Finite collisional age effects on electrostatic shocks*

In strictly collisionless electrostatic shocks the ion distribution function can develop discontinuities along phase space separatrices, due to the ion reflection. However the ions in laser-plasma experiments operating with dense and/or high-Z targets are not completely collisionless. In a semi-analytical orbit-averaged kinetic time-dependent treatment we consider the effect of a small but finite collisionality, which acts to smoothen these discontinuities into growing boundary layers. Ions diffusing into the previously empty trapped regions of phase space upset the charge balance, and affect the downstream properties of the shock. Importantly, this is a cumulative process, thus the collisional age of the shock, which can become significant during the lifetime of the shock, is more relevant for the process than the collision frequency, even if the latter is small. After comparisons to the theory, simulations with the continuum solver Gkeyll [J. Juno et al 2018 J. Comp. Phys. 353, 110] are used to study the evolution of weakly collisional shocks, at order unity collisional age.