Effects of plasma density on relativistic self-injection for electron laser wake-field acceleration

Density effects on the dynamics of a cavity produced in the wake of an ultraintense (a(0)=eE/mcomega>1) and short (omega(pl)tau/pi<1) laser pulse and on the duration of accelerated electrons are studied via two-dimensional particle-in-cell simulation. Formation of a nonbreaking cavity is a crucial part of relativistic self-injection of plasma electrons from the front of a laser pulse and their further acceleration leading to a beam-quality femtosecond bunch. This self-injection appears in a uniform plasma when the group velocity of the pulse becomes smaller than the maximal electron velocity accelerated in the ponderomotive bias, Phi=mc(2)a(0)(2)/2. However with increasing density, this mechanism starts to contend with relativistic wave breaking. Though additional injection due to the relativistic wave breaking increases the total charge of energetic electrons, the duration of the bunch increases to the picosecond range and its energy distribution becomes a Maxwellian. (C) 2004 American Institute of Physics.