[1] We introduce a computational model for high-resolution simulations of particle-laden gravity currents. The features of the computational model are described in detail, and validation data are discussed. Physical results are presented that focus on the influence of particle entrainment from the underlying bed. As turbulent motions detach particles from the bottom surface, resuspended particles entrained over the entire length of the current are transferred to the current's head, causing it to become denser and potentially accelerating the front of the current. The conditions under which turbidity currents may become self-sustaining through particle entrainment are investigated as a function of slope angle, current and particle size, and particle concentration. The effect of computational domain size and initial aspect ratio of the current on the evolution of the current are also considered. Applications to flows traveling over a surface of varying slope angle, such as turbidity currents spreading down the continental slope, are modeled via a spatially varying gravity vector. Particular attention is given to the resulting particle deposits and erosion patterns.

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We present a modal analysis of the paraxial wave equation for plasma x-ray lasers. Gain guiding and refractive antiguiding are identified as the essential mechanisms which influence the coherence of the output radiation. Scaling laws for the number of guided modes and the coherence are given, depending on three parameters: the gain-dependent Fresnel number, the strength of refraction relative to gain, and the amplification length. The importance of controlling excess spontaneous emission to obtain coherence is identified. We suggest an experimental effort to verify various guiding regimes and to produce coherent output in the range between 45 and 200 A.PACS numbers: 42.50. Ar, 42.60.Da, 52.25.Nr The rapid development of soft-x-ray lasers 1 and the intent to use them for holography 2 has stimulated much interest in their coherence properties. The longitudinal coherence required for holography should be easy to obtain, based on estimates of line profiles. Good transverse coherence appears to be more difficult to achieve. 3 Although there have been several previous studies of x-ray laser (XRL) propagation, 4,5 none have identified the physical mechanisms that influence the coherence properties of the output. In this paper, we identify gain guiding and refractive defocusing for a relatively small number of modes as the essential features of XRL transverse coherence. Deriving scaling laws and using semianalytical calculations, we demonstrate a way to reduce the number of guided modes and increase the coherence for several characteristic plasma profiles. We suggest experimental measurements toward the goal of a fully coherent XRL.The wave-propagation problem in XRL's is similar to that in a broader class of mirrorless lasers. Discussions of the transverse coherence in superfluorescent 6 and stimulated Raman 7 sources have been reported. Propagator methods, expansions in free-space modes and orthogonal transverse modes, and numerical methods have been used to solve the wave equation.We present a steady-state, wave-optics model describing the development of the radiation from spontaneous emission for arbitrary transverse profiles. Our method differs from the previous ones 5 " 7 in that we use a modal decomposition developed in recent papers on "excess spontaneous emission" in lasers, 8 " 10 and consider higher-order modes. The transverse modes are determined by the gain and refraction profiles, and are in general nonorthogonal.We begin with the paraxial wave equation, including polarization by free electrons-a specific mechanism for refraction-and polarization by the atoms, causing spontaneous emission and gain: |vi-2/-^-Mp)+/g( P ) 6(r)--4;rifcP S p(r). (1) electric field, specific to a unit bandwidth dco at angular frequency co. The free-space wave number is k, V ± is the transverse Laplacian, p is the transverse position vector, h(p) = Q)p(p)/kc 2 is the refraction strength, where co p is the electron plasma frequency ((Op oc electron density), and g(p;(o) = (47t 2 d 2 k/3h)y/ (0 AN is the frequencydependent gain co...

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Cavitation bubbles accompany explosive vaporization of water following pulsed energy deposition in liquid media. Bubbles collapsing at the tip of a surgical endoprobe produce a powerful and damaging water jet propagating forward in the axial direction of the probe. We studied interaction of such jet with tissue using fast flash photography and modeled the flow dynamics using a two-dimensional Rayleigh-type hydrodynamic simulation. Maximal velocity of the jet generated at pulse energies of up to 1 mJ was about 80 m/s. The jet can produce tissue damage at a distance exceeding the radius of the cavitation bubble by a factor of 4. We demonstrate that formation of this flow and associated tissue damage can be prevented by application of the concave endoprobes that slow down the propagation of the back boundary of the bubble. Similar effect can be achieved by positioning an obstacle to the flow, such as a ring behind the tip.