Generation of Secondary Particles From Subnanosecond Laser Irradiation of Targets at Intensities of $10^{16}~{\rm W}~{\rm cm}^{-2}$

Emission of several tens of mA/cm² currents carried by MeV-protons is reported for plasmas produced by a beam of the 3-TW kJ-class Prague Asterix Laser System (PALS) focused onto foil targets of various thicknesses. The maximum proton energy of 4.2 MeV is observed at a modest irradiance (Iλ² ~ 2×10¹⁶ W cm^-2μm²). According to the maximum ion energy scaling EM ~ (Iλ²)^0.4 found for picosecond lasers such a value should be reached at Iλ² ~ 3×10¹⁸ W cm^-2 μm². Enhancement of the maximum proton energy can be explained by nonlinear interaction of the PALS laser beam with the generated plasma. Nonlinear effects also result in enhancement in the yield of fusion neutrons from CD₂ plasma. The obtained yield of 4×10⁵ DD-neutrons per laser energy is comparable to those obtained by the use of ps- and fs-lasers irradiating targets with Iλ² ~ 10¹⁹ W cm^-2 μm².