Magnetic centrifuge effects in ultrafast laser ablation plasmas

Abstract

A self-consistent model is developed to explain the anomalously large enrichment of nickel isotopes observed in ablation plumes from ultrafast laser irradiation of solid surfaces. The model is based on the spontaneous creation of a magnetic centrifuge in the ablation plume and the associated cyclotron rotation of plasma ions with effective rotation rates of the order of 109 rad/s. Mass separation occurs around the radial coordinate of cylindrical symmetry with the longitudinal axis normal to the ablating surface. A Gaussian shaped radial magnetic field Beff is extracted for Ni isotopes which is shown to be a combination of an axial Bz component and a second contribution Bibw that represents the equivalent of an “effective magnetic field” contributing to the isotopic separation due to broad spectrum Ion Bernstein Waves (IBWs) providing electrostatic acceleration to the cyclotron orbits. These IBWs are also responsible for a profound resonance of enrichment observed for certain specific charge states. In addition to cyclotron rotation of ions, a rigid-rotor model is also presented that is associated with the hydrodynamic rotation of the entire plasma and is shown to be of little consequence for the isotope enrichment. Cyclotron rotation and IBWs dominate the process.