5,204 research outputs found
Effect of turbulence on electron cyclotron current drive and heating in ITER
Non-linear local electromagnetic gyrokinetic turbulence simulations of the
ITER standard scenario H-mode are presented for the q=3/2 and q=2 surfaces. The
turbulent transport is examined in regions of velocity space characteristic of
electrons heated by electron cyclotron waves. Electromagnetic fluctuations and
sub-dominant micro-tearing modes are found to contribute significantly to the
transport of the accelerated electrons, even though they have only a small
impact on the transport of the bulk species. The particle diffusivity for
resonant passing electrons is found to be less than 0.15 m^2/s, and their heat
conductivity is found to be less than 2 m^2/s. Implications for the broadening
of the current drive and energy deposition in ITER are discussed.Comment: Letter, 5 pages, 5 figures, for submission to Nuclear Fusio
Herschel PACS and SPIRE spectroscopy of the Photodissociation Regions associated with S 106 and IRAS 23133+6050
Photodissociation regions (PDRs) contain a large fraction of all of the
interstellar matter in galaxies. Classical examples include the boundaries
between ionized regions and molecular clouds in regions of massive star
formation, marking the point where all of the photons energetic enough to
ionize hydrogen have been absorbed. In this paper we determine the physical
properties of the PDRs associated with the star forming regions IRAS 23133+6050
and S 106 and present them in the context of other Galactic PDRs associated
with massive star forming regions. We employ Herschel PACS and SPIRE
spectroscopic observations to construct a full 55-650 {\mu}m spectrum of each
object from which we measure the PDR cooling lines, other fine- structure
lines, CO lines and the total far-infrared flux. These measurements are then
compared to standard PDR models. Subsequently detailed numerical PDR models are
compared to these predictions, yielding additional insights into the dominant
thermal processes in the PDRs and their structures. We find that the PDRs of
each object are very similar, and can be characterized by a two-phase PDR model
with a very dense, highly UV irradiated phase (n 10^6 cm^(-3), G
10^5) interspersed within a lower density, weaker radiation field phase
(n 10^4 cm^(-3), G 10^4). We employed two different numerical
models to investigate the data, firstly we used RADEX models to fit the peak of
the CO ladder, which in conjunction with the properties derived yielded
a temperature of around 300 K. Subsequent numerical modeling with a full PDR
model revealed that the dense phase has a filling factor of around 0.6 in both
objects. The shape of the CO ladder was consistent with these components
with heating dominated by grain photoelectric heating. An extra excitation
component for the highest J lines (J > 20) is required for S 106.Comment: 20 pages, 10 figures, A&A Accepte
Snake orbits and related magnetic edge states
We study the electron motion near magnetic field steps at which the strength
and/or sign of the magnetic field changes. The energy spectrum for such systems
is found and the electron states (bound and scattered) are compared with their
corresponding classical paths. Several classical properties as the velocity
parallel to the edge, the oscillation frequency perpendicular to the edge and
the extent of the states are compared with their quantum mechanical
counterpart. A class of magnetic edge states is found which do not have a
classical counterpart.Comment: 8 pages, 10 figure
Exciton trapping in magnetic wire structures
The lateral magnetic confinement of quasi two-dimensional excitons into wire
like structures is studied. Spin effects are take into account and two
different magnetic field profiles are considered, which experimentally can be
created by the deposition of a ferromagnetic stripe on a semiconductor quantum
well with magnetization parallel or perpendicular to the grown direction of the
well. We find that it is possible to confine excitons into one-dimensional (1D)
traps. We show that the dependence of the confinement energy on the exciton
wave vector, which is related to its free direction of motion along the wire
direction, is very small. Through the application of a background magnetic
field it is possible to move the position of the trapping region towards the
edge of the ferromagnetic stripe or even underneath the stripe. The exact
position of this 1D exciton channel depends on the strength of the background
magnetic field and on the magnetic polarisation direction of the ferromagnetic
film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte
Precession-torque-driven domain-wall motion in out-of-plane materials
Domain-wall (DW) motion in magnetic nanostrips is intensively studied, in
particular because of the possible applications in data storage. In this work,
we will investigate a novel method of DW motion using magnetic field pulses,
with the precession torque as the driving mechanism. We use a one dimensional
(1D) model to show that it is possible to drive DWs in out-of-plane materials
using the precession torque, and we identify the key parameters that influence
this motion. Because the DW moves back to its initial position at the end of
the field pulse, thereby severely complicating direct detection of the DW
motion, depinning experiments are used to indirectly observe the effect of the
precession torque. The 1D model is extended to include an energy landscape in
order to predict the influence of the precession torque in the depinning
experiments. Although preliminary experiments did not yet show an effect of the
precession torque, our calculations indicate that depinning experiments can be
used to demonstrate this novel method of DW motion in out-of-plane materials,
which even allows for coherent motion of multiple domains when the
Dzyaloshinskii-Moriya interaction is taken into account
Instability due to long range Coulomb interaction in a liquid of polarizable particles (polarons, etc.)
