Influence of the Focus Position on Preplasma Formation and Ion Acceleration in Laser Irradiation of a Flat Target

It has been demonstrated experimentally and in the theoretical calculation that the energy of protons accelerated by a 60-TW femtosecond laser pulse can be increased by choosing the optimal position of the focus spot relative to the surface of the irradiated target at a given nanosecond pedestal due to amplified spontaneous emission. This position is determined as the best focusing option that ensures a sufficiently efficient matching of incident and preplasma-trapped laser radiation under the conditions of emerging self-consistent spatial profiles of the preplasma and crater (i.e., the remaining thickness of the target) on the irradiated surface. At the same time, the focal spot does not lie on the surface of the target, but is shifted into the preplasma, which makes it possible to raise the maximum proton energy by a factor of 1.5.