75,914 research outputs found
Observation of fast stochastic ion heating by drift waves
Anomalously fast ion heating has been observed in the Caltech Encore tokamak [Phys. Rev. Lett. 59, 1436 (1987)], with the use of laser-induced fluorescence. This heating was found to be independent of electron temperature, but was well correlated with the presence of large-amplitude drift-Alfvén waves. Evidence is presented that suggests that the heating is stochastic and occurs when the ion displacement due to polarization drift becomes comparable to the perpendicular wavelength, i.e., when k[perpendicular] (mik[perpendicular] phi0/qB^2)~1. Stochastic heating may also be the cause of the anomalously high ion temperatures observed in reversed-field pinches
Coherent structures in an electron beam
The formation and evolution of coherent structures in a low-energy electron
beam produced in a Malmberg-Penning trap is investigated by means of CCD
diagnostics. The electrons are emitted from a thermionic cathode and their
energy is controlled by an acceleration grid. By varying the spatial
distribution of the energy of emitted electrons, different space charge effects
are observed, as, e. g., a sharp or a gradual transition to a space charge
dominated regime. The variation of the coherent structures along the beam is
studied by varying the electron density or/and the value of the confined
magnetic field. The observed processes are interpreted using a tridimensional
particle-in-cell code which solves the Vlasov-Poisson system in zeroth order
drift approximation.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Polarization Drift Channel Model for Coherent Fibre-Optic Systems
A theoretical framework is introduced to model the dynamical changes of the
state of polarization during transmission in coherent fibre-optic systems. The
model generalizes the one-dimensional phase noise random walk to higher
dimensions, accounting for random polarization drifts, emulating a random walk
on the Poincar\'e sphere, which has been successfully verified using
experimental data. The model is described in the Jones, Stokes and real
four-dimensional formalisms, and the mapping between them is derived. Such a
model will be increasingly important in simulating and optimizing future
systems, where polarization-multiplexed transmission and sophisticated digital
signal processing will be natural parts. The proposed polarization drift model
is the first of its kind as prior work either models polarization drift as a
deterministic process or focuses on polarization-mode dispersion in systems
where the state of polarization does not affect the receiver performance. We
expect the model to be useful in a wide-range of photonics applications where
stochastic polarization fluctuation is an issue.Comment: 15 pages, 4 figure
An Empirical Model for the Radio Emission from Pulsars
A model for slow radio pulsars is proposed which involves the entire
magnetosphere in the production of the observed radio emission. It is argued
that observations of pulsar profiles suggest that a feedback mechanism exists
between the star surface and the null charge surface, requiring particle flow
in both directions. In their flow to and from the surface the particles execute
an azimuthal drift around the magnetic pole, thereby creating a ring of
discrete `emission nodes' close to the surface. Motion of the nodes is observed
as the well-known subpulse `drift', but is interpreted here as a small residual
component of the real particle drift. The nodes can therefore move in either
direction, or even remain stationary. A precise fit is found for the pulsar
PSR0943+10. Azimuthal interactions between different regions of the
magnetosphere depend on the angle between the magnetic and rotation axes and
influence the conal type, as observed. The requirement of intermittent weak
pair-production in an outergap suggests a natural evolutionary link between
radio and gamma-ray pulsars.Comment: 17 pages 8 figure
Coherent structures in a turbulent environment
A systematic method is proposed for the determination of the statistical
properties of a field consisting of a coherent structure interacting with
turbulent linear waves. The explicit expression of the generating functional of
the correlations is obtained, performing the functional integration on a
neighbourhood in the function space around the soliton. The results show that
the non-gaussian fluctuations observed in the tokamak plasma edge can be
explained by the intermittent formation of nonlinear coherent structures.Comment: Revtex 21 pages includes 6 EPS figure
Coherent Pair Production by Photons in the 20-170 GeV Energy Range Incident on Crystals and Birefringence
The cross section for coherent pair production by linearly polarised photons
in the 20-170 GeV energy range was measured for photon aligned incidence on
ultra-high quality diamond and germanium crystals. The theoretical description
of coherent bremsstrahlung and coherent pair production phenomena is an area of
active theoretical debate and development. However, under our experimental
conditions, the theory predicted the combined cross section and polarisation
experimental observables very well indeed. In macroscopic terms, our experiment
measured a birefringence effect in pair production in a crystal. This study of
this effect also constituted a measurement of the energy dependent linear
polarisation of photons produced by coherent bremsstrahlung in aligned
crystals. New technologies for manipulating high energy photon beams can be
realised based on an improved understanding of QED phenomena at these energies.
In particular, this experiment demonstrates an efficient new polarimetry
technique. The pair production measurements were done using two independent
methods simultaneously. The more complex method using a magnet spectrometer
showed that the simpler method using a multiplicity detector was also viable.Comment: 10 pages, 13 figures, 1 table, REVTeX4 two column, Version for
publicatio
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