140 research outputs found
Explicit Green's Function of a Boundary Value Problem for a Sphere and Trapped Flux Analysis in Gravity Probe B Experiment
Magnetic flux trapped on the surface of superconducting rotors of the Gravity
Probe B (GP-B) experiment produces some signal in the SQUID readout. For the
needs of GP-B error analysis and simulation of data reduction, this signal is
calculated and analyzed in the paper. We first solve a magnetostatic problem
for a point source (fluxon) on the surface of a sphere, finding the closed form
elementary expression for the corresponding Green's function. Second, we
calculate the flux through the pick-up loop as a function of the fluxon
position. Next, the time dependence of a fluxon position, caused by rotor
motion according to a symmetric top model, and thus the time signature of the
flux are determined, and the spectrum of the trapped flux signal is analyzed.
Finally, a multi-purpose program of trapped flux signal generation based on the
above results is described, various examples of the signal obtained by means of
this program are given, and their features are discussed.Comment: 14 pages, including 7 figures. Submitted to: "Journal of Applied
Physics
Multi-focal laser surgery: cutting enhancement by hydrodynamic interactions between cavitation bubbles
Transparent biological tissues can be precisely dissected with ultrafast
lasers using optical breakdown in the tight focal zone. Typically, tissues are
cut by sequential application of pulses, each of which produces a single
cavitation bubble. We investigate the hydrodynamic interactions between
simultaneous cavitation bubbles originating from multiple laser foci.
Simultaneous expansion and collapse of cavitation bubbles can enhance the
cutting efficiency by increasing the resulting deformations in tissue, and the
associated rupture zone. An analytical model of the flow induced by the bubbles
is presented and experimentally verified. The threshold strain of the material
rupture is measured in a model tissue. Using the computational model and the
experimental value of the threshold strain one can compute the shape of the
rupture zone in tissue resulting from application of multiple bubbles. With the
threshold strain of 0.7 two simultaneous bubbles produce a continuous cut when
applied at the distance 1.35 times greater than that required in sequential
approach. Simultaneous focusing of the laser in multiple spots along the line
of intended cut can extend this ratio to 1.7. Counter-propagating jets forming
during collapse of two bubbles in materials with low viscosity can further
extend the cutting zone - up to a factor of 1.54.Comment: 16 pages, 8 figures. Paper is accepted for publication in Physical
Review
Normal Modes of Black Hole Accretion Disks
This paper studies the hydrodynamical problem of normal modes of small adiabatic oscillations of relativistic barotropic thin accretion disks around black holes (and compact weakly magnetic neutron stars). Employing WKB techniques, we obtain the eigenfrequencies and eigenfunctions of the modes for different values of the mass and angular momentum of the central black hole. We discuss the properties of the various types of modes and examine the role of viscosity, as it appears to render some of the modes unstable to rapid growth
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