3,568 research outputs found
Strong "quantum" chaos in the global ballooning mode spectrum of three-dimensional plasmas
The spectrum of ideal magnetohydrodynamic (MHD) pressure-driven (ballooning)
modes in strongly nonaxisymmetric toroidal systems is difficult to analyze
numerically owing to the singular nature of ideal MHD caused by lack of an
inherent scale length. In this paper, ideal MHD is regularized by using a
-space cutoff, making the ray tracing for the WKB ballooning formalism a
chaotic Hamiltonian billiard problem. The minimum width of the toroidal Fourier
spectrum needed for resolving toroidally localized ballooning modes with a
global eigenvalue code is estimated from the Weyl formula. This
phase-space-volume estimation method is applied to two stellarator cases.Comment: 4 pages typeset, including 2 figures. Paper accepted for publication
in Phys. Rev. Letter
Dressed test particles, oscillation centres and pseudo-orbits
A general semi-analytical method for accurate and efficient numerical
calculation of the dielectrically screened ("dressed") potential around a
non-relativistic test particle moving in an isotropic, collisionless,
unmagnetised plasma is presented. The method requires no approximations and is
illustrated using results calculated for two cases taken from the MSc thesis of
the first author: test particles with velocities above and below the ion sound
speed in plasmas with Maxwellian ions and warm electrons. The idea that the
fluctuation spectrum of a plasma can be described as a superposition of the
fields around \emph{non-interacting} dressed test particles is an expression of
the quasiparticle concept, which has also been expressed in the development of
the oscillation-centre and pseudo-orbit formalisms.Comment: 14 pages to Plasma Physics and Controlled Fusion for publication with
a cluster of papers associated with workshop Stability and Nonlinear Dynamics
of Plasmas, October 31, 2009 Atlanta, GA on occasion of the 65th birthday of
R.L. Dewar. Version 2: Reference [27] added in Sec. 5. Version 3: Revised in
response to referee
Model Data Fusion: developing Bayesian inversion to constrain equilibrium and mode structure
Recently, a new probabilistic "data fusion" framework based on Bayesian
principles has been developed on JET and W7-AS. The Bayesian analysis framework
folds in uncertainties and inter-dependencies in the diagnostic data and signal
forward-models, together with prior knowledge of the state of the plasma, to
yield predictions of internal magnetic structure. A feature of the framework,
known as MINERVA (J. Svensson, A. Werner, Plasma Physics and Controlled Fusion
50, 085022, 2008), is the inference of magnetic flux surfaces without the use
of a force balance model. We discuss results from a new project to develop
Bayesian inversion tools that aim to (1) distinguish between competing
equilibrium theories, which capture different physics, using the MAST spherical
tokamak; and (2) test the predictions of MHD theory, particularly mode
structure, using the H-1 Heliac.Comment: submitted to Journal of Plasma Fusion Research 10/11/200
A possibility to measure elastic photon--photon scattering in vacuum
Photon--photon scattering in vacuum due to the interaction with virtual
electron-positron pairs is a consequence of quantum electrodynamics. A way for
detecting this phenomenon has been devised based on interacting modes generated
in microwave waveguides or cavities [G. Brodin, M. Marklund and L. Stenflo,
Phys. Rev. Lett. \textbf{87} 171801 (2001)]. Here we materialize these ideas,
suggest a concrete cavity geometry, make quantitative estimates and propose
experimental details. It is found that detection of photon-photon scattering
can be within the reach of present day technology.Comment: 7 pages, 3 figure
A comparison of incompressible limits for resistive plasmas
The constraint of incompressibility is often used to simplify the
magnetohydrodynamic (MHD) description of linearized plasma dynamics because it
does not affect the ideal MHD marginal stability point. In this paper two
methods for introducing incompressibility are compared in a cylindrical plasma
model: In the first method, the limit is taken, where
is the ratio of specific heats; in the second, an anisotropic mass
tensor is used, with the component parallel to the magnetic
field taken to vanish, . Use of resistive MHD reveals
the nature of these two limits because the Alfv\'en and slow magnetosonic
continua of ideal MHD are converted to point spectra and moved into the complex
plane. Both limits profoundly change the slow-magnetosonic spectrum, but only
the second limit faithfully reproduces the resistive Alfv\'en spectrum and its
wavemodes. In ideal MHD, the slow magnetosonic continuum degenerates to the
Alfv\'en continuum in the first method, while it is moved to infinity by the
second. The degeneracy in the first is broken by finite resistivity. For
numerical and semi-analytical study of these models, we choose plasma
equilibria which cast light on puzzling aspects of results found in earlier
literature.Comment: 14 pages, 10 figure
Humanising communication between stroke unit practitioners (SUP) and patients with communication impairment (CI) to support therapeutic relationships
Introduction Research into the lived experience of CI describes discomfort, feeling isolated and ignored when interacting with SUP. Negative feelings are also described by SUP and relatives when attempting to communicate with patients with CI. Study Aim: To explore a humanising relationship-centred approach to support the development of positive relationships between patients, relatives and SUP. This abstract will present data specific to patients with CI. Methods A two-phase action research study with two stroke units over 20 months. Phase 1 explored the experiences of positive relationships with SUP, patients and relatives, and collaborated with SUP to develop and evaluate practices that supported relationships. Phase 2 used the practices developed in Phase 1 to explore translation to a second setting. Methods were participant observation, interviews, story-telling and group discussions. Processes of sense-making and immersion crystallisation were used to analyse data with staff as co-analysts. Results Three themes described the processes in clinical practice to support positive relationships: 1. Intentions by SUP or relatives to reach out towards, to try and connect with those with CI, was most meaningful for patients with CI. Successful transaction of information was less important. 2. Relatives and SUP used vicarious storytelling to share and maintain the uniqueness of the patient with CI. 3. Increasing SUP sensitivity to communicating beyond words â drawing on their emotional or tacit response during encounters. Conclusion This study has shown SUP using a humanising relationship-centred focus and wordless narratives support positive, therapeutic relationships. It offers new insights into practice developments beyond conventional supportive communication strategies
A Study of the Interactions Between the Double Bonds in Unsaturated Ketones
The interactions between C=C and C=O double bonds in
several unsaturated ketones have been studied by comparing
MIND0/2 caloulations wiith ionisation potentials determined by
photoelectron spectroscopy (PES). With one exception (norbornadienone)
the direct through-space interactions in conjugated .ketones
appear to be negligible, the double bonds couple hyperoonjugatively
via the intervening a bonds. This kind of approach should prove
useful for studying other long range interactions
Turbulent edge structure formation in complex configurations
Formation of nonlinear structures in drift-Alfvén turbulence is investigated in the often complex edge geometries of stellarator and tokamak configurations, by analysis of drift waveturbulence simulations using a model in which three-dimensional magnetic geometries are approximated. The structures of parallel mode extension, radially sheared zonal flows and perpendicular mode spectra are highlighted in particular for three-dimensional stellaratormagnetic fields and shaped tokamaks. Specific characteristics of advanced stellarators in comparison to (lower aspect ratio) circular tokamaks are a less pronounced ballooning structure of the modes, a strong influence of local magnetic shear on amplitude structure and average, and stronger level of zonal flows due to lower geodesic curvature.This work was partly funded by grants within the
ââAustralian-German Joint Research Co-operation schemeââ
~PPP project no. D/0205403!
- âŠ