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UDC 538.9"71;535.212 When the characteristic surface-roughness scale corresponds to the wavelength of the incident radiation, the surface of the target is modulated by a set of nonlinear oscillators (clusters). The dynamic-chaos criterion is calculated near the dissociation limit.It is shown that the value of the external fieM that controls the dynamicchaos state is then related to the boundary power density of the laser radiation, which separates the cluster and dropping mechanisms of particle separation from the surface. The numerical estimate obtained for the boundary power density agrees with observations.In the classical nonlinear systems, disruption of regular motion in the phase plane begins in the neighborhood of the separatrix [1, 2]. In the case of a Duffing oscillator in a biharmonic field, clusters of states with chaotic properties appear near the dissociation limit, i.e., near the separatrix [3]. Using a nonlinear oscillator model, we shall show that the dynamic chaos excited on interaction of a laser pulse with the surface of a high-temperature superconductor (HTSC) target makes it possible to explain certain observational facts.It was shown experimentally in [4] that when laser pulses interact with the surface of an HTSC target, which in that case was a polycrystalline YBa2Cu307_ ~ ceramic superconductor, there is a certain boundary value W c below which target particles are dislodged from the surface by a dropping mechanism and above which separation occurs in clusters without drops.The appearance of W c is quite interesting and a bit unexpected, since it would appear almost obvious that melting of the target surface should be more efficient as the power density W is increased and that an increase of W should result in separation of more drops from the surface. From this standpoint, no boundary power density W e should exist. This "obvious" picture of the absorption of laser light by the surface of a solid is based intuitively on the assumption that the surface is absolutely smooth and the absorbed radiation itself penetrates to a depth of the order of a -1 (~r is the absorption coefficient) and is dissipated in thermal lattice vibrations. Surface melting of the target material may occur when the incident energy reaches a certain value.However, it has been shown [4] that the surface of a polycrystalline ceramic target cannot be treated as smooth because the lengths of the inhomogeneities L -1-10/zm and are of the order of magnitude of the incident wavelength. It was suggested in [4] that this surface mi,,ht be treated as a fractal. The fractal nature of the surface is of fundamental value when the interaction of the laser light with the target is considered. The surface can then be represented as a set of numerous cluster-atom groups that are not bound as strongly to the framework of the crystal as they would be if they (the cluster) were in the interior of the material. Under exposure to a laser pulse, a cluster will vibrate as a single entity, so that the fractal surface can be represented as a ...