Method for the calculation of electrical field in laser-generated plasma for ion stream production

Ion streams are produced by hot plasmas laser generated at INFN-LNS of Catania by using a 9 ns Nd:YAG laser which irradiates heavy element targets (Ta and Au) placed in vacuum. The ion emission from the plasma was measured with an electrostatic ion energy analyzer either for the energy-to-charge ratio or for the ion energy distributions. The ion emission occurs mainly along the normal to the target surface; the ion stream has a maximum charge state of 10+; the ions' kinetic energy ranges from hundreds of eV up to values of about 6 keV. In order to evaluate the production and acceleration processes a theoretical framework was defined and cross-checked with experimental results. The ion energy distributions can be fitted with Boltzmann-Coulomb-shifted functions giving the ion plasma temperature, the thermal velocity, the adiabatic expansion velocity of the plasma, and the Coulomb velocity. The ion energy distribution increased with the charge state (about 600 eV/charge state). This result and the emission normal to the target surface indicate the presence of an electrical field inside the plasma which can be calculated by the method described hereinafter. Assuming that the electrical field acts for a distance comparable with the Debye length, the calculated electrical field is of the order of 5 MV/cm, in good agreement with the literature data. Measurements and calculation will be presented and discussed in detail