104 research outputs found
A practical technique for the generation of highly uniform LIPSS
AbstractLaser-induced periodic surface structures (LIPSS) can be reliably produced with ultrashort (<10ps) laser pulses given fluence near the ablation threshold. Neat, parallel, uniform structures are harder to reproduce. Electrodynamic models show a field at normal incidence interacts with the surface resulting in periodicity in intensity along the surface in the direction of the incident E-field producing ridges and toughs on the surface orthogonal to the E-field. A completely smooth surface offers nothing to perturb the eventual periodic feature formation but is very difficult to achieve: we have demonstrated that simply avoiding surface roughness components near the frequency and direction of the emergent features significantly improves uniform feature production. An appropriate unidirectional polishing process can be realised using an inexpensive spinning cloth wheel. By using a cylindrical lens we were also able to process stainless steel surfaces at 5mm2s−1 so indicating useful industrial potential
Extending the modeling of the anisotropic galaxy power spectrum to
We present a new model for the redshift-space power spectrum of galaxies and
demonstrate its accuracy in modeling the monopole, quadrupole, and hexadecapole
of the galaxy density field down to scales of .
The model describes the clustering of galaxies in the context of a halo model
and the clustering of the underlying halos in redshift space using a
combination of Eulerian perturbation theory and -body simulations. The
modeling of redshift-space distortions is done using the so-called distribution
function approach. The final model has 13 free parameters, and each parameter
is physically motivated rather than a nuisance parameter, which allows the use
of well-motivated priors. We account for the Finger-of-God effect from centrals
and both isolated and non-isolated satellites rather than using a single
velocity dispersion to describe the combined effect. We test and validate the
accuracy of the model on several sets of high-fidelity -body simulations, as
well as realistic mock catalogs designed to simulate the BOSS DR12 CMASS data
set. The suite of simulations covers a range of cosmologies and galaxy bias
models, providing a rigorous test of the level of theoretical systematics
present in the model. The level of bias in the recovered values of
is found to be small. When including scales to ,
we find 15-30\% gains in the statistical precision of relative to
and a roughly 10-15\% improvement for the
perpendicular Alcock-Paczynski parameter . Using the BOSS DR12
CMASS mocks as a benchmark for comparison, we estimate an uncertainty on that is 10-20\% larger than other similar Fourier-space RSD
models in the literature that use , suggesting
that these models likely have a too-limited parametrization.Comment: Submitted to JCA
Galaxy power spectrum in redshift space: combining perturbation theory with the halo model
Theoretical modeling of the redshift-space power spectrum of galaxies is
crucially important to correctly extract cosmological information from redshift
surveys. The task is complicated by the nonlinear biasing and redshift space
distortion effects, which change with halo mass, and by the wide distribution
of halo masses and their occupations by galaxies. One of the main modeling
challenges is the existence of satellite galaxies that have both radial
distribution and large virial velocities inside halos, a phenomenon known as
the Finger-of-God effect. We present a model for the galaxy power spectrum of
in which we decompose a given galaxy sample into central and satellite galaxies
and relate different contributions to 1- and 2-halo terms in a halo model. Our
primary goal is to ensure that any parameters that we introduce have physically
meaningful values, and are not just fitting parameters. For the 2-halo terms we
use the previously developed RSD modeling of halos in the context of
distribution function and perturbation theory approach. This term needs to be
multiplied by the effect of radial distances and velocities of satellites
inside the halo. To this one needs to add the 1-halo terms, which are
non-perturbative. We show that the real space 1-halo terms can be modeled as
almost constant, with the finite extent of the satellites inside the halo
inducing a small k^2R^2 term, where R is related to the size of the halo. We
adopt a similar model for FoG in redshift space, ensuring that FoG velocity
dispersion is related to the halo mass. For FoG k^2 type expansions do not work
and FoG resummation must be used instead. We test several damping functions to
model the velocity dispersion FoG effect. Applying the formalism to mock
galaxies modeled after the "CMASS" sample of the BOSS survey, we find that our
predictions for the redshift-space power spectra are accurate up to k~0.4Mpc/h.Comment: 16 pages, 9 figures, 1 table, minor changes made with respect to
published versio
Imprint of DESI fiber assignment on the anisotropic power spectrum of emission line galaxies
The Dark Energy Spectroscopic Instrument (DESI), a multiplexed fiber-fed
spectrograph, is a Stage-IV ground-based dark energy experiment aiming to
measure redshifts for 29 million Emission-Line Galaxies (ELG), 4 million
Luminous Red Galaxies (LRG), and 2 million Quasi-Stellar Objects (QSO). The
survey design includes a pattern of tiling on the sky and the locations of the
fiber positioners in the focal plane of the telescope, with the observation
strategy determined by a fiber assignment algorithm that optimizes the
allocation of fibers to targets. This strategy allows a given region to be
