3,346 research outputs found
Nonperturbative resonant strong field ionization of atomic hydrogen
We investigate resonant strong field ionization of atomic hydrogen with
respect to the 1s-2p-transition. By "strong" we understand that Rabi-periods
are executed on a femtosecond time scale. Ionization and AC Stark shifts modify
the bound state dynamics severely, leading to nonperturbative signatures in the
photoelectron spectra. We introduce an analytical model, capable of predicting
qualitative features in the photoelectron spectra such as the positions of the
Autler-Townes peaks for modest field strengths. Ab initio solutions of the
time-dependent Schroedinger equation show a pronounced shift and broadening of
the left Autler-Townes peak as the field strength is increased. The right peak
remains rather narrow and shifts less. This result is analyzed and explained
with the help of exact AC Stark shifts and ionization rates obtained from
Floquet theory. Finally, it is demonstrated that in the case of finite pulses
as short as 20fs the Autler-Townes duplet can still be resolved. The fourth
generation light sources under construction worldwide will provide bright,
coherent radiation with photon energies ranging from a tenth of a meV up to
tens of keV, hence covering the regime studied in the paper so that
measurements of nonperturbative, relative AC Stark shifts should become
feasible with these new light sources.Comment: 16 pages, 11 figures, IOP styl
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A study of nucleate boiling and critical heat flux with EHD enhancement
The paper describes results from an experimental and theoretical study of the effect of an electric field on nucleate boiling and the critical heat flux (CHF) in pool boiling of R123 at atmospheric pressure on a horizontal wall with a smooth surface. Two designs of electrode (parallel rods and wire mesh) were used.
The experimental data exhibit some differences from the data obtained by other researchers in similar experiments on a wall with a different surface finish and with a slightly different design of wire mesh electrode. The hydrodynamic model for EHD enhancement of CHF cannot reconcile the differences.
A theoretical model has been developed for the growth of a single vapour bubble on a superheated wall in an electric field, leading to a numerical simulation based on the level-set method. The model includes matching of sub-models for the micro- and macro- regions, conduction in the wall, distortion of the electric field by the bubble, the temperature dependence of electrical properties and free-charge generation. In the present form of the model, some of these effects are realised in an approximate form. The capability to investigate dry-spot formation and wall temperature changes that might lead to CHF has been demonstrated
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Modelling of the growth and detachment of a vapour bubble and the effect of a electric field in the nucleate boiling regime
A comprehensive model predicting the deformation, growth and detachment of a vapour bubble in the nucleate boiling regime with an applied electric field is described in this paper. The model takes into account the full electrohydrodynamics of the phenomenon including the influence of local temperature on the generation of free charges in the liquid. Solution of the model by the level set method has been successfully implemented with a commercial CFD code. Aspects of the code and the graphical software requiring further development are noted.
Sample results are presented to demonstrate the effect of the electric field on the growth and detachment of the bubble, for a bubble initially protruding through a thermal boundary layer on a horizontal wall. The bubble is elongated under the influence of electrical forces, the effect being more pronounced for stronger electrical fields. The electric field is found to promote earlier detachment of the bubble at a smaller volume, thus increasing the bubble frequency. The wall heat flux during the process of detachment is not much affected by the electric field
Does distributed leadership have a place in destination management organisations? A policy-makers perspective
Within an increasingly networked environment and recent transitions in the landscape of funding for destination management organisations (DMOs) and destinations, pooling knowledge and resources may well be seen as a prerequisite to ensuring the long-term sustainability of reshaped, yet financially constrained DMOs facing severe challenges to deliver value to destinations, visitors and member organisations. Distributed Leadership (DL) is a recent paradigm gaining momentum in destination research as a promising response to these challenges. Building on the scarce literature on DL in a DMO context, this paper provides a policy-makers’ perspective into the place of DL in reshaped DMOs and DMOs undergoing transformation and explores current challenges and opportunities to the enactment and practice of DL. The underpinned investigation used in-depth, semi-structured interviews with policy-makers from VisitEngland following an interview agenda based on the DMO Leadership Cycle. Policy-makers within VisitEngland saw a multitude of opportunities for DMOs with regards to DL, but equally, they emphasised challenges acting as barriers to realising the potential benefits of introducing a DL model to DMOs as a response to uncertainty in the funding landscape
Nonlinear Terms of MHD Equations for Homogeneous Magnetized Shear Flow
We have derived the full set of MHD equations for incompressible shear flow
of a magnetized fluid and considered their solution in the wave-vector space.
The linearized equations give the famous amplification of slow magnetosonic
waves and describe the magnetorotational instability. The nonlinear terms in
our analysis are responsible for the creation of turbulence and self-sustained
spectral density of the MHD (Alfven and pseudo-Alfven) waves. Perspectives for
numerical simulations of weak turbulence and calculation of the effective
viscosity of accretion disks are shortly discussed in k-space.Comment: 13 pages, no figures; AIP Conference Proceedings 1356, Proceedings of
the School and Workshop on Space Plasma Physics (1--12 September 2010, Kiten,
Bulgaria), American Institute of Physics, Melville, NY, 201
On BPS bounds in D=4 N=2 gauged supergravity II: general matter couplings and black hole masses
We continue the analysis of BPS bounds started in arXiv:1110.2688, extending
it to the full class of N=2 gauged supergravity theories with arbitrary vector
and hypermultiplets. We derive the general form of the asymptotic charges for
asymptotically flat (M_4), anti-de Sitter (AdS_4), and magnetic anti-de Sitter
(mAdS_4) spacetimes. Some particular examples from black hole physics are given
to explicitly demonstrate how AdS and mAdS masses differ when solutions with
non-trivial scalar profiles are considered.Comment: 21 pages; v2 added reference, published version; v3 minor correction
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