  Three-dimensional particle-in-cell simulation is used to investigate the
witness proton acceleration in underdense plasma with a short intense
Laguerre-Gaussian (LG) laser pulse. Driven by the LG10 laser pulse, a special
bubble with an electron pillar on the axis is formed, in which protons can be
well-confined by the generated transversal focusing field and accelerated by
the longitudinal wakefield. The risk of scattering prior to acceleration with a
Gaussian laser pulse in underdense plasma is avoided, and protons are
accelerated stably to much higher energy. In simulation, a proton beam has been
accelerated to 7 GeV from 1 GeV in underdense tritium plasma driven by a
2.14x1022 W/cm2 LG10 laser pulse.
