The impact of contaminants on laser-driven light ion acceleration

Abstract

The impact of contaminants on laser-driven ion acceleration is investigated using particle-in-cell simulations. The conventional ion acceleration mechanism, target normal sheath acceleration, has been revisited for targets with proton-rich contaminants in the form of water vapor. The targets considered have a deuterated plastic layer on the rear surface of an aluminum target, and the influence of the contaminant layer on the deuteron acceleration is investigated. In the early stage of ion acceleration, the space-charge electrostatic field on the rear target surface accelerates only the outermost, proton-rich layer of ions, which inhibits the deuteron acceleration by shielding it from the field. When the proton layer is depleted, the deuterons become exposed to the space-charge field and are promptly accelerated. This scenario was tested with a two-dimensional particle-in-cell simulation model by varying the contaminant layer thickness and laser fluence (laser energy per unit area). For laser fluences Flaser<1 J/μm2, the contamination layer over the surface inhibits the deuteron acceleration from the rear surface, while in the opposite case of laser fluences Flaser>1 J/μm2 deuterons and heavier ions can be successfully accelerated with conversion efficiency of laser energy into ions of more than 1%. Experimental data from a 6 μm thick aluminum foil coated with a 1 μm deuterated plastic layer on the back surface are suggestive of the detrimental role of contaminants on deuteron acceleration.