Experimental and theoretical studies of the time and space development of plasma parameters in a laser induced spark in helium

An experimental study has been made of laser induced breakdown of a gas. 0⋅2 J of optical frequency radiation was absorbed in helium at 4 atm initial pressure from the focused beam of a Q switched ruby laser. Space and time resolved quantitative data on the plasma development has been compared with theoretical models and some new results obtained. Two main experimental techniques were used. Image converter streak photography gave information on the axial and radial growth of the plasma and the time variation of the emission spectrum. Quantitative spectroscopy techniques were used to study the plasma parameters, leading to electron density, temperature and spatial dimensions. Two phases in the plasma evolution are discussed. The first, during which absorption of the incident laser pulse occurs, is characterized by the formation of a radiation driven breakdown wave. The front propagation and the axial dynamic processes behind the front are discussed. New results concerning the structure behind the front and the shape of the front itself are presented and the axial front propagation is consistent with an existing theoretical model. The second phase is a purely thermally driven expansion. A model for the radial development of the plasma in this phase is outlined, based on blast wave theory but including a treatment of ionization. From the experimental evidence, ionization is only of secondary importance in the basic radial expansion. The data suggest a self similar expansion occurring at a rate consistent with the old blast wave theory but involving radial distributions of parameters departing at the centre from those of the old theory. Good agreement was found between experimental data and predictions of this modified blast wave theory concerning the plasma dimensions and electron number density.