Ionization dynamics of laser ablation and a laser-ablation-assisted-plasma-discharge ion source.

Abstract

A new type of ion source has been developed which combines KrF excimer laser ablation and a pulsed power discharge. The ionization dynamics produced in the laser ablation of aluminum and iron, along with the laser-ablation-assisted-plasma-discharge (LAAPD) ion source have been characterized by numerous plasma diagnostics including a newly developed resonant ultraviolet interferometry (RUVI) diagnostic. Focusing a KrF excimer laser (248 nm, 1 J, 4-10 J/cm$\sp2$, 40 ns) onto a solid iron or aluminum target generates the ablated material. The LAAPD ion source configuration employs an annular electrode in front of the grounded target. Simultaneous to the excimer laser striking the target, a pulse-forming network is discharged across 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-ablation plume. Both time-integrated and time-resolved optical emission spectroscopy show the laser ablation plume spectra dominated by neutral atom transitions with ion emission only occurring at early times ($<$100 ns). Discharge electron temperatures are inferred in the range of 1-3 eV while the laser ablation plume electron temperature is approximately 0.5 eV. With application of the discharge, ion optical emission is detected with significant contribution from Al III and Fe II transitions. The optical emission evolution closely follows the magnitude of the discharge plasma current, indicative of ohmic heating. Absorption photography shows decreased neutral atom absorption and enhanced ion absorption over that from only laser ablation when the discharge is applied. Resonant photographs also show the iron plume is very forward directed, while the aluminum plume expands more strongly in the radial direction. Iron neutral atom line-densities are measured with the RUVI diagnostic by tuning the dye laser near the 271.903 nm ground-state and 273.358 nm excited-state transitions, while iron singly-ionized line-densities are measured using the 273.955 and 263.105 nm excited-state transitions. The line-density, expansion velocity, temperature, and number of each species has been characterized as a function of time for laser ablation and the LAAPD. Data analysis, assuming a Boltzmann distribution, yields the ionization ratio $\rm(n\sb{i}/n\sb{n})$ and indicates substantial laser ablation plume ionization. With application of the discharge, the plume ionization ratio increases by a factor of $\sim$5 to $\rm n\sb{i}/n\sb{n}$ of approximately 20. Ion line-densities in excess of $1 \times 10\sp{15}$ cm$\sp{-2}$ have been measured implying peak ion densities of ${\sim}1\times 10\sp{15}$ cm$\sp{-3}$ consistent with values obtained from a Langmuir probe measurement.