Comparative study of CO2 laser-produced tin-droplet plasma with and without pre-pulse laser has been presented. A pre-pulse laser and the CO2 laser was combined and focused to tin-droplet with a diameter of 180 µm. The emitted Sn ions were detected by several Faraday cups to obtain angular distribution of ions in the laser-produced tin-droplet plasma. The influence of pre-pulse laser energy and delay time between pumping laser and pre-pulse laser on the ion characteristics was investigated. It is illustrated that ion average kinetic energy from CO2 laser-produced plasma (LPP) can be reduced when the tin-droplet target has been replaced by the preformed Sn plasma. The obtained optimal delay time with the lowest ion average kinetic energy is about hundreds of nanoseconds. The ion time-of-flight spectra show a twin peak structure in laser-irradiating preformed Sn plasma. And a superimposed Maxwell–Boltzmann (MB) distribution is proposed to describe this twin peak ion time-of-flight spectra. The fitting results quite agree with the raw ion time-of-flight spectra in current experiment. Then, the fitted plasma temperatures and mass-center velocities with various delay times in laser-irradiating preformed plasma are obtained, and the fitted plasma temperatures can be comparable with ion average kinetic energy in double-pulse LPP, which justified the rationality using this superimposed MB distribution.

A scheme for the improvement of proton beam quality by the optimized dragging field from the interaction of ultraintense laser pulse with a complex double-layer target is proposed and demonstrated by one-dimensional particle-in-cell (Opic1D) simulations. The complex double-layer target consists of an overdense proton thin foil followed by a mixed hydrocarbon (CH) underdense plasma. Because of the existence of carbon ions, the dragging field in the mixed CH underdense plasma becomes stronger and flatter in the location of the proton beam than that in a pure hydrogen (H) underdense plasma. The optimized dragging field can keep trapping and accelerating protons in the mixed CH underdense target to high quality. Consequently, the energy spread of the proton beam in the mixed CH underdense plasma can be greatly reduced down to 2.6% and average energy of protons can reach to 9 GeV with circularly polarized lasers at intensities 2.74 × 1022 W/cm2.