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- ItemTerm-dependent hybridization of the 5 f-wave functions of Ba and Ba++(American Physical Society, 1982) Connerade, J. P.; Mansfield, Michael W. D.
Show more It is shown that, unlike in neutral Ba, the 4d→5f transitions cannot be neglected in the interpretation of the 4d spectrum of Ba++. A term-dependent hybridization of the 5f wave functions occurs, the effects of which reverse between Ba and Ba++, and oscillator strength reappears in the 4d→nf (n>~5) transitions. A second kind of wave-function collapse is identified and its effects are described.Show more - ItemObservation of a burst of cosmic-rays at energies above 7x10(13)eV(American Physical Society, 1983) Fegan, David J.; McBreen, B.; O'Sullivan, Colm T.
Show more The authors report on an unusual simultaneous increase in the cosmic-ray shower rate at two recording stations separated by 250 km. The event lasted for 20 s. This event was the only one of its kind detected in three years of observation. The duration and structure of this event is different from a recently reported single-station cosmic-ray burst. The simultaneity of the coincident event suggests that it was caused by a burst of cosmic gamma rays. There is a possibility that this event may be related to the largest observed glitch of the pulsar in the Crab Nebula.Show more - ItemHow large can a star be?(American Physical Society, 1986) Ó Murchadha, Niall
Show more In a recent article Schoen and Yau develop an inequality which shows that there exists a relationship between the minimum energy density ρ0 in a star and the size R of the star, of the form ρ0R2≤π6. This article shows that this inequality is valid for two different measures of the size of the star and conjectures that the inequality can be improved to ρ0R2≤3π32.Show more - ItemAverage energy density and the size of the universe(American Physical Society, 1987) Ó Murchadha, Niall
Show more This paper shows that there is a relationship between the maximum volume of any cosmological solution of the Einstein equations and its average energy density. An inequality is derived which shows that the average energy density multiplied by the volume to the ⅔, power must be greater than a fixed constant. Equivalently, the total energy content, divided by the volume to the ⅓ power, is bounded below.Show more - ItemTrapped surfaces in spherical stars(American Physical Society, 1988) Bizon, Piotr; Malec, Edward; Ó Murchadha, Niall
Show more We give necessary and sufficient conditions for the existence of trapped surfaces in spherically symmetric spacetimes. These conditions show that the formation of trapped surfaces depends on both the degree of concentration and the average flow of the matter. The result can be considered as a partial validation of the cosmic-censorship hypothesis.Show more - ItemTrapped surfaces due to concentration of gravitational radiation(American Physical Society, 1991) Beig, Robert; Ó Murchadha, Niall
Show more Sequences of nonsingular, asymptotically flat initial data for general relativity (GR) in vacuo, called critical sequences, are defined which approach the strong-field limit of GR in a precise sense. It is proven to Penrose, the spacetimes evolving from all such configurations must develop singularities. In the course of the proof a new and conceptually simple proof of the positivity of the Arnowitt-Deser-Misner mass in the strong-field regime is obtained.Show more - ItemWeak-field gravity of revolving circular cosmic strings(American Physical Society, 1993) McManus, Des J.; Vandyck, Michel A.
Show more A weak-field solution of Einstein's equations is constructed. It is generated by a circular cosmic string revolving in its plane about the center of the circle. (The revolution is introduced to prevent the string from collapsing.) This solution exhibits a conical singularity, and the corresponding deficit angle is the same as for a straight string of the same linear energy density, irrespective of the angular velocity of the string.Show more - ItemThere are no R(3) x S(1) vacuum gravitational instantons(American Physical Society, 1993) Ó Murchadha, Niall; Shanahan, Hugh
Show more Gravitational instantons, solutions to the Euclidean Einstein equations, with topology R3 x S1 arise naturally in finite-temperature quantum gravity. It is shown here that all such instantons must have the same asymptotic structure as the Schwarzschild instanton. From this follows that if the Ricci tensor of such a manifold is non-negative it must be flat. Hence there is no nontrivial vacuum gravitational instanton on R3 x S1. This places a significant restriction on the instabilities of hot flat space. Another consequence is that any static vacuum Lorentzian Kaluza-Klein solution is flat.Show more - ItemTrapped surfaces and spherical closed cosmologies(American Physical Society, 1993) Malec, Edward; Ó Murchadha, Niall
Show more This Article (peer-reviewed) gives necessary and sufficient conditions for the formation of trapped surfaces in spherically symmetric initial data defined on a closed manifold. Such trapped surfaces surround a region in which there occurs an enhancement of matter over the average. The conditions are posed directly in terms of physical variables and show that what one needs is a relatively large amount of excess matter confined to a small volume. The expansion of the Universe and an outward flow of matter oppose the formation of trapped surfaces; an inward flow of matter helps. The model can be regarded as a Friedmann-Lemaitre-Walker cosmology with localized spherical inhomogeneities. We show that the total excess mass cannot be too large.Show more - ItemWeak-field gravity of circular cosmic strings(American Physical Society, 1993) Hughes, Shane J.; Mcmanus, Des J.; Vandyck, Michel A.
