Electron Acceleration In Wake Field By Intense Combined-pulse Laser In Underdense Plasma

When an intense short-pulse laser propagates in plasma, the electrons in the plasma can be endued with considerably high energy. Up to now, people usually focus on two mechanisms associated with the acceleration of electrons: the ponderomotive acceleration in the laser pulses and the wake field acceleration in the plasma. When the peak intensity of the laser pulse exceeds the relativistic laser intensity, the electric field parallel to the propagation direction (longitudinal component) of the laser will emerge by rather large amplitude under the effect of the ponderomotive force (optical pressure). The phase velocity of the plasma wave approaches the velocity of light due to the action of the longitudinal electric field. The electrons injected along the propagating direction of the plasma wave (or the electrons in the plasma) will be trapped by the plasma. Those electrons that happen to be in the accelerating phase of the strong electric field of the plasma wave will be accelerated to obtain extremely high energy. This procedure is so-called ponderomotive acceleration. On the other hand, the laser-excited wake plasma oscillation is produced by the separation between the electrons and the ions. The spatial displacement of the electrons can give rise to a large space charge field, whose relaxation after the pulse moves away can lead to strong electrostatic wake oscillations. A small number of electrons are released when wave breaking occurs. For the foregoing two accelerated mechanisms, some investigators have exposed that the relative efficiencies of the two acceleration processes depend on the response of the electrons to the pulse and is governed by the laser intensity and the pulse width, etc. In this paper we recur the relative efficiencies of the two acceleration processes using a one-dimensional (1D) analysis when an intense short-pulse laser propagates in an underdense plasma. For a relatively longer intense short-pulse laser, especially when local phase matching in the laser pulse between the ponderomotive force and the wake plasma oscillation occurs, there is strong electron acceleration in the laser-excited wake field and weak acceleration inside the laser pulse. For a sufficiently short and intense laser, there is strong acceleration of the electron inside the laser pulse, and the corresponding wake field acceleration becomes much weaker. For the same laser intensity, it is shown that the optimum energy of accelerated electrons in the wake field is much higher than that of the ponderomotively accelerated electrons inside the laser pulse when a pulse laser interacts with a plasma. Especially considering the maximum momentum synchronized by the maximum density of wake accelerated electrons, we are apt to invoke the acceleration in the laser-excited wake field.The most direct way of promoting the energy of the electrons is to increase the peak intensity of the pulse laser. However, limited by the experimental condition, the promoting of the laser intensity is not an easy task in practice. The main task of this paper is to provide an alternative method of accelerating electrons. We propose the employment of an definitely arranged group of pulse laser which is called combined-pulse laser, instead of a single pulse laser for the enhancement of electron acceleration. It is found that the energy attained by the electrons under the action of combined-pulse laser is several times larger than that attained under a single pulse laser. The paper hammers at the momentum and density of the electrons in the wake field induced by the interaction of the intense short combined-pulse laser with an underdense plasma. Here the term 'combined-pulse' refers to a group of laser pulse with definite space-time sequence. We set circularly polarized two combined-pulse laser as an example. When we add a second pulse laser at the instant that the momentum of the electrons in the oscillating wake field reaches its positive maximum, the optimum value of the momentum and density of the accelerated electrons are alm...