  We have investigated the role that the transverse electric field of the laser
plays in the acceleration of electrons in a laser wakefield accelerator (LWFA)
operating in the quasi-blowout regime through particle-in-cell code
simulations. In order to ensure that longitudinal compression and/or transverse
focusing of the laser pulse is not needed before the wake can self-trap the
plasma electrons, we have employed the ionization injection technique.
Furthermore, the plasma density is varied such that at the lowest densities,
the laser pulse occupies only a fraction of the first wavelength of the wake
oscillation (the accelerating bucket), whereas at the highest density, the same
duration laser pulse fills the entire first bucket. Although the trapped
electrons execute betatron oscillations due to the ion column in all cases, at
the lowest plasma density they do not interact with the laser field and the
energy gain is all due to the longitudinal wakefield. However, as the density
is increased, there can be a significant contribution to the maximum energy due
to direct laser acceleration (DLA) of those electrons that undergo betatron
motion in the plane of the polarization of the laser pulse. Eventually, DLA can
be the dominant energy gain mechanism over acceleration due to the longitudinal
field at the highest densities.
