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Effect of Knudsen‐layer formation on the initial expansion and angular distribution of a laser‐produced copper plasma at reduced pressure of air

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1 Author(s)
Pietsch, W. ; CEA Saclay, DCC/DPE/SPEA/SPS, Analytical Laser Spectroscopy Group, F‐91191 Gif‐sur‐Yvette, France

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The initial expansion of a copper plasma generated at reduced pressure (20 mTorr air) by XeCl excimer laser irradiation of 308 nm has been studied by fast intensified charge‐coupled device photography of the overall visible plume emission with a time resolution of 3 ns. The spatial distribution and its temporal development along the target normal as well as in a plane parallel to the sample surface were measured in a single laser shot (28 ns). We report the first in situ measurement and observation of a one‐dimensional plasma expansion with very small divergence during the initial stage (δt≪150 ns) at reduced pressure for low laser fluence (5.4 J cm-2). The related angular distribution of ablated particles has a cosn Θ form where n≳36 for δt≤150 ns and Θ≪20°. The observed effect corresponds to a highly collisional plasma, where strongest forward peaking beyond the Knudsen‐layer edge takes place. For times greater than 150 ns, the plasma switches into a three‐dimensional expansion corresponding to an effusion model with recondensation. A nearly constant center‐of‐mass velocity in the range of 6×103 m s-1 has been observed. A particle velocity at the surface of uk=2×103 m s-1, in good agreement with the theory of unsteady adiabatic expansion, has been confirmed through a measured excitation temperature of T≊104 K revealing an image of the kinetic energy. © 1996 American Institute of Physics.

Published in:

Journal of Applied Physics  (Volume:79 ,  Issue: 3 )