  Density dowmramp injection has been demonstrated to be an elegant and
efficient approach for generating high quality electron beams in laser
wakefield accelerators. Yet, the charge of the produced beam is tens of pC per
Joule of laser energy, still limiting its use for a wider range of
applications. The possibility of generating high charge beam while keeping a
good beam quality, stays to be explored. Moreover, despite previous studies
focused on separate physical processes such as beam loading which affects the
uniformity of the acceleration field and thus the energy spread of the trapped
electrons, repulsive force from the rear spike of the bubble which reduces the
transverse momentum $p_\perp$ of the trapped electrons and results in small
beam emmittance, and the laser evolution when travelling in plasma. A more
general investigation of the plasma density parameters on the final beam
properties is required. In this work, we demonstrate that the current profile
of the injected electron beam is directly correlated with the density
transition parameters, which further affects the beam charge and energy spread.
By fine-tuning the plasma density parameters, high-charge (up to several
hundreds of pC) and low-energy-spread (around 1\% FWHM) electron beams can be
obtained. All these results are supported by large-scale three-dimensional
particle-in-cell simulations.
