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.
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