This paper is mainly concerned with Nd:YAG laser induced plasmas in air at different pressures. A modelling of the formation of laser plasmas is presented. The preionization phase is described using time differential equations that are solved in the basis of the Crank–Nicolson second order scheme. Moreover, the following dynamic phase, described as a strong shock wave expanding out of the focal volume, is simulated by using a two-dimensional compressible flow calculation code. The evolution of pressure, temperature, densities and velocities are analysed and compared with experimental measurements. The mean electron density of the plasma measured through an interferometric method leads to ( ), which correlates with computed results.

Nd : YAG laser triggering of a centimetric air gap in a homogeneous electric field is studied for pressures ranged from 0.5 to 5 bar. Breakdown transient voltage and current are measured and the formative time lag dependence on reduced field and laser intensity is analysed. It is quantitatively shown that the laser field strongly influences the time lag to breakdown through the effective reduced field.

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