Self-similar motion of laser half-space plasmas. II. Thermal wave and intermediate regimes

The one-dimensional self-similar motion of an initially cold, half-space plasma of electron density n0, produced by the (anomalous) absorption of a laser pulse of irradiation φ=φ0t/τ (0<t⩽τ) at the critical density nc, is considered; the analysis, which allows for electron heat conduction and ion-electron energy exchange, involves three dimensionless numbers: ε≡ nc/n0 assumed small, Zi (ion charge number), and a parameter α∝ (n02τ/φ0)2/3. For α≪1, a weak discontinuity develops, separating a thermal wave (where convection is negligible) moving into the undisturbed plasma, from a much thinner isothermal flow expanding into the vacuum. For 1≪α≪ε−4/3, there is an isentropic compression flow between a shock, bounding the undisturbed plasma, and a very thin transition layer bounding an expansion flow, much larger in extent than the compression region. In both regimes, the critical plane lies far in the expansion tail. The results break down when the density is so small that the plasma becomes collisionless. The analysis is also invalid for α too small. Using results previously found for α≫ε−4/3, a qualitative discussion of how plasma behavior changes with α, is given.