mode dispersion relation indicates that frequency shift of Raman backward scattering is much sensitive to the magnetic field than the Raman forward scattering. It is shown that such a difference between RBS and RFS can be used for measuring the magnetic field and plasma density simultaneously. The result is compared with 1D PIC simulation. In the final topic, I will discuss an improved theoretical condition of electron trapping into the ellipsoidal bubble in laser wake field acceleration. The improved theory describes better the electron injection in a more generally shaped bubble than just the previous spherical bubble model, and also exhibits good agreement with the 3D PIC simulations. In addition these topics require different PIC techniques. I will also briefly introduce various numerical techniques used in simulating those topics.