44 research outputs found
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Secondary electron yield engineering of copper surfaces by 532 nm ultrashort laser pulses
Nanostructured surfaces exhibit outstanding properties and enable manifold industrial applications. In this study the laser surface processing of polycrystalline, flat copper surfaces by 532 nm picosecond laser irradiation for secondary electron yield (SEY) reduction is reported. The laser beam was scanned in parallel lines across the sample surface in order to modify large surface areas. Morphology and SEY are characterized in dependence of the process parameters to derive correlations and mechanisms of the laser-based SEY engineering process. The nano- and microstructure morphology of the laser-modified surface was characterized by scanning electron microscopy and the secondary electron yield was measured. In general, an SEY reduction with increasing accumulated laser fluence was found. In particular, at low scanning speed (1 mm/s - 10 mm/s) and “high” laser power (~ 1 W) compact nanostructures with a very low SEY maximum of 0.7 are formed
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Influence of wavelength and accumulated fluence at picosecond laser-induced surface roughening of copper on secondary electron yield
Ultrashort-pulse laser processing of copper is performed in air to reduce the secondary electron yield (SEY). By UV (355 nm), green
(532 nm), and IR (1064 nm) laser-light induced surface modification, this study investigates the influence of the most relevant experimental
parameters, such as laser power, scanning speed, and scanning line distance (represented as accumulated fluence) on the ablation depth,
surface oxidation, topography, and ultimately on the SEY. Increasing the accumulated laser fluence results in a gradual change from a Cu 2 O
to a CuO-dominated surface with deeper micrometer trenches, higher density of redeposited surface particles from the plasma phase, and a
reduced SEY. While the surface modifications are less pronounced for IR radiation at low accumulated fluence (,1000 J/cm2 ), analogous
results are obtained for all wavelengths when reaching the nonlinear absorption regime, for which the SEY maximum converges to 0.7.
Furthermore, independent of the extent of the structural transformations, an electron-induced surface conditioning at 250 eV allows a
reduction of the SEY maximum below unity at doses of 5×10 -4 C/mm2 . Consequently, optimization of processing parameters for application
in particle accelerators can be obtained for a sufficiently low SEY at controlled ablation depth and surface particle density, which are factors
that limit the surface impedance and the applicability of the material processing for ultrahigh vacuum systems. The relations between pro-
cessing parameters and surface features will provide guidance in treating the surface of vacuum components, especially beam screens of
selected magnets of the Large Hadron Collider or of future colliders
Radiation enhancement and "temperature" in the collapse regime of gravitational scattering
We generalize the semiclassical treatment of graviton radiation to
gravitational scattering at very large energies and finite
scattering angles , so as to approach the collapse regime of impact
parameters . Our basic tool is the
extension of the recently proposed, unified form of radiation to the ACV
reduced-action model and to its resummed-eikonal exchange. By superimposing
that radiation all-over eikonal scattering, we are able to derive the
corresponding (unitary) coherent-state operator. The resulting graviton
spectrum, tuned on the gravitational radius , fully agrees with previous
calculations for small angles but, for sizeable angles
acquires an exponential cutoff of the large
region, due to energy conservation, so as to emit a finite fraction
of the total energy. In the approach-to-collapse regime of we find
a radiation enhancement due to large tidal forces, so that the whole energy is
radiated off, with a large multiplicity and a
well-defined frequency cutoff of order .
The latter corresponds to the Hawking temperature for a black hole of mass
notably smaller than .Comment: 5 pages, 2 figures, talk presented at the European Physical Society
Conference on High Energy Physics, 5-12 July, Venice, Ital
IceCube Search for Neutrinos Coincident with Compact Binary Mergers from LIGO-Virgo's First Gravitational-Wave Transient Catalog
Using the IceCube Neutrino Observatory, we search for high-energy neutrino
emission coincident with compact binary mergers observed by the LIGO and Virgo
gravitational wave (GW) detectors during their first and second observing runs.
