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Small‐scale laser effects experiments on graphite: Coupling coefficient, lateral loss, and effective heat of ablation

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4 Author(s)
Klein, Claude A. ; Research Division, Raytheon Company, Lexington, Massachusetts 02173 ; Menefee, Richard F. ; Krenek, Brendan D. ; Berry, Michael J.

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This work concerns laser‐interaction experiments performed on a fine‐grained, nearly isotropic graphite grade known as GraphNOL. They were carried out with a cw/DF laser of 100‐W output power, at peak irradiances varying from 10 to 50 kW/cm2 on targets of, typically, 1.5‐mm diameter. One of the major objectives was to accept the difficulties associated with such small sizes and rely on analytical techniques for estimating the lateral heat loss, developing a scaling law, and formulating an improved, parametric representation of the effective heat of ablation (Q*). The procedure rests on Breaux’s formula (Ballistic Research Laboratories report no. 1834) heuristically extended to accommodate the concept of a lateral loss parameter, which relates linearly to a scaling parameter that combines target thickness, target diameter, and spot size, thus specifying how geometrical factors correlate in terms of their impact on radial losses. Our investigation demonstrates that a small‐scale, low‐cost laser ablation experiment can generate a rich set of well‐characterized, highly accurate data, which are amenable to a comprehensive analytical evaluation. The coupling coefficient of GraphNOL to 3.8‐μm radiation is 86±1%, averaged over the duration of the burn, in a burnthrough experiment; this result leads to the conclusion that the intrinsic Q* should be close to 38 kJ/g at the DF‐laser wavelength.

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Journal of Applied Physics  (Volume:61 ,  Issue: 5 )