Ion acceleration with few-cycle relativistic laser pulses from foil targets

Ion acceleration resulting from the interaction of 11 fs laser pulses of similar to 35 mJ energy with ultrahigh contrast (<10(-10)) and 1019 W cm(-2) peak intensity with foil targets made of various materials and thicknesses at normal (0 degrees) and 45 degrees laser incidence is investigated. The maximum energy of the protons reached similar to 1.4 MeV accelerated in the laser propagation direction and similar to 1.2 MeV in the opposite direction from a formvar target. The energy conversion efficiency from the laser to the proton beam is estimated to be as high as similar to 1.4% at 45 degrees laser incidence using a 51 nm thick Al target. The high laser contrast indicates the predominance of vacuum heating via Brunel's effect as an absorption mechanism involving a tiny pre-plasma at the target front. The experimental results are in reasonable agreement with theoretical estimates, where proton acceleration from the target front side in the backward direction is well explained by the Coulomb explosion of a charged cavity formed in a tiny pre-plasma, while forward proton acceleration is likely to be a two-step process: protons are first accelerated in the target front-side cavity and then further boosted in energy through the target back side via the target normal sheath acceleration (TNSA) mechanism.