Confinement and acceleration of pellet material by one-sided laser irradiation

Abstract

A simple model is developed to predict the shape of the laser–pellet interface for pellet material being accelerated by one-sided laser irradiation. The ablation pressure is assumed to be much greater than the yield strength of the pellet material which is treated as a gas. The model predicts a shape of the critical surface contour that is consistent with that inferred from experimental data. The key new concept used is that, while the laser light is on, the freely flowing pellet material is not only accelerated but also completely confined by the laser ablation. In addition, it is shown that a hollow laser intensity profile, with the scale length for intensity change about equal to the pellet size, not only keeps the pellet material centered on the beam but also stabilizes the Rayleigh–Taylor instability over most of the laser–material interface. Thus concerns about overstressing or shock heating initially very soft pellet material, such as solid hydrogen, with very high acceleration pressure are alleviated. A possible method for accelerating millimeter-sized hydrogen pellets to very high velocity (∼200 km/sec) by laser irradiation on one side is described.