Target-normal sheath acceleration (TNSA) of monoenergetic proton bunches using a triple-layer target consisting of a long slightly-above-critical-density carbon preplasma, a thin solid-density carbon main target layer, and a thin hydrogen layer attached to its back, is considered. The target-back space-charge field the consists of mainly the preplasma electrons that have been accelerated and heated by the relativistic laser pulse. The backside electron cloud is thus denser and more favorably profiled for TNSA of the protons from the hydrogen layer. It is found that a proton beam with a ∼460 MeV monoenergetic peak energy can be generated. The corresponding energy and angular spread are ∼33 MeV and ∼3°, respectively.

We report an efficient scheme to improve the proton acceleration and energy conversion efficiency by using double laser pulses with foil interaction. We find a significant increase in the peak energy, the total number, and the maximum energy of the accelerated protons for the double laser pulses with foil interaction compared to those in the single laser pulse case, while the total laser energy is kept constant. The role of the first pulse (pre-pulse) is to change the target electron distribution and reduce the reflection of succeeding laser pulse and hence enhance the laser absorption so that more energy of the second laser pulse (main pulse) is converted into the particle energy. The main pulse preferentially accelerates the slower electrons located deeper in the plasma, and it also accelerates the fast electrons due to volumetric heating. Finally, the protons are accelerated to high energy due to the laser break-out afterburner, when the target becomes relativistically transparent to the laser pulse.

The acceleration of protons from the interaction of two intersecting laser pulses in a double-layer target, consisting of a carbon slab with a thin hydrogen layer behind it, is investigated. It is found that a bunch of well-collimated, up to 270 MeV, quasi-monoenergetic protons can be generated. The affected target electrons are heated and accelerated forward by the laser pulses so that the target gradually becomes transparent as the lasers bore into it. Eventually, the thin target becomes fully transparent. The converging laser pulses can then pass through it and the hot backside electron cloud, together with the electrons trapped in them. As a result, behind the target, an inhomogeneous sheath electric field is formed, which not only accelerates the protons but also merges them into a quasi-monoenergetic bunch with low divergence. The energy and spatial spread of the bunch, as well as the peak energy, can be controlled by manipulating the locations of the two lasers.