29 research outputs found
Nonlinear vortex light beams supported and stabilized by dissipation
We describe nonlinear Bessel vortex beams as localized and stationary
solutions with embedded vorticity to the nonlinear Schr\"odinger equation with
a dissipative term that accounts for the multi-photon absorption processes
taking place at high enough powers in common optical media. In these beams,
power and orbital angular momentum are permanently transferred to matter in the
inner, nonlinear rings, at the same time that they are refueled by spiral
inward currents of energy and angular momentum coming from the outer linear
rings, acting as an intrinsic reservoir. Unlike vortex solitons and dissipative
vortex solitons, the existence of these vortex beams does not critically depend
on the precise form of the dispersive nonlinearities, as Kerr self-focusing or
self-defocusing, and do not require a balancing gain. They have been shown to
play a prominent role in "tubular" filamentation experiments with powerful,
vortex-carrying Bessel beams, where they act as attractors in the beam
propagation dynamics. Nonlinear Bessel vortex beams provide indeed a new
solution to the problem of the stable propagation of ring-shaped vortex light
beams in homogeneous self-focusing Kerr media. A stability analysis
demonstrates that there exist nonlinear Bessel vortex beams with single or
multiple vorticity that are stable against azimuthal breakup and collapse, and
that the mechanism that renders these vortexes stable is dissipation. The
stability properties of nonlinear Bessel vortex beams explain the experimental
observations in the tubular filamentation experiments.Comment: Chapter of boo
The quest for the solar g modes
Solar gravity modes (or g modes) -- oscillations of the solar interior for
which buoyancy acts as the restoring force -- have the potential to provide
unprecedented inference on the structure and dynamics of the solar core,
inference that is not possible with the well observed acoustic modes (or p
modes). The high amplitude of the g-mode eigenfunctions in the core and the
evanesence of the modes in the convection zone make the modes particularly
sensitive to the physical and dynamical conditions in the core. Owing to the
existence of the convection zone, the g modes have very low amplitudes at
photospheric levels, which makes the modes extremely hard to detect. In this
paper, we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation of the
g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the
techniques that have been used to try to detect g modes. We review results in
the literature, and finish by looking to the future, and the potential advances
that can be made -- from both data and data-analysis perspectives -- to give
unambiguous detections of individual g modes. The review ends by concluding
that, at the time of writing, there is indeed a consensus amongst the authors
that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie
Detection of unsafety in families with parental and/or child developmental problems at the start of family support
Background Risk assessment is crucial in preventing child maltreatment as it can identify high-risk cases in need of child protection intervention. Despite this importance, there have been no validated risk assessment instruments available in the Netherlands for assessing the risk of child maltreatment. Therefore, the predictive validity of the California Family Risk Assessment (CFRA) was examined in Dutch families who received family support. In addition, the added value of a number of experimental items was examined. Finally, it was examined whether the predictive value of the instrument could be improved by modifying the scoring procedure. Methods Dutch families who experienced parenting and/or child developmental problems and were referred by the Centres for Youth and Family for family support between July 2009 and March 2011 were included. This led to a sample of 491 families. The predictive validity of the CFRA and the added value of the experimental items were examined by calculating AUC values. A CHAID analysis was performed to examine whether the scoring procedure could be improved. Results About half of the individual CFRA items were not related to future reports of child maltreatment. The predictive validity of the CFRA in predicting future reports of child maltreatment was found to be modest (AUC = .693). The addition of some of the experimental items and the modification of the scoring procedure by including only items that were significantly associated with future maltreatment reports resulted in a ‘high’ predictive validity (AUC = .795). Conclusions This new set of items might be a valuable instrument that also saves time because only variables that uniquely contribute to the prediction of future reports of child maltreatment are included. Furthermore, items that are perceived as difficult to assess by professionals, such as parental mental health problems or parents’ history of abuse/neglect, could be omitted without compromising predictive validity. However, it is important to examine the psychometric properties of this new set of items in a new dataset