Collisionless absorption in sharp-edged plasmas

The absorption of subpicosecond, obliquely incident laser light is studied using a 11/2D particle-in-cell code. Density scale lengths from L/λ=0.01 to 2 and laser irradiances between I${\lambda }^2$=${10}^{14}$ and ${10}^{18}$ W ${\mathrm{cm}}^{\mathrm{-}2}$ μ${\mathrm{m}}^2$ are considered. ‘‘Vacuum heating’’ [F. Brunel, Phys. Rev. Lett. 59, 52 (1987)] dominates over resonance absorption for scale lengths L/λ<0.1, and is most efficient when ${\mathit{v}}_{\mathrm{osc}}$/c≃3.1(L/λ${)}^2$. Absorbed energy is carried mainly by a ‘‘superhot’’ electron population with ${\mathit{U}}_{\mathrm{hot}}$∼(I${\lambda }^2$ ${)}^{1/3–1/2}$.