Show more A weak-field solution of Einstein's equations is constructed. It is generated by a circular cosmic string externally supported against collapse. The solution exhibits a conical singularity, and the corresponding deficit angle is the same as for a straight string for the same linear energy density. This confirms the deficit-angle assumption made in the Frolov-Israel-Unruh derivation of the metric describing a string loop at a moment of time symmetry.Show more - ItemSwift-Hohenberg equation for lasers(American Physical Society, 1994) Lega, J.; Moloney, J. V.; Newell, A. C.
Show more Pattern formation in large aspect ratio, single longitudinal mode, two-level lasers with flat end reflectors, operating near peak gain, is shown to be described by a complex Swift-Hohenberg equation for class A and C lasers and by a complex Swift-Hohenberg equation coupled to a mean flow for the case of a class B laser.Show more - ItemTrapped surfaces in spherical expanding open universes(American Physical Society, 1994) Uwe Brauer; Malec, Edward; Ó Murchadha, Niall
Show more Consider spherically symmetric initial data for a cosmology which, in the large part, approximates an open k = - 1, LAMBDA = 0 Friedmann-Lemeitre universe. further assume that the data are chosen so that the trace of the extrinsic curvature is a constant and that the matter field is at rest at this instant of time. One expects that no trapped surfaces appear in the data if no significant clump of excess matter is to be found. This Brief Report confirms this belief by displaying a necessary condition for the existence of trapped surfaces. This necessary condition, simply stated, says that a relatively large amount of excess-matter must be concentrated in a small volume for trapped surfaces to appear.Show more - ItemEvent horizons and apparent horizons in spherically symmetrical geometries(American Physical Society, 1994) Malec, Edward
Show more Spherical configurations that are very massive must be surrounded by apparent horizons. These, in turn, when placed outside a collapsing body, have a fixed area and must propagate outward with a velocity equal to the velocity of radially outgoing photons. That proves, within the framework of the 1+3 formalism and without resorting to the Birkhoff theorem, that apparent horizons coincide with event horizons in an electrovacuum. The existence of the maximal slicing of electrovacuum is proved and an explicit line element is found in the maximal foliation.Show more - ItemTrapped surfaces and the Penrose inequality in spherically symmetrical geometries(American Physical Society, 1994) Malec, Edward; Ó Murchadha, Niall
Show more We demonstrate that the Penrose inequality is valid for spherically symmetric geometries even when the horizon is immersed in matter. The matter field need not be at rest. The only restrict.ion is that the source satisfies an energy condition outside the horizon. No restrictions are placed on the matter inside the horizon. The proof of the Penrose inequality gives a new necessary condition for the formation of trapped surfaces. This formulation may also be adapted to give a sufficient condition. We show that a modification of the Penrose inequality proposed by Gibbons for charged black holes can be broken in early stages of gravitational collapse. This investigation is based exclusively on the initial data formulation of general relativity.Show more - ItemReversal of ferroelectric domains by ultrashort optical pulses(American Physical Society, 1994) Fahy, Stephen B.; Merlin, Roberto
