Study of bubble dynamics in laser wakefield acceleration in pettawatt regime

The bubble regime of the laser wakefield acceleration is one of the most recent and promising mechanism for generating quasi-monoenergetic electron beams. In this work, we propose to study the dynamics of bubble (a wake structure devoid of electrons created in underdense plasma by a highly-intense ultrashort laser pulse) in petawatt regime. The dependence of the electron beam energy and the quality of the electron beam on the shape of the bubble is the main motivation behind this work. The bubble length as well as bubble shape have been investigated using PIC simulations. The evolution of bubble with time, and the correlation of bubble length (longitudinal and transverse radius) with the intensity of laser pulse have been revealed in this study. It has been observed that the bubble longitudinal length grows until the dephasing length. The rate of change of bubble dimensions can be estimated by various determining factors such as the laser pulse focusing, the beam loading, the residual electrons, and the bubble velocity. It has also been confirmed that the shape of the bubble cannot be predicted using fixed shape models as spherical or elliptical. Simulations unveil that as the bubble traverses in plasma, it evolves from spherical shape to the highly elliptical shape. And, as it approaches the dephasing length, the eccentricity decreases further. Consequently, the self-injection of plasma electrons in the bubble is seriously affected by the bubble evolution. Comparison of the electron energy gain at different intensities of laser pulse has also been provided. The study has been carried out using two-dimensional Particle-in-Cell simulation code. (author)