An efficient pumping scheme that involves direct excitation of the upper lasing level of the Nd(3+) ion is demonstrated experimentally. The results obtained for direct upper laser level pumping of Nd:YAG R2 (869 nm) and Nd:YVO(4) (880 nm) were compared with traditional approximately 808-nm pump band excitation. A tunable cw Ti:sapphire laser was used as the pump source. In Nd:YAG, the oscillator slope efficiency increased by 10% and the threshold decreased by 11%. In Nd:YVO(4), the slope efficiency increased by 5% and the threshold decreased by 11%. These results agree with theory. The increase in optical efficiency indicates that laser material thermal loading can be substantially reduced.
The sudden expansion into vacuum of a gas cloud, with initial centrally symmetric density and temperature profiles, is studied theoretically for different values of the specific heat ratio γ. Models treating the expansion are discussed, in particular, a model for isentropic expansion and a model for spatially isothermal expansion. For γ→1, the density of the gas obtained from the former model for late stages of the expansion, approaches a Gaussian spatial profile which is the exact solution to the latter model. A description by a Gaussian profile can be, for some important cases, approximately correct even for large deviations of γ from one. For a spherically symmetric flow, the maximum difference (for any given time and distance from the center of symmetry) between the densities obtained from the above two models is 11% for γ=7/5. For γ=1.28, which corresponds to the expansion of lead azide detonation products previously studied in the author’s laboratory, the difference is 9%. It is also shown that in practice it is more convenient to use the model for isothermal expansion to describe the density profile since it does not depend on γ, which is very often not exactly known. Finally, for γ→1, a relation between the density and the temperature is obtained which is not dependent on their initial distributions.
The present paper is the first of a series reporting on a comprehensive study of the hydrodynamics, kinetics, and spectroscopy of the transient species formed following the detonation of lead azide (LA). The spatial and temporal behavior of the detonation products expanding into vacuum is obtained via high-speed framing photography, transmission of a HeNe laser beam, and chemiluminescence from excited Pb atoms. The photography reveals that following the initiation of LA the products form an expanding, bell-shaped cloud. The HeNe beam is attenuated when the cloud of products traverses its route. The attenuation starts 4–15 μs after initiation and depends on the height of the beam above the LA sample. The chemiluminescence consists of two components: the first, appearing 1–2 μs after initiation, is obtained from excited products formed by the detonation near the surface of the sample, while the second, starting 2–14 μs after initiation, originates from the expanding cloud of products. The intensity and the temporal behavior of the second component of chemiluminescence depend on the distance to a barrier placed above the LA sample. The cloud contains gaseous products and solid particles which propagate perpendicular to the LA surface with a maximum velocity of 4.48±0.10 km/s and 3.78±0.18 km/s, respectively. To reproduce the experimental results, two alternative hydrodynamic models are applied: Stanyukovich’s model [K. P. Stanyukovich, Unsteady Motion of Continuous Media (Pergamon, London, 1960), pp. 498–501] for isentropic expansion and London and Rosen’s model [R. A. London and M. D. Rosen, Phys. Fluids 29, 3813 (1986)] for exploding foil. The latter model is preferred and when incorporating Beer’s law and Mie’s theory [C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light in Small Particles (Wiley, New York, 1983), p. 77] it reproduces very well both the temporal behavior of the second component of chemiluminescence and the attenuation of the HeNe beam and suggests that lead particles with radius of 0.05–0.15 μm are involved in the attenuation. The model also provides an estimate of the composition of the product cloud and of the density of the gaseous and solid species as a function of time and distance from the LA sample.
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