A Sufficient Condition for Power Flow Insolvability with Applications to Voltage Stability Margins

For the nonlinear power flow problem specified with standard PQ, PV, and slack bus equality constraints, we present a sufficient condition under which the specified set of nonlinear algebraic equations has no solution. This sufficient condition is constructed in a framework of an associated feasible, convex optimization problem. The objective employed in this optimization problem yields a measure of distance (in a parameter set) to the power flow solution boundary. In practical terms, this distance is closely related to quantities that previous authors have proposed as voltage stability margins. A typical margin is expressed in terms of the parameters of system loading (injected powers); here we additionally introduce a new margin in terms of the parameters of regulated bus voltages.Comment: 12 pages, 7 figure

Quantum Quench of an Atomic Mott Insulator

We study quenches across the Bose-Hubbard Mott-insulator-to-superfluid quantum phase transition using an ultra-cold atomic gas trapped in an optical lattice. Quenching from the Mott insulator to superfluid phase is accomplished by continuously tuning the ratio of Hubbard tunneling to interaction energy. Excitations of the condensate formed after the quench are measured using time-of-flight imaging. We observe that the degree of excitation is proportional to the fraction of atoms that cross the phase boundary, and that the quantity of excitations and energy produced during the quench have a power-law dependence on the quench rate. These phenomena suggest an excitation process analogous to the Kibble-Zurek (KZ) mechanism for defect generation in non-equilibrium classical phase transitions

The Metaphysics of Improvisation

In The Metaphysics of Improvisation, I criticize wrongheaded metaphysical views of, and theories about, improvisation, and put forward a cogent metaphysical theory of improvisation, which includes action theory, an analysis of the relevant genetic and aesthetic properties, and ontology (work-hood). The dissertation has two Parts. Part I is a survey of the history of many improvisational practices, and of the concept of improvisation. Here I delineate, sketch, and sort out the often vague boundaries between improvising and non-improvising within many art forms and genres, including music, dance, theatre, motion pictures, painting, and literature. In addition, I discuss the concept of non-artistic improvisation in various contexts. I attempt to portray an accurate picture of how improvisation functions, or does not function, in various art forms and genres. Part II addresses metaphysical issues in, and problems and questions of, improvisation in the arts. I argue that that continuum and genus-species models are the most cogent ways to understand the action-types of improvising and composing and their relations. I demonstrate that these models are substantiated by an informed investigation and phenomenology of improvisational practice, action theory conceptual analysis, cognitive neuroscience studies and experiments, cognitive psychology studies and models, and some theories of creativity. In addition, I provide a constraint based taxonomy for classifying improvisations that is compatible with, and supports, the continuum model. Next, I address epistemological and ontological issues involving the genetic properties of improvisations, and the properties improvisatory, and as if improvised. Finally, I show that arguments against treating, or classifying, improvisations as works are weak or erroneous, and by focusing on music, I provide a correct ontological theory of work-hood for artistic improvisations

Bose-Einstein condensates in RF-dressed adiabatic potentials

Bose-Einstein condensates of $^{87}$Rb atoms are transferred into radio-frequency (RF) induced adiabatic potentials and the properties of the corresponding dressed states are explored. We report on measurements of the spin composition of dressed condensates. We also show that adiabatic potentials can be used to trap atom gases in novel geometries, including suspending a cigar-shaped cloud above a curved sheet of atoms

Limits to Sympathetic Evaporative Cooling of a Two-Component Fermi Gas

We find a limit cycle in a quasi-equilibrium model of evaporative cooling of a two-component fermion gas. The existence of such a limit cycle represents an obstruction to reaching the quantum ground state evaporatively. We show that evaporatively the \beta\mu ~ 1. We speculate that one may be able to cool an atomic fermi gas further by photoassociating dimers near the bottom of the fermi sea.Comment: Submitted to Phys. Rev