Show more The response of a soft-phonon ferroelectric material subjected to a high-intensity optical pulse of duration much shorter than the period of the phonon is modeled using a classical, finite-temperature simulation. It is found that complete, permanent reversal of the orientation of the ferroelectric domains may occur even when the energy per atom imparted by the light pulse is much less than the average thermal energy. The result raises the possibility of using the effect to create optical switches or data storage media with switching times less than 10 psec.Show more - ItemOptical scalars and singularity avoidance in spherical spacetimes(American Physical Society, 1994) Malec, Edward; Ó Murchadha, Niall
Show more Consider a spherically symmetric spacelike slice through a spacetime. One can derive universal bounds on any such slice assuming that the matter sources satisfy an energy condition and that the slice be regular. These bounds are used to derive the horizon formation conditions and to show how a regular spacelike slicing may avoid singularities. The results hold true even when the matter has a distribution on a shell or blows up at the origin so as to give a conical singularity.Show more - ItemObservation of continuous divertor detachment in H-mode discharges in ASDEX upgrade(American Physical Society, 1995) Gruber, O.; Kallenbach, A.; Kaufmann, M.; Lackner, K.; Mertens, V.; Neuhauser, J.; Ryter, F.; Zohm, H.; Bessenrodt-Weberpals, M.; Büchl, K.; Fiedler, S.; Field, A.; Fuchs, Ch.; Garcia-Rosales, C.; Haas, G.; Herrmann, A.; Herrmann, W.; Hirsch, S.; Köppendörfer, W.; Lang, P.; Lieder, G.; Mast, K. F.; Pitcher, C. S.; Schittenhelm, M.; Stober, J.; Suttrop, W.; Troppmann, M.; Weinlich, M.; Albrecht, M.; Alexander, M.; Asmussen, K.; Ballico, M.; Behler, K.; Behringer, K.; Bosch, H. S.; Brambilla, M.; Carlson, A.; Coster, D.; Cupido, L.; DeBlank, H. J.; De Pena Hempel, S.; Deschka, S.; Dorn, C.; Drube, R.; Dux, R.; Eberhagen, A.; Engelhardt, W.; Fahrbach, H. U.; Feist, H. U.; Fieg, D.; Fußmann, G.; Gehre, O.; Gernhardt, J.; Ignacz, P.; Jüttner, B.; Junker, W.; Kass, T.; Kiemer, K.; Kollotzek, H.; Kornherr, M.; Krieger, K.; Kurzan, B.; Lang, R.; Laux, M.; Manso, M. E.; Maraschek, M.; Mayer, H. M.; McCarthy, Patrick J.; Meisel, D.; Merkel, R.; Murmann, H.; Napiontek, B.; Naujoks, D.; Neu, G.; Neu, R.; Noterdaeme, J. M.; Pautasso, G.; Poschenrieder, W.; Raupp, G.; Richter, H.; Richter, T.; Röhr, H.; Roth, J.; Salmon, N.; Salzmann, H.; Sandmann, W.; Schilling, H. B.; Schneider, H.; Schneider, R.; Schneider, W.; Schönmann, K.; Schramm, G.; Schumacher, U.; Schweinzer, J.; Seidel, U.; Serra, F.; Silva, A.; Sokoll, M.; Speth, E.; Stäbler, A.; Steuer, K. H.; Streibl, B.; Treutterer, W.; Ulrich, M.; Varela, P.; Vernickel, H.; Vollmer, O.; Wedler, H.; Wenzel, U.; Wesner, F.; Wunderlich, R.; Zasche, D.; Zehrfeld, H. P.
