Studies Of Electrons Acceleration By Chirped Laser Pulse

As a clean and simple physical system, vacuum laser acceleration has received considerable attention in recent years. However, a laser field propagating in an unlimited free space is periodic and its phase velocity is normally faster than the light speed c. If fast phase slippage appears between the electron and the laser field, this will induce no net energy gain in the electron because of the oscillation of the electron between the acceleration phase and the deceleration phase. In fact, the key problem in vacuum laser acceleration is how to break down the symmetry between acceleration and deceleration so that the former becomes greater than the latter. There are two main types of vacuum laser acceleration:the capture and acceleration scenario (CAS) and Ponderomotive acceleration. Several people have proposed schemes using chirped laser pulses in vacuum. Their simulation results show that the chirped laser pulses could dramatically enhance the acceleration effect.In this paper, we study the mechanism of electron acceleration by chirped laser pulse. We found that, although the chirp mechanism is similar to CAS since electron be trapped in acceleration phase for a long times and accelerated to very high energy in main acceleration stage, the cause of the physical process is different. Electron with initial high-energy moving in subluminous phase velocity region just be trapped in main acceleration stage in CAS. In chirp mechanism, electron with initial low-energy is injected into high phase velocity region, because of the chirp effect, a region exists where the laser wave phase experienced by the electron varies slowly, so that the electron can be accelerated for a long time. The influence of chirp coefficient and beam waist width over electron acceleration was also analyzed in this paper. It is found that chirped laser pulses could also enhance the proton acceleration effect.In order to enhance our understanding of the nature of chirp mechanism, we study the phase velocity of laser field in this paper. The evolution of the phase velocity of the wave field along the electron trajectory is also investigated. We proposed the minimum phase velocity along the trajectory of relativistic electron in moving coordinate.