The interaction Hamiltonian for a system of polarons a la Feynman in the
presence of long range Coulomb interaction is derived and the dielectric
function is computed in mean field. For large enough concentration a liquid of
such particles becomes unstable. The onset of the instability is signaled by
the softening of a collective optical mode in which all electrons oscillate in
phase in their respective self-trapping potential. We associate the instability
with a metallization of the system. Optical experiments in slightly doped
cuprates and doped nickelates are analyzed within this theory.
We discuss why doped cuprates matallize whereas nickelates do not.Comment: 5 pages,1 figur
The two electron artificial molecule
Exact results for the classical and quantum system of two vertically coupled
two-dimensional single electron quantum dots are obtained as a function of the
interatomic distance (d) and with perpendicular magnetic field. The classical
system exhibits a second order structural transition as a function of d which
is smeared out and shifted to lower d values in the quantum case. The
spin-singlet - spin-triplet oscillations are shifted to larger magnetic fields
with increasing d and are quenched for a sufficiently large interatomic
distance.Comment: 4 pages, 4 ps figure
Metal artefact reduction sequences for a piezoelectric bone conduction implant using a realistic head phantom in MRI
Industry standards require medical device manufacturers to perform
implant-induced artefact testing in phantoms at a pre-clinical stage to define
the extent of artefacts that can be expected during MRI. Once a device is
commercially available, studies on volunteers, cadavers or patients are
performed to investigate implant-induced artefacts and artefact reduction
methods more in-depth. This study describes the design and evaluation of a
realistic head phantom for pre-clinical implant-induced artefact testing in a
relevant environment. A case study is performed where a state-of-the-art
piezoelectric bone conduction implant is used in the 1.5 T and 3 T MRI
environments. Images were acquired using clinical and novel metal artefact
reducing (MARS) sequences at both field strengths. Artefact width and length
were measured in a healthy volunteer and compared with artefact sizes obtained
in the phantom. Artefact sizes are reported that are similar in shape between
the phantom and a volunteer, yet with dimensions differing up to 20% between
both. When the implant magnet is removed, the artefact size can be reduced
below a diameter of 5 cm, whilst the presence of an implant magnet and splint
creates higher artefacts up to 20 cm in diameter. Pulse sequences have been
altered to reduce the scan time up to 7 minutes, while preserving the image
quality. These results show that the anthropomorphic phantom can be used at a
preclinical stage to provide clinically relevant images, illustrating the
impact of the artefact on important brain structures.Comment: 17 pages, 5 figure
Structural and dynamical properties of a quasi-one-dimensional classical binary system
The ground state configurations and the \lq{}\lq{}normal\rq{}\rq{} mode
spectra of a -one-dimensional (Q1D) binary system of charged particles
interacting through a screened Coulomb potential are presented. The minimum
energy configurations were obtained analytically and independently through
molecular dynamic simulations. A rich variety of ordered structures were found
as a function of the screening parameter, the particle density, and the ratio
between the charges of the distinct types of particles. Continuous and
discontinuous structural transitions, as well as an unexpected symmetry
breaking in the charge distribution are observed when the density of the system
is changed. For near equal charges we found a disordered phase where a mixing
of the two types of particles occurs. The phonon dispersion curves were
calculated within the harmonic approximation for the one- and two-chain
structures.Comment: 11 pages, 11 fig
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