Pulsed laser ablation of pyrolytic graphite with a 4-J / cm2 KrF laser in backgrounds of air, argon, nitrogen, and helium at pressures up to 10 Torr was performed to study the plume dynamics. Optical emission imaging with a 2-ns gated intensified charged-coupled device camera was used to determine shock front positions and plume trajectories for characterization by free expansion, Sedov–Taylor (ST) blast, and drag models. The plume expands with initial Mach numbers of M = 55, decreasing to M ∼ 20 as the emission becomes too weak to detect. The plumes begin with a planar shock front and thickness of a few mean free paths, but evolve to higher dimensionality, n, depending on pressure and mass of the background gas. The ST energy released in the sudden ablation is typically 33% the laser pulse energy. Blast energy and plume dimensionality are correlated with stopping distances, which are typically greater than 103 mean free paths. Upper bounds for the mass ablated (1 to 2 μg / pulse) and hole depth (∼88 to 183 nm) are inferred from the shock kinetic energy relative to laser pulse energy. The inferred hole depth ranges from 0.32 to 0.67 percent of the thermal diffusion length as the pressure increases from 1 to 10 Torr for these conditions where the fluence is just above twice the ablation threshold.
Porous graphite plates, cylinders and cones with densities of 1.55-1.82 g/cm3 were irradiated by a 10 kW fiber laser at 0.075 –3.525 kW/cm2 for 120 s to study mass removal and crater formation. Surface temperatures reached steady state values as high as 3767 K. The total decrease in sample mass ranged from 0.06 to 6.29 g, with crater volumes of 0.52 - 838 mm3, and penetration times for 12.7 mm thick plates as short as 38 s. Minor contaminants in the graphite samples produced calcium and iron oxide to be re-deposited on the graphite surface. Significantly increased porosity of the sample is observed even outside of the laser-irradiated region. Total mass removed increases with deposited laser energy at a rate of 4.83 g/MJ for medium extruded graphite with an apparent threshold of 0.15 MJ. Visible emission spectroscopy reveals C2 Swan and CN red, CN violet bands and Li, Na, and K 2P3/2,1/2 – 2S1/2 doublets. The reacting boundary layer is observed using a mid-wave imaging Fourier transform spectrometer (IFTS) at 2 cm-1 spectral resolution, 0.5 mm/pixel spatial resolution, and 0.75 Hz data cube rate. A two-layer radiative transfer model was used to determine plume temperature, CO, and CO2 concentrations from spectral signatures. The new understanding of graphite combustion and sublimation during laser irradiation is vital to the more complex behavior of carbon composites.
Pulsed laser ablation of Al and Ti with a < 3.3 J/cm2 KrF laser and Ar background pressure of up to 1 Torr was performed to study the ablated plume. Mass loss experiments revealed the number of ablated atoms per pulse increases by ~30% for Ti and ~20% for Al as pressure decreases from 1 Torr to vacuum. Optical emission imaging performed using a gated ICCD revealed a strong dependence of shock front parameters, defined by the Sedov-Taylor blast and classical drag models, on background pressure. Spatially resolved optical emission spectroscopy from Al I, Al II, Ti I, and Ti II revealed ion temperatures of 104 K that decreased away from the target surface along the surface normal and neutral temperatures of 103 K independent of target distance. Comparison between kinetic energy in the shock and internal excitation energy reveals that nearly 100% of the energy is partitioned into shock front kinetic energy and ~1% into internal excitation.
Porous graphite samples were irradiated with up to 3.5 kW/cm2 and 1 MJ deposited energy from a continuous wave ytterbium 1.07-μm fiber laser. Visible emission spectroscopy reveals C2 Swan (d3Πg−a3Πu) Δv=±2,±1, and 0 sequences, CN red (A2Π−X2Σ+) Δv=−4,−3 sequences, CN violet (B2Σ+−X2Σ+) Δv=+1,0 sequences, and Li, Na, and K2P3/2,1/2−S1/22 doublets. Surface temperatures increased from ∼2500 K at 0.7 kW/cm2 to ∼4000 K at 3.5 kW/cm2. Spectral emissivity at 3.9 μm ranging from 0.74 to 0.93 increases by ∼8% after laser irradiation. Spectral simulations demonstrate that the ratio of C2(d) and CN(A) column densities are independent of sample porosity. Column densities increase from 0.00093 to 1.6×1012 molecules/cm2 for CN(A) and 0.00014 to 1.4×109 molecules/cm2 for C2(d) as laser intensity increases from 1.4 to 3.5 kW/cm2. Surface temperatures increase by 134 K and CN(A) and C2(d) emissions increase by 100% and 4200%, respectively, in stagnation air flow of 5 m/s.
The various effects of laser heating of carbon materials are key to assessing laser weapon effectiveness. Porous graphite plates, cylinders, and cones with densities of 1.55 to 1.82 g/cm3 were irradiated by a 10-kW fiber laser at 0.075 to 3.525 kW/cm2 for 120 s to study mass removal and crater formation. Surface temperatures reached steady state values as high as 3767 K. The total decrease in sample mass ranged from 0.06 to 6.29 g, with crater volumes of 0.52 to 838 mm3, and penetration times for 12.7-mm-thick plates as short as 38 s. Minor contaminants in the graphite samples produced calcium and iron oxide to be redeposited on the graphite surface. Dramatic graphite crystalline structures are also produced at higher laser irradiances. Significantly increased porosity of the sample is observed even outside the laser-irradiated region. Total mass removed increases with deposited laser energy at a rate of 4.83 g/MJ for medium extruded graphite with an apparent threshold of 0.15 MJ. At ∼3.5 kW/cm2, the fractions of the mass removed from the cylindrical samples in the crater, surrounding trench, and outer region of decreased porosity are 38%, 47%, and 15%, respectively. Graphite is particularly resistant to damage by high power lasers. The new understanding of graphite combustion and sublimation during laser irradiation is vital to the more complex behavior of carbon composites.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.