Effect of Prepulses on the Generation of Fast Protons in a Flat Target Under the Action of a High-Power Picosecond Laser Pulse

We analyze experimental results of the acceleration of protons at the rear side of an aluminum target under the action of a picosecond laser pulse (an intensity of 2 · 1018 W/cm2 and a wavelength of 1.06 μm). The picosecond pulse has two prepulses, picosecond and nanosecond (a prepulse of amplified spontaneous emission of 8 ns duration). We examine a target of thickness 10 μm, at which the energy of protons is maximum. The decrease in the energy of protons with increase in the target thickness is due to the increase in ionization losses of the energy of fast electrons in passing from the region of their generation to the rear side of the target. A drop in energy of protons at target thicknesses less than 10 μm is due to the breakdown of the target rear side owing to a split-off phenomenon at the exit of the shock wave to the surface. We consider various cases of the target rear side breakdown depending on the aggregate state of the target material behind the front of the shock wave (solid, liquid, and gaseous states).