Control of ion-velocity distributions in laser–target interaction experiments

The width of ion-velocity distributions from laser-produced plasmas can be controlled experimentally by varying the size of the laser spot. This ion-velocity width is determined primarily by whether the ions are mostly in the rarefaction or steady-state regime. It is not dominated by the thermal distribution of the ions, as is sometimes assumed. A simple analytic theory shows that the distribution width is governed by the scaling parameter rs/cτ, where rs is the focal spot radius, c is the sound speed, and τ is the laser pulse length.