Two-stage acceleration of protons from relativistic laser-solid interaction

A two-stage proton acceleration scheme using present-day intense lasers and a unique target design is proposed. The target system consists of a hollow cylinder with conical inner wall, which is followed by the main target with a flat front and a dishlike flared rear surface. At the center of the latter is a tapered proton layer, which is surrounded by side proton layers at an angle to it. In the first acceleration stage, protons in both layers are accelerated by target normal sheath acceleration. The center-layer protons are accelerated forward along the axis while the side protons are accelerated and focused towards them. As a result, the side-layer protons radially compress as well as axially further accelerate the front part of the center-layer protons in the second stage. Two-dimensional (2D) particle-in-cell (PIC) simulations show that a quasimonoenergetic proton bunch with the maximum energy over 250 MeV and energy spread similar to 17% can be generated when such a target is irradiated with an 80 fs laser pulse with focused intensity 3.1 x 10(20) W/cm(2). Three-dimensional (3D) PIC simulation gives the reduced maximum energy similar to 112 MeV but even smaller energy spread similar to 3% under the same laser conditions due to anisotropic electron acceleration with linearly polarized lasers.