727 research outputs found
Cost-effective aperture arrays for SKA Phase 1: single or dual-band?
An important design decision for the first phase of the Square Kilometre
Array is whether the low frequency component (SKA1-low) should be implemented
as a single or dual-band aperture array; that is, using one or two antenna
element designs to observe the 70-450 MHz frequency band. This memo uses an
elementary parametric analysis to make a quantitative, first-order cost
comparison of representative implementations of a single and dual-band system,
chosen for comparable performance characteristics. A direct comparison of the
SKA1-low station costs reveals that those costs are similar, although the
uncertainties are high. The cost impact on the broader telescope system varies:
the deployment and site preparation costs are higher for the dual-band array,
but the digital signal processing costs are higher for the single-band array.
This parametric analysis also shows that a first stage of analogue tile
beamforming, as opposed to only station-level, all-digital beamforming, has the
potential to significantly reduce the cost of the SKA1-low stations. However,
tile beamforming can limit flexibility and performance, principally in terms of
reducing accessible field of view. We examine the cost impacts in the context
of scientific performance, for which the spacing and intra-station layout of
the antenna elements are important derived parameters. We discuss the
implications of the many possible intra-station signal transport and processing
architectures and consider areas where future work could improve the accuracy
of SKA1-low costing.Comment: 64 pages, 23 figures, submitted to the SKA Memo serie
Dominant Secondary Nuclear Photoexcitation with the X-ray Free Electron Laser
The new regime of resonant nuclear photoexcitation rendered possible by x-ray
free electron laser beams interacting with solid state targets is investigated
theoretically. Our results unexpectedly show that secondary processes coupling
nuclei to the atomic shell in the created cold high-density plasma can dominate
direct photoexcitation. As an example we discuss the case of Mo isomer
depletion for which nuclear excitation by electron capture as secondary process
is shown to be orders of magnitude more efficient than the direct laser-nucleus
interaction. General arguments revisiting the role of the x-ray free electron
laser in nuclear experiments involving solid-state targets are further deduced.Comment: 6 pages, 2 figures; v2 updated to published version; results
unchange
Direct and secondary nuclear excitation with x-ray free-electron lasers
The direct and secondary nuclear excitation produced by an x-ray free
electron laser when interacting with a solid-state nuclear target is
investigated theoretically. When driven at the resonance energy, the x-ray free
electron laser can produce direct photoexcitation. However, the dominant
process in that interaction is the photoelectric effect producing a cold and
very dense plasma in which also secondary processes such as nuclear excitation
by electron capture may occur. We develop a realistic theoretical model to
quantify the temporal dynamics of the plasma and the magnitude of the secondary
excitation therein. Numerical results show that depending on the nuclear
transition energy and the temperature and charge states reached in the plasma,
secondary nuclear excitation by electron capture may dominate the direct
photoexcitation by several orders of magnitude, as it is the case for the 4.8
keV transition from the isomeric state of Mo, or it can be negligible,
as it is the case for the 14.4 keV M\"ossbauer transition in
. These findings are most relevant for future nuclear quantum
optics experiments at x-ray free electron laser facilities.Comment: 17 pages, 7 figures; minor corrections made; accepted by Physics of
Plasma
Influence of functional rider and horse asymmetries on saddle force distribution during stance and in sitting trot
Asymmetric forces exerted on the horse's back during riding are assumed to have a negative effect on rider–horse interaction, athletic performance, and health of the horse. Visualized on a saddle pressure mat, they are initially blamed on a nonfitting saddle. The contribution of horse and rider to an asymmetric loading pattern, however, is not well understood. The aim of this study was to investigate the effects of horse and rider asymmetries during stance and in sitting trot on the force distribution on the horse's back using a saddle pressure mat and motion capture analysis simultaneously. Data of 80 horse-rider pairs (HRP) were collected and analyzed using linear (mixed) models to determine the influence of rider and horse variables on asymmetric force distribution. Results showed high variation between HRP. Both rider and horse variables revealed significant relationships to asymmetric saddle force distribution (P < .001). During sitting trot, the collapse of the rider in one hip increased the force on the contralateral side, and the tilt of the rider's upper body to one side led to more force on the same side of the pressure mat. Analyzing different subsets of data revealed that rider posture as well as horse movements and conformation can cause an asymmetric force distribution. Because neither horse nor rider movement can be assessed independently during riding, the interpretation of an asymmetric force distribution on the saddle pressure mat remains challenging, and all contributing factors (horse, rider, saddle) need to be considered
Method For Making 2-Electron Response Reduced Density Matrices Approximately N-representable
In methods like geminal-based approaches or coupled cluster that are solved
using the projected Schr\"odinger equation, direct computation of the
2-electron reduced density matrix (2-RDM) is impractical and one falls back to
a 2-RDM based on response theory. However, the 2-RDMs from response theory are
not -representable. That is, the response 2-RDM does not correspond to an
actual physical -electron wave function. We present a new algorithm for
making these non--representable 2-RDMs approximately -representable, i.e.
it has the right symmetry and normalization and it fulfills the -, - and
-conditions. Next to an algorithm which can be applied to any 2-RDM, we have
also developed a 2-RDM optimization procedure specifically for seniority-zero
2-RDMs. We aim to find the 2-RDM with the right properties that is the closest
(in the sense of the Frobenius norm) to the non-N-representable 2-RDM by
minimizing the square norm of the difference between the initial 2-RDM and the
targeted 2-RDM under the constraint that the trace is normalized and the 2-RDM,
- and -matrices are positive semidefinite, i.e. their eigenvalues are
non-negative. Our method is suitable for fixing non-N-respresentable 2-RDMs
which are close to being N-representable. Through the N-representability
optimization algorithm we add a small correction to the initial 2-RDM such that
it fulfills the most important N-representability conditions.Comment: 13 pages, 8 figure
- …