  Laser-driven ion accelerators have the advantages of compact size, high
density, and short bunch duration over conventional accelerators. Nevertheless,
it is still challenging to generate ion beams with quasi-monoenergetic peak and
low divergence in the experiments with the current ultra-high intensity laser
and thin target technologies. Here we propose a scheme that a Laguerre Gaussian
laser irradiates a near-critical-density (NCD) plasma to generate a
quasi-monoenergetic and low-divergence proton beam. The Laguerre Gaussian laser
pulse in NCD plasma excites a moving longitudinal electrostatic field with a
large amplitude, and it maintains the inward bowl-shape for dozens of laser
durations. This special distribution of the longitudinal electrostatic field
can accelerate and converge the protons simultaneously. Our particle-in-cell
(PIC) simulation shows that the efficient proton acceleration can be realized
with the Laguerre Gaussian laser intensity ranging from 3.9-10^21 Wcm^-2
-1.6-10^22 Wcm^-2 available in the near future, e.g., a quasi-monoenergetic
proton beam with peak energy ~115 MeV and divergence angles less than 50 can be
generated by a 5.3-10^21 Wcm^-2 pulse. This work could provide a reference for
the high-quality ion beam generation with PW-class laser systems available
recently.
