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NIA Seminar by Alex Povitsky |
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Date: March 28, 2006
Time: 10:30am
Location: NIA, Rm 137
Additional Information: Presentation (PDF)
Combined Thermal and Gas Dynamics Numerical Model for Laser Ablation
Alex Povitsky, University of Akron
The process of short-pulse (fs to ns) laser ablation can be roughly divided into two parts. During the first several nanoseconds, fast laser energy deposition leads to the overheating, melting, and, at sufficiently high laser fluencies, explosive boiling and disintegration of the surface region of the target leading to a massive ejection of vapor loaded with liquid-phase droplets and molecular clusters and explosive propagation of plume in the following micro- to mille- second time interval. Modeling of ablated plume is important in micromachining, synthesis of nanotubes, surface cleaning, and laser vision correction. The above stages of laser ablation are strongly inter-related especially for multiple plume ejection in a multi-pulse irradiation regime. The plume shielding represents a major problem of multiple laser pulse technologies. The shielding effect prevents laser technologies from reaching the goal of precise delivery of high-density energy since the laser energy is either absorbed or scattered by the plume shield.
The major complication in the computational description of this ablation plume dynamics is due to the presence of droplets that can exhibit condensation due to the pressure and temperature drop. Evaporation occurs in the expanding plume due to interactions of plume with next laser pulses. In turn, the dynamics of the plume expansion largely affect the degree of laser irradiation shielding of different parts of the target. The ablated mass per laser pulse is non-monotonic in time and non-uniform throughout the target area. As a result, the large disparity of the processes occurring at above-listed stages of laser ablation does not allow one to describe this phenomenon within a single material medium (gas, vapor, liquid, or droplets) and a single computational approach that makes computational investigation of laser ablation challenging.
A combined thermal and gas dynamics model is developed, implemented and tested. The proposed model is based on combined conduction heat transfer within the solid target, sublimation process, and process of plume development described by continuous gas dynamics. The carbon sublimation model is based on Clausius-Clapeyron equation and conservation of energy for differential control volume. The validity of turbulent, viscous and inviscid models of plume dynamics for millisecond-range plume dynamics is discussed.
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