Characteristics of Electron Acceleration in a Self-modulated Laser Wakefield

Acceleration of electrons by an electron plasma wave has been of great current interest because of its much larger acceleration gradient than that of conventional rf linacs [1]. Several methods have been proposed for driving a largeamplitude fast-phase-velocity plasma wave [1]. In the Laser Wake-Field Accelerator (LWFA), an electron plasma wave is driven resonantly by a short laser pulse through longitudinal laser ponderomotive force [2]. In the SelfModulated Laser Wake-Field Accelerator (SMLWFA), an electron plasma wave is excited by a relatively long laser pulse undergoing stimulated Raman forward scattering instability [3, 4, 5]. The injection of electrons can occur by trapping of hot background electrons, which are preheated by other processes such as Raman backscattering and sidescattering instabilities [6, 7, 8], or by wavebreaking (longitudinal [1] or transverse [9]). It can also be achieved by specific injection schemes [10, 11] in order to control the characteristics of the generated electron beam. Several groups have observed the generation of MeV electrons from the SMLWFA [7, 8, 12, 13, 14, 15]. In this experiment, the electron beam produced from a selfmodulated laser wakefield accelerator injected with selftrapping of electrons was characterized in detail. The observations of up-to-three-component electron-beam profiles and up-to-two discrete changes in the slope of electron energy distribution are reported. In addition, dark spots that form regular modes were observed in the first beam-profile component. These new observations provide us important new clues to the underlying dynamics of electron acceleration in a three-dimensional (3-D) plasma wave.