No abstract

Carbon plasmas produced by radiation from a ruby laser (wavelength 694.3 nm) focussed onto a carbon target in vacuum are studied spectroscopically with a time resolution of 40 ns. Measured line profiles of several ionic species (CI-CIV) were used to infer electron density and temperature at several positions above the target surface as function of time elapsed after the beginning of the laser pulse. The particle density at several positions above the target surface as function of time was judged from corrected line intensities. Experimental data are compared with theoretical predictions made with the effusion model of plasma expansion (Kelly R and Braren B 1991 Appl. Phys. B 53 160). The effusion model provided the relative particle density in the expanding plasma cloud as a function of initial target temperature. By comparing predicted and measured time evolution of particle density, an initial target temperature of about 125eV was inferred. The coupling of the laser beam energy to the plasma itself was inferred from the failure of the model of the direct target surface heating (Andreić Z, Henč-Bartolić V and Kunze H-J 1993 Physica Scripta 48 331) to produce the required target temperature.

Titanium plasmas produced in vacuum and in air by radiation from a nitrogen laser focused onto a solid titanium target are studied spectroscopically. The energy deposition is more effective than in other cases since the wavelength of the laser is in the vicinity of Ti resonance lines.

Pulses from a CO,-and a N,-laser are focused onto a zirconium target, and the released atomic beams are analysed by the laser-induced fluorescence technique at various distances from the target surface. The average beam velocity exceeds quite drastically the velocity expected from any thermal model; the populations of the fine structure levels of the ground-state, on the other hand, correspond to a temperature below the expected lattice temperature. The influence of surface oxygen is investigated.

13mJ laser pulses from a nitrogen laser were focused onto an aluminum target in air. The target surface was perpendicular to the axis of the laser beam. A peak energy density of 1.3 J/cm2 and a power density of 80 MW/ anz were achieved with a laser pulse duration of 1611s. The high power density produced a transient plasma cloud that expanded explosively into the surrounding atmosphere. An initial electron density of about 1 x 10'9cm3 and an electron temperature of about 2eV were determined by optical spectroscopy. The line of sight was parallel to the surface and perpendicular to the laser beam axis. The height of the line of sight above the target surface was varied in order to gather data about the whole plasma cloud. In about 50011s the plasma cloud expands to about OSmm above the target surface, cools down to about 1.2eV and is tenfold reduced in electron density. The initial expansion velocity was determined to be about 2 km/s. The experimentally determined plasma parameters were input into numerical models of target heating and plasma expansion. The numerical results outrule the so called outflow model of plasma expansion and show reasonable agreement with an effusion model. The observed discrepancies in observed and calculated plasma parameters are attributed to the fact that the theoretical models describe the plasma expansion in vacuum only.