7A9--Density and temperature of a laser induced plasma

The plasma "blow-off vapor," produced when a high power laser beam strikes a solid surface, has been studied experimentally and theoretically. Electron densities and plasma optical thicknesses have been measured both in time and position resolved manners for plasmas produced from carbon. The ruby laser system consisted of a "Q-switched" oscillator and one amplifier, both 7 inches long and 0.6 inch in diameter. The system energy output was between 8 and 13 joules in a 0.055 μ-second pulse width at half height. The electron density was measured using a gas laser as a probe light source and a Mach-Zender interferometer to measure the phase shifts due to plasma refractivity. The experiments were performed with incident laser energies of 10 to 1000 joules/cm²(by focusing). Electron densities as high as 10¹⁹cm^-3were observed, with correspondingly high plasma optical thicknesses. There were strong indications of heating by the incident laser beam of the front edge of the plasma, which was not expected on the basis of plasma heating by free-free and bound-free absorption due to the low electron density existing there. In general, a theoretical treatment of plasma heating by the bound-free and free-free absorption agreed well with experiments.