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 c⊥ here, R is the scale length for density falloff, c is the velocity of light, and ⊥ 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.