IONIZATION DYNAMICS OF IRON PLUMES GENERATED BY LASER-ABLATION VERSUSA LASER-ABLATION-ASSISTED-PLASMA DISCHARGE ION-SOURCE

The ionization dynamics (iron ion and neutral atom absolute line densi ties) produced in the KrF excimer laser ablation of iron and a laser-a blation-assisted plasma discharge (LAAPD) ion source have been charact erized by a new dye-laser-based resonant ultraviolet interferometry di agnostic. The ablated material is produced by focusing a KrF excimer l aser (248 nm,< 1 J, 40 ns) onto a solid iron target. The LAAPD ion sou rce configuration employs an annular electrode in front of the grounde d target. Simultaneous to the excimer laser striking the target, a thr ee-element, inductor-capacitor, pulse-forming network is discharged ac ross the electrode-target gap. Peak discharge parameters of 3600 V and 680 A yield a peak discharge power of 1.3 MW through the laser ablati on plume. Iron neutral atom line densities are measured by tuning the dye laser near the 271.903 nm (a D-5-y (5)p(0)) ground-state and 273.3 58 mm (a F-5-w D-5(0)) excited-state transitions while iron singly ion ized line densities are measured using the 263.105 nm (a D-6-z D-6(0)) and 273.955 nm (a D-4-z D-4(0)) excited-state transitions. The line d ensity, expansion velocity, temperature, and number of each species ha ve been characterized as a function of time for laser ablation and the LAAPD. Data analysis assuming a Boltzmann distribution yields the ion ization ratio (n(i)/n(n)) and indicates that the laser ablation plume is substantially ionized. With application of the discharge, neutral i ron atoms are depleted from the plume, while iron ions are created, re sulting in a factor of similar to 5 increase in the plume ionization r atio. Species temperatures range from 0.5 to 1.0 eV while ion line den sities in excess of 1 x 10(15) cm(-2) have been measured, implying pea k ion densities of similar to 1 X 10(15) cm(-3). (C) 1996 American Ins titute of Physics.