A complex target (CT) configuration tailored for generating high quality proton bunch by circularly polarized laser pulses at intensities of 1020-21 W/cm2 is proposed. Two-dimensional particle-in-cell simulations show that both the collimation and mono-energetic qualities of the accelerated proton bunch obtained using a front-shaped thin foil can be greatly enhanced by the backside inhomogeneous plasma layer. The main mechanisms for improving the accelerated protons are identified and discussed. These include stabilization of the photon cavity, providing hole-boring supplementary acceleration and suppressing the thermal-electron effects. A theory for tailoring the CT parameters is also presented.

Multistaged acceleration of solid-density thin foils by ultraintense circularly polarized laser pulse is investigated. A stable radiation pressure acceleration (RPA) stage is first established. Higher dimensional effects such as transverse instabilities and enhanced electron heating then gradually make the initially opaque foil transparent to the laser light. Accordingly, the dominant acceleration mechanism changes smoothly from RPA to target normal sheath acceleration (TNSA). The transition can therefore enhance the maximum energy of the accelerated ions but broaden their energy spectrum. For a double-layer target, however, the light ions (protons) in the backlayer can be efficiently accelerated in the RPA and TNSA regimes nearly monoenergetically. Two-dimensional particle-in-cell simulations show that with this scheme a circularly polarized laser pulse of peak intensity 3.9×1022 W/cm2 can produce a collimated proton bunch that persists for many Rayleigh lengths and its peak energy can reach 4.2 GeV with FWHM of 200 MeV.

Backward Raman amplification (BRA) in plasmas has been promoted as a means for generating ultrapowerful laser pulses. For the purpose of achieving the maximum intensities over the shortest distances, an envelope matching between the seed pulse and the amplification gain is required, i.e., the seed pulse propagates at the same velocity with the gain such that the peak of the seed pulse can always enjoy the maximum gain. However, such an envelope matching is absent in traditional BRA because in the latter the amplification gain propagates at superluminous velocity while the seed pulse propagates at the group velocity, which is less than the speed of light. It is shown here that, by using self-ionizing plasmas, the speed of the amplification gain can be well reduced to reach the envelope matching regime. This results in a favorable BRA process, in which higher saturated intensity, shorter interaction length and higher energy-transfer efficiency are achieved. V C 2014 AIP Publishing LLC. [http://dx.

Two-dimensional particle-in-cell simulation is used to investigate the acceleration of foil ions by the radiation pressure of an ultraintense short laser pulse in the presence of a background plasma of proper density and length behind the foil. It is shown that under appropriate conditions a central foil segment with a transverse dimension ϳ L / 2, where L is the laser spot size, can be stably accelerated. In this plasma backed acceleration scheme, foil electron heating and ion expansion are greatly suppressed by the cool backside electrons that replace the expelled fast electrons in the target, so that a monoenergetic collimated GeV ion sheet is produced. The simulation results agree with that from a physical model for the stably accelerated foil segment.