  Proton acceleration by ultra-intense laser pulse irradiating a target with
cross-section smaller than the laser spot size and connected to a parabolic
density channel is investigated. The target splits the laser into two parallel
propagating parts, which snowplow the back-side plasma electrons along their
paths, creating two adjacent parallel wakes and an intense return current in
the gap between them. The radiation-pressure pre-accelerated target protons
trapped in the wake fields now undergo acceleration as well as collimation by
the quasistatic wake electrostatic and magnetic fields. Particle-in-cell (PIC)
simulation shows that stable long-distance acceleration can be realized, and a
30 fs monoenergetic ion beam of > 10 GeV peak energy and < 2degree divergence
can be produced by a 9.8 *10^21 W/cm2 circularly polarized laser pulse.
