  The dynamic process of a laser or particle beam propagating from vacuum into
underdense plasma has been investigated theoretically. Our theoretical model
combines a Lagrangian fluid model with the classic quasistatic wakefield
theory. It is found that background electrons can be injected into wakefields
because sharp vacuum-plasma transitions can reduce the injection threshold. The
injection condition, injection threshold as well as the injection length can be
given theoretically by our model and are compared with results from computer
simulations. Moreover, electron beams of high qualities can be produced near
the injection thresholds and the proposed scheme is promising in reducing the
injection threshold and improving the beam qualities of plasma based
accelerators.

Plasma Physics;Accelerator Physics