CHAR FORMATION IN TISSUE IRRADIATED WITH A CO2-LASER - MODEL AND SIMULATIONS

A model predicting temperature and damage fields in soft tissue expose d to CO2 laser radiation has been devised and previously described. It failed, in its previous version, to explain the following experimenta l phenomenon: under low laser irradiances, a black substance defined a s char is accumulated at the bottom of the crater. We modified the bas ic model in the attempt to provide an explanation to the phenomenon of char formation. The basic model solves the heat equation over a 2-D g rid assuming cylindrical symmetry. The terms participating in the basi c heat equation are as follows: laser radiation as a source, radiation emission from the sample, conduction between neighboring grid cells, and convection terms that may be responsible for char formation under low laser irradiance. A series of assumptions regarding these convecti on mechanisms and the characteristics of char, in particular its absor ption coefficient, are made in this work and their implications are in vestigated.