Cooling mechanism of expanding plasmas produced by optical-field-induced ionization

Particle-in-cell simulation is carried out to investigate the cooling proce ss of a multicharged dense plasma produced by optical-field-induced ionizat ion. All coulomb collisions among charged particles are taken into consider ation using Nanbu's theory of coulomb collisions and its recent extension t o the case when the weight assigned to a simulated particle varies from par ticle to particle. The effect of coulomb collisions on the expanding plasma is very large. Two cases are considered: expansion of a source plasma 1) i nto a vacuum and 2) into an ambient cold plasma. The results obtained for t he first case agree well with the self-similar hydrodynamic solution. In th e second case, the rapid cooling of the multicharged plasma, which is essen tial for x-ray lasing, is accomplished. The mechanism of this cooling is cl arified by the simulation. It is also shown that the expansion is accompani ed by a dense wave front. Near the wave front, a double layer of electric c harge is formed.