New optical diagnostics for studying laser ablation and induced combustion for carbon materials are key to monitoring the evolving, spatial distribution of the gas plume. We are developing high speed imaging FTIR and gated ICCD imagery for materials processing, manufacture process control, and high energy laser applications. The results from two projects will be discussed. First, an imaging Fourier Transform Spectrometer with a 320 x 256 InSb focal plane array frames at 1.9 kHz with a spatial resolution of 1 mm and spectral resolution of up to 0.25 cm-1. Gas phase plumes above the surface of laser-irradiated black plexiglass, fiberglass and painted thin metals have been spectrally resolved. Molecular emission from CO, CO2, H2O, and hydrocarbons is readily identified. A line-by-line radiative transfer model is used to derive movies for specie concentrations and temperatures. Second, excimer laser pulsed ablation of bulk graphite into low-pressure (0.05 - 1 Torr) argon generates highly ionized, high speed (M>40) plumes. A gated, intensified CCD camera with band pass filtering has been used to generate plume imagery with temporal resolution of 10ns. The Sedov-Taylor shock model characterizes the propagation of the shock front if the dimensionality of the plume is allowed to deviate from ideal spherical expansion. A drag model is more appropriate when the plume approaches extinction (~10 μs) and extends the characterization into the far field. Conversion of laser pulse energy to the shock is efficient.
Plumes were generated by ablation of graphite using a 248 nm excimer laser in the presence of low-pressure argon at
50-1000 mTorr. The pulsed laser deposition of energy on carbon/graphite targets at fluences of 1-5 J/cm2 in low pressure
argon backgrounds yields emissive plumes with large kinetic energies (estimated between 10-200 eV ), driving the formation
of a shock front with large Mach numbers (M). The plumes were investigated using element specific imaging (filtered
and gated ICCD camera), time-of-flight experiments, and
UV - VIS - IR spectroscopy. We expect to see contributions
from atomic carbon as well as the C2 diatomic. Studies showed the importance of plume/substrate interaction in causing
secondary excitation/interaction phenomena. The propagation of the shock front is independent of ionization species and
adequately characterized by the Sedov-Taylor shock model during the early life-time of the plume if the dimensionality
is allowed to deviate from ideal spherical expansion. The ideal efficiency of energy conversion from laser pulse to shock
expansion is investigated. The low background pressures between 50 and 1000 mTorr are sufficient for the generation of a
strong shock front with significant thickness, but may be too low to develop three-dimensional flow. It can be shown that
shock strength is proportional to the Mach number.