This Chapter presents a theoretical analysis of radiation interaction with a semi-transparent metalized energetic material. Main regularities of the laser pulse interaction with metalized compositions are considered within the framework of non-resonant interaction of radiation with matter. The large variety of metalized composite propellants with different properties of the components, their ratio and dispersion can be divided into two classes, depending on the ratio of the laser irradiation's characteristic time (tr) and the thermal relaxation time of the propellant characteristic cell containing one metal particle (tm). Analysis of the role of metallic particles shape shows that in the case of spherical metal particles, duration of the laser pulse corresponds to the optimal size of particles, heated to a maximum temperature. In the case of flat metallic particles and constant pulse duration, the critical radiation flux and the critical density of ignition energy significantly decrease with decreasing thickness of the particle.
TopIntroduction
Experimental and theoretical study of interaction of thermal radiation with energetic materials have a great history starting from the pioneering works of P. Pokhil and A. Kovalskiy with co-workers in the fifties in ICP (Pokhil, 1982) (Koval'skiy, Khlevnoy, & Мikheev, 1967) (Pokhil, Maltsev, & Zaitsev, 1969), and Beyer, Fishman, Ohlemiller, and Summerfield in the USA (Beyer & Fishman, 1960) (Ohlemiller & Summerfield, 1968) at the same time.
Main purpose of first investigations was mechanism of processes in gas and condensed phases of solid propellants during their ignition and combustion. One of main findings of those investigations was ability of the double-based (DB) propellants to burn in deep vacuum (Pokhil, Maltsev, & Zaitsev, 1969). This means that chemical destruction of condensed phase of DB-propellant is exothermal enough to support self-destruction of the propellant (in contrast to theoretical models of solid propellants ignition and combustion existed at that time (see the review (Librovich, 1963)). This result led to the development of the solid-phase models of ignition and combustion of solid propellants (see the reviews (Merzhanov & Averson, 1971; Vilyunov & Zarko, 1989; Ohlemiller & Summerfield, 1968).
The new period of investigations of ignition and initiation of energetic materials by thermal radiation has been started, since creation of first lasers, from pioneering work of A.A. Brish with employees (Brish, Galeev, & Zaitsev, 1969). The effect of laser radiation on energetic materials (rocket propellants, explosives, pyrotechnics, etc.) is nowadays the subject of large number of publications. The main purposes of the majority of these studies are the mechanisms and regularities of the emergence of wave modes of exothermic reactions propagation. In spite of long history of the study of the laser ignition (LI) of metalized solid propellants (MSP) there is no commonly accepted opinion on mechanism of the laser initiation. It is due to wide variety of energetic materials and sources of laser irradiation. Meanwhile, the examined characteristics of the process, which are specific and sometimes inconsistent for various materials, are related by a number of quite general regularities. The main of them are the thermal mechanism of ignition and the spatial inhomogeneity of chemical conversions in the materials. A chemical reaction is localized at optically dense particles and at other inhomogeneities related to specific features of the structure, irradiation, and transformation of the material (see the reviews, e.g., in (Kondrikov, Ohlemiller, & Summerfield, 1970; DeLuca, Ohlemiller, Caveny, & Summerfield, 1976; DeLuca, Ohlemiller, Caveny, & Summerfield, 1976; Karabanov & Bobolev, 1981; Assovskiy, 1994; Tarzhanov, 1998; Assovskiy, 2000; Aluker et al., 2008). These facts show on effective methods to control laser ignition of composite propellants.
In this chapter, we consider main regularities of the laser pulse interaction with MSP within the framework of non-resonant (thermal) interaction of radiation with MSP (Assovskiy, 2000). Certain attention is given to features of a local initiation of metallized energetic materials by short laser pulses of small energy. A role of shape and size of optical discontinuities in the pulse intensity dependence on pulse duration is considered in detail.