covered on average five times for a five-year survey, but with coverage varying
between zero and twelve, which imprints a spatially-dependent pattern on the
galaxy clustering. We investigate the systematic effects of the fiber
assignment coverage on the anisotropic galaxy clustering of ELGs and show that,
in the absence of any corrections, it leads to discrepancies of order ten
percent on large scales for the power spectrum multipoles. We introduce a
method where objects in a random catalog are assigned a coverage, and the mean
density is separately computed for each coverage factor. We show that this
method reduces, but does not eliminate the effect. We next investigate the
angular dependence of the contaminated signal, arguing that it is mostly
localized to purely transverse modes. We demonstrate that the cleanest way to
remove the contaminating signal is to perform an analysis of the anisotropic
power spectrum and remove the lowest bin, leaving
modes accurate at the few-percent level. Here, is the cosine of the angle
between the line-of-sight and the direction of . We also investigate
two alternative definitions of the random catalog and show they are comparable
but less effective than the coverage randoms method.Comment: Submitted to JCA
nbodykit: an open-source, massively parallel toolkit for large-scale structure
We present nbodykit, an open-source, massively parallel Python toolkit for
analyzing large-scale structure (LSS) data. Using Python bindings of the
Message Passing Interface (MPI), we provide parallel implementations of many
commonly used algorithms in LSS. nbodykit is both an interactive and scalable
piece of scientific software, performing well in a supercomputing environment
while still taking advantage of the interactive tools provided by the Python
ecosystem. Existing functionality includes estimators of the power spectrum, 2
and 3-point correlation functions, a Friends-of-Friends grouping algorithm,
mock catalog creation via the halo occupation distribution technique, and
approximate N-body simulations via the FastPM scheme. The package also provides
a set of distributed data containers, insulated from the algorithms themselves,
that enable nbodykit to provide a unified treatment of both simulation and
observational data sets. nbodykit can be easily deployed in a high performance
computing environment, overcoming some of the traditional difficulties of using
Python on supercomputers. We provide performance benchmarks illustrating the
scalability of the software. The modular, component-based approach of nbodykit
allows researchers to easily build complex applications using its tools. The
package is extensively documented at http://nbodykit.readthedocs.io, which also
includes an interactive set of example recipes for new users to explore. As
open-source software, we hope nbodykit provides a common framework for the
community to use and develop in confronting the analysis challenges of future
LSS surveys.Comment: 18 pages, 7 figures. Feedback very welcome. Code available at
https://github.com/bccp/nbodykit and for documentation, see
http://nbodykit.readthedocs.i
Laser microsculpting for the generation of robust diffractive security markings on the surface of metals
AbstractWe report the development of a laser-based process for the direct writing (‘microsculpting’) of unique security markings (reflective phase holograms) on the surface of metals. In contrast to the common approaches used for unique marking of the metal products and components, e.g., polymer holographic stickers which are attached to metals as an adhesive tape, our process enables the generation of the security markings directly onto the metal surface and thus overcomes the problems with tampering and biocompatibility which are typical drawbacks of holographic stickers. The process uses 35ns laser pulses of wavelength 355nm to generate optically-smooth deformations on the metal surface using a localised laser melting process. Security markings (holographic structures) on 304-grade stainless steel surface are fabricated, and their resulted optical performance is tested using a He–Ne laser beam of 632.8nm wavelength
Simulation and implementation of heat load shifting in a low carbon building
A predictive load shifting control system for a heat pump has been developed and installed in a low carbon test house located at the BRE Innovation Park, Motherwell, near Glasgow. The house features an exhaust-air source heat pump supplying an under floor heating system. The controller predicted the day-ahead space heating requirements for the house, based on forecast air temperatures and solar radiation levels and then automatically set the heat pump’s start and stop times for the following day. The heat pump’s operation was restricted where possible to off-peak electricity tariff periods (00:00-07:00). The controller’s operating parameters were pre-set using a calibrated building simulation model. After installation, the controller’s performance was monitored during September 2015 and analysis of test data showed that the predictive control maintained indoor air temperatures between 18-23oC for around 87% of notional occupied hours between 07:00-22:00; this was better than predicted by simulation. However, the energy performance of the heat pump was extremely poor as it did not function well under intermittent load-shifting operation, with the majority of the heat was delivered primarily by an auxiliary immersion coil rather than the heat pump itself. The paper concludes with suggestions for refinements to the controller and further work
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