  Three-dimensional Particle-in-Cell (PIC) simulations with the code QuickPIC
are used to illustrate the typical accelerating structures associated with the
interaction of an intense laser beam with an underdense plasma in the blowout
regime. Our simulations are performed with an externally injected electron
beam, positioned in the region of maximum accelerating gradients. As the laser
propagates in the plasma, almost complete electron cavitation occurs, leading
to the generation of accelerating fields in excess of 1 GeV/cm.
