Detailed dynamics of electron beams self-trapped and accelerated in a self-modulated laser wakefield

The electron beam generated in a self-modulated laser-wakefield accelerator is characterized in detail. A transverse normalized emittance of 0.06 pi m m mrad, the lowest ever for an electron injector, was measured for 2 MeV el ectrons. The electron beam was observed to have a multicomponent beam profi le and energy distribution. The latter also undergoes discrete transitions as the laser power or plasma density is varied. In addition, dark spots tha t form regular modes were observed in the electron beam profile. These feat ures are explained by analysis and test particle simulations of electron dy namics during acceleration in a three-dimensional plasma wakefield. (C) 199 9 American Institute of Physics. [S1070-664X(99)03612-5].