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

Abstract

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.