Ultrashort laser pulses having 50 fs duration were used to cause planer fracture, i.e., spallation, in pure aluminum foils at intensities of (0.07–1)×1015 W/cm2. A linear relationship was obtained between spall depth and foil thickness, approximately corresponding to the proportions that were obtained in other relations for 300 ps and longer laser pulse irradiation. Spall thicknesses less than 5 μm were obtained in foils thinner than 50 μm, and revealed small-scale spallation at a strain rate exceeding 108 s−1. This ultrashort laser pulse evidently produced a shock wave followed by a steep unloading stress profile.

No abstract

Equivalent edge currents (EECs) for high-frequency diffraction analysis are intermediate concepts between ray theory and wave optics [physical optics (PO)]. The methods of EEC present some ambiguity in the definition of currents at general edge points, which do not satisfy the diffraction law. Modified edge representation (MER) is a unique concept for a complete definition of EEC with simple and classical Keller-type knife-edge diffraction coefficients. Although singularities in the definition of EEC exist, the line integration of MER EEC results uniform and accurate fields everywhere including geometrical boundaries except for the case of grazing incidence, where reflection and incidence shadow boundaries (RSB and ISB) are close to each other. In this paper, fringe wave (FW) part of MER EEC is modified by introducing the weighting in terms of Fresnel zone number (FZN) distance from the reflection point to improve the accuracy at and near shadow boundaries even in grazing incidence. Next, the corner diffraction, a contribution coming from edge point which does not satisfy the diffraction law, is extracted in MER by introducing another weighting function using the FZN distance from scattering centers on edge. Its remarkable accuracy fully validates MER EECs at general edge points. The dipole wave scattering from flat square and triangular plates are discussed with numerical examples. Index Terms-Corner diffraction, equivalent edge current (EEC), Fresnel zone, geometrical theory of diffraction (GTD), modified edge representation (MER), uniform theory of diffraction (UTD).

No abstract

No abstract