Transition to efficient, unsuppressed bulk-target ion acceleration via high-fluence laser irradiation

A high-intensity laser irradiating a few-μm solid foil will accelerate ions from the bulk of the target as well as protons from a surface contaminant layer. Experimental measurements of ion spectra using the OMEGA EP laser (0.25–1 kJ, 10 ps) show, as suggested previously [Petrov et al., Phys. Plasmas 17, 103111 (2010)], that at a laser fluence exceeding 1 J/μm2, the contaminant layer is accelerated enough that ions from the bulk of the target are more effectively accelerated. When using CD2 as a target, the high fluence results in a 100-fold increase in deuteron acceleration efficiency (near 1% of laser energy) compared to subthreshold fluence. This is found to be due to the fact that the deuterons have a higher density at many locations during acceleration, allowing a larger electric field to develop, leading to improved efficiency. Using a pitcher-catcher setup, these deuterons, as well as protons from the contaminant layer, strike a LiF target and generate neutrons via (d,n) and (p,n) nuclear reactions. CR39 plastic and nuclear activation detectors measured broadband neutron yields of 4E9/sr and yields of 1E8/sr for neutrons above 11 MeV.