Show more Feedback-controlled puffing of neon and deuterium has been applied to control the edge-localized-mode behavior and the target plate power deposition during high-power H-mode discharges in ASDEX Upgrade. A regime has been found in which more than 90% of the heating power is lost through radiation and divertor detachment occurs, without deterioration of the energy confinement. The plasma remains in the H mode, exhibiting small-amplitude, high-frequency ELM's, which do not penetrate to the target plates in the strike zone region.Show more - ItemConstraints in spherically symmetric classical general relativity. II. Identifying the configuration space: A moment of time symmetry(American Physical Society, 1995) Guven, Jemal; Ó Murchadha, Niall
Show more We continue our investigation of the configuration space of general relativity begun in the preceding paper. Here we examine the Hamiltonian constraint when the spatial geometry is momentarily static (MS). We begin with a heuristic description of the presence of apparent horizons and singularities. A peculiarity of MS configurations is that not only do they satisfy the positive quasilocal mass (QLM) theorem, they also satisfy its converse: the QLM is positive everywhere, if and only if the (nontrivial) spatial geometry is nonsingular. We derive an analytical expression for the spatial metric in the neighborhood of a generic singularity. The corresponding curvature singularity shows up in the traceless component of the Ricci tenser. As a consequence of the converse, if the energy density of matter is monotonically decreasing, the geometry cannot be singular. A. supermetric on the configuration space which distinguishes between singular geometries and nonsingular ones is constructed explicitly. Global necessary and sufficient criteria for the formation of trapped surfaces and singularities are framed in terms of inequalities which relate some appropriate measure of the material energy content on a given support to a measure of its volume. The sufficiency criteria are cast in the following form: if the material energy exceeds some universal constant times the proper radius l(0) of the distribution, the geometry will possess an apparent horizon for one constant and a singularity for some other larger constant. A more appropriate measure of the material energy for casting the necessary criteria is the maximum value of the energy density of matter rho(max): if rho(max)l(0)(2) < some constant the distribution of matter will not possess a singularity for one constant and an apparent horizon for some other smaller constant. These inequalities provide an approximate characterization of the singular (nonsingular) and trapped (nontrapped) partitions on the configuration space. Their strength is gauged by exploiting the exactly solvable piecewise constant density star as a template. Finally, we provide a more transparent derivation of the lower bound on the binding energy conjectured by Arnowitt, Deser, and Misner and proven by Bizon, Malec, and O Murchadha and speculate on possible improvements.Show more - ItemConstraints in spherically symmetrical classical general-relativity .1. Optical scalars, foliations, bounds on the configuration-space variables, and the positivity of the quasi-local mass(American Physical Society, 1995) Guven, Jemal; Ó Murchadha, Niall
Show more This is the first of a series of papers in which we examine the constraints of spherically symmetric general relativity with one asymptotically hat region. Our approach is manifestly invariant under spatial diffeomorphisms, exploiting both traditional metric variables as well as the optical scalar variables introduced recently in this context. With respect to the latter variables, there exist two linear combinations of the Hamiltonian and momentum constraints one of which is obtained from the other by time reversal. Boundary conditions on the spherically symmetric three-geometries and extrinsic curvature tensors are discussed. We introduce a one-parameter family of foliations of spacetime involving a linear combination of the two scalars characterizing a spherically symmetric extrinsic curvature tenser. We can exploit this gauge to express one of these scalars in terms of the other and thereby solve the radial momentum constraint uniquely in terms of the radial current. The values of the parameter yielding potentially globally regular gauges correspond to the vanishing of a timelike vector in the superspace of spherically symmetric geometries. We define a quasilocal mass (QLM) on spheres of fixed proper radius which provides observables of the theory. When the constraints are satisfied the QLM can be expressed as a volume integral over the sources and is positive. We provide two proofs of the positivity of the QLM. If the dominant energy condition (DEC) and the constraints are satisfied positivity can be established in a manifestly gauge-invariant way. This is most easily achieved exploiting the optical scalars. In the second proof we specify the foliation. The payoff is that the weak energy condition replaces the DEC and the Hamiltonian constraint replaces the full constraints. Underpinning this proof is a bound on the derivative of the circumferential radius of the geometry with respect to its proper radius. We show that, when the DEC is satisfied, analogous bounds exist on the optical scalar variables and, following on from this, on the extrinsic curvature tenser. We compare the difference between the values of the QLM and the corresponding material energy to prove that a reasonable definition of the gravitational binding energy is always negative. Finally, we summarize our understanding of the constraints in a tentative characterization of the configuration space of the theory in terms of closed bounded trajectories on the parameter space of the optical scalars.Show more - Item3d absorption-spectra of Sr I through Sr IV(American Physical Society, 1995) McGuinness, C.; O'Sullivan, G.; Carroll, P. K.; Audley, D.; Mansfield, Michael W. D.
Show more The extreme ultraviolet photoabsorption spectra of neutral to three-times-ionized strontium have been recorded in a comprehensive series of experiments with the dual laser-produced plasma technique. Striking differences were found in the spectra, which can be attributed to the transfer of oscillator strength from 3d→np to 3d→nf transitions at Sr2+ due to nf wave-function contraction. In Sr and Sr+, 3d→5p transitions dominate; in Sr2+, 3d→nf transitions are most intense, while in Sr3+ the 4p subshell opens and 3d→4p transitions are the strongest features. Partial cross sections for 3d→ɛf and 3d→ɛp photoionization were calculated and compared with experiment.Show more