High energy conversion efficiency in laser-proton acceleration by controlling laser-energy deposition onto thin foil targets.

An all-optical approach to laser-proton acceleration enhancement is investigated using the simplest of target designs to demonstrate application-relevant levels of energy conversion efficiency between laser and protons. Controlled deposition of laser energy, in the form of a double-pulse temporal envelope, is investigated in combination with thin foil targets in which recirculation of laser-accelerated electrons can lead to optimal conditions for coupling laser drive energy into the proton beam. This approach is shown to deliver a substantial enhancement in the coupling of laser energy to 5-30 MeV protons, compared to single pulse irradiation, reaching a record high 15% conversion efficiency with a temporal separation of 1 ps between the two pulses and a 5 lm-thick Au foil. A 1D simulation code is used to support and explain the origin of the observation of an optimum pulse separation of ~1 ps. [ABSTRACT FROM AUTHOR]