| This thesis details research on the interaction of femtosecond laser pulses with materials. Specifically, the temporal and spatial evolution of the ablation plume is the focus, starting from the creation of the plasma state as a damage threshold phenomenon, continuing to the absorption of additional laser energy by the plasma, and ending with the expansion of the plasma as part of the ablation plume.; The main focus of this thesis is on mass separation driven by self-generated electromagnetic fields in the ablation plume. The separation of stable isotopes from a solid target and metallic elements from an alloy target are studied. Results are presented concerning the spatial distributions of ions in ultrafast laser ablation plasmas and their relationship to the temporal and spatial evolution of the plume. The isotopic enrichment of light ions in the central portion of the plume is similar to the results from a conventional plasma centrifuge, in particular a wire-wound, vacuum-arc, plasma centrifuge. In the laser-ablation results, the relative independence of the mass separation among different elements and other similarities in the ion distributions have indicated a universality to this effect that lends credence to and helps to develop the plasma centrifuge model. The externally-applied axial magnetic field in the conventional plasma centrifuge is replaced with a self-generated longitudinal magnetic field in the laser ablation plasma. Ions execute centrifugal motion about the longitudinal field and are separated by mass. Beyond this simple comparison, the plasma-centrifuge model is developed for the specific case of ultrafast ablation, with the full evolution of the plume. The angular dependence of the mass separation can be mapped into a spatial dependence across a deposited thin film. Research is presented for ablation targets of ceramics (BN, GaN), metals (Cu, Zn, Ti), and an alloy (Cu:Ni).; Of particular importance to applications is the total enrichment yield. The dependence of the mass separation on laser parameters is studied, including the cases of picosecond and nanosecond pulsed-laser ablation. Methods for enhancing the mass separation are discussed with a particular emphasis on results using secondary laser pulses to further ionize and pump the expanding plasma plume. |
