OPTICAL ABSORPTION AND EXPANSION OF LASER-PRODUCED PLASMAS

Calculations of the absorption, of laser radiation by an inhomogeneous overdense plasma have been carried out. Such a plasma is produced when a high-powered laser irradiates a solid particle. Nearly complete absorption is found as long as 2R is greater than cτ; here, R is the scale length for density falloff, c is the velocity, of light, and τ is the electron-ion collision time for a plasma frequency equal to the incident laser frequency. For hydrogen this gives complete absorption up to temperatures of 5 keV if R is greater than 5 × 10-2 cm; higher Z materials absorb better. The expansion of a laser-produced plasma was also investigated. In the case of a plasma produced from a thin metallic film, it is found that the expansion preferentially takes place normal to the film which is in agreement with observation.