  The phase velocity of the wakefield of a laser wakefield accelerator can,
theoretically, be manipulated by shaping the longitudinal plasma density
profile, thus controlling the parameters of the generated electron beam. We
present an experimental method where using a series of shaped longitudinal
plasma density profiles we increased the mean electron peak energy by more than
50%, from 174.8 +/- 1.3 MeV to 262 +/- 9.7 MeV and the maximum peak energy from
182.1 MeV to 363.1 MeV. The divergence follows closely the change of mean
energy and decreases from 58.95 +/- 0.45 mrad to 12.63 +/- 1.17 mrad along the
horizontal axis and from 35.23 +/- 0.27 mrad to 8.26 +/- 0.69 mrad along the
vertical axis. Particle-in-cell simulations show that a ramp in a plasma
density profile can affect the evolution of the wakefield, thus qualitatively
confirming the experimental results. The presented method can increase the
electron energy for a fixed laser power and at the same time offer an energy
tunable source of electrons.
