Gasdynamic processes in heating of a substance by laser radiation

Abstract

Processes of heating and gasdynamic motion of substances under the action of strong laser radiation are studied. The case of heating of a transparent mass of gas is examined. Also the problem of heating by radiation of an initially cold and stationary gas occupying a volume adjacent to a vacuum is solved. The existence of self-similar state of flow is pointed out. Results of self-similar solution and numerical calculations are presented and compared. The possibility is discussed to use the obtained results for describing the heating and vaporization of material from the surface of a solid body in the case of action of strong laser radiation with modulated streangth. In connection with investigation of the possibility of heating metrial to high temperatures by laser radiation with modulated strength, recently the examination of a number of gasdynamic problems is of substantial interest which take place on focusing the radiation on the surface of condensed media. The gasdynamic approach to the solution of indicated problems is dictated by the circumstance that for sufficiently powerful fluxes of laser radiation the temperature increase is associated with an initiation of gasdynamic motion of material (“vaporization”), which in turn exerts substantial influence on the entire process of heating. An essential peculiarity of the process under examination, which complicates the solution of the problem, is the dependence of the absorption power on the state of the substance in its gasdynamic motion. Indeed, if the absorption coefficient per unit mass is a constant value, then the main part of the incident flux is absorbed in a layer of approximately constant mass with optical thickness ~ 1, and in this case it is not dependent on the state of the substance. However, if the coefficient of absorption depends on the density and temperature, which is realized for example in an ionized gas, then the optical density of the substance strongly depends on the distribution of gasdynamic quantities. In the latter case equations of radiation transfer become substantially more complicated because of dependence on gasdynamic quantities. Gasdynamic motion was taken into consideration for heating by laser radiation of small volumes of material with constant mass in [1 and 2]. However, in all these papers gasdynamic equations were not examined. The motion of matter was described by means of quantities averaged over the volume. Heating of matter with varying mass without taking into account gasdynamic motion was examined in [3]. In papers [4 and 5] gasdynamic motion was examined for a given mass of material the optical thickness of which was much smaller than one, while the absorption depended on temperature and density. One-dimensional gasdynamic problem of heating of material is examined below for the case of incident strong monochromatic laser radiation on the surface of gas which is initially cold and immobile and which prior to heating fills a halfspace adjacent to a vacuum. The special feature of the problem examined is, in distinction from those mentioned above, the exact solution of gasdynamic equations in case of changing mass which is included by the motion in the process of action of a laser radiation pulse. The solution of such a problem can be used for approximate description of the process of vaporization and heating of a solid substance placed in a vacuum under the action of laser radiation.