We present results from two searches targeting emission coincident with the sky
localization of each gravitational wave event within a 1000 second time window
centered around the reported merger time. One search uses a model-independent
unbinned maximum likelihood analysis, which uses neutrino data from IceCube to
search for point-like neutrino sources consistent with the sky localization of
GW events. The other uses the Low-Latency Algorithm for Multi-messenger
Astrophysics, which incorporates astrophysical priors through a Bayesian
framework and includes LIGO-Virgo detector characteristics to determine the
association between the GW source and the neutrinos. No significant neutrino
coincidence is seen by either search during the first two observing runs of the
LIGO-Virgo detectors. We set upper limits on the time-integrated neutrino
emission within the 1000 second window for each of the 11 GW events. These
limits range from 0.02-0.7 . We also set limits on the
total isotropic equivalent energy, , emitted in high-energy
neutrinos by each GW event. These limits range from 1.7 10 -
1.8 10 erg. We conclude with an outlook for LIGO-Virgo
observing run O3, during which both analyses are running in real time
Characteristics of the diffuse astrophysical electron and tau neutrino flux with six years of IceCube high energy cascade data
We report on the first measurement of the astrophysical neutrino flux using
particle showers (cascades) in IceCube data from 2010 -- 2015. Assuming
standard oscillations, the astrophysical neutrinos in this dedicated cascade
sample are dominated () by electron and tau flavors. The flux,
observed in the sensitive energy range from to
, is consistent with a single power-law model as expected
from Fermi-type acceleration of high energy particles at astrophysical sources.
We find the flux spectral index to be and a flux
normalization for each neutrino flavor of
at , in agreement with IceCube's complementary muon
neutrino results and with all-neutrino flavor fit results. In the measured
energy range we reject spectral indices at
significance level. Due to high neutrino energy resolution and low atmospheric
neutrino backgrounds, this analysis provides the most detailed characterization
of the neutrino flux at energies below compared to
previous IceCube results. Results from fits assuming more complex neutrino flux
models suggest a flux softening at high energies and a flux hardening at low
energies (p-value ). The sizable and smooth flux measured below remains a puzzle. In order to not violate the isotropic
diffuse gamma-ray background as measured by the Fermi-LAT, it suggests the
existence of astrophysical neutrino sources characterized by dense environments
which are opaque to gamma-rays.Comment: 4 figures, 4 tables, includes supplementary materia
Multimessenger Gamma-Ray and Neutrino Coincidence Alerts using HAWC and IceCube sub-threshold Data
The High Altitude Water Cherenkov (HAWC) and IceCube observatories, through
the Astrophysical Multimessenger Observatory Network (AMON) framework, have
developed a multimessenger joint search for extragalactic astrophysical
sources. This analysis looks for sources that emit both cosmic neutrinos and
gamma rays that are produced in photo-hadronic or hadronic interactions. The
AMON system is running continuously, receiving sub-threshold data (i.e. data
that is not suited on its own to do astrophysical searches) from HAWC and
IceCube, and combining them in real-time. We present here the analysis
algorithm, as well as results from archival data collected between June 2015
and August 2018, with a total live-time of 3.0 years. During this period we
found two coincident events that have a false alarm rate (FAR) of
coincidence per year, consistent with the background expectations. The
real-time implementation of the analysis in the AMON system began on November
20th, 2019, and issues alerts to the community through the Gamma-ray
Coordinates Network with a FAR threshold of coincidences per year.Comment: 14 pages, 5 figures, 3 table
Untersuchung von Zugscherversuchen bezüglich der Eignung für GFK-Verklebungen im Schienenfahrzeugbau
Pulse Duration Dependence of Infrared Laser-Induced Secondary Electron Yield Reduction of Copper Surfaces
The irradiation of metals with ultrashort laser pulses enables the rapid and cost-effective
production of nanostructured surfaces with a wide range of industrial applications. The laser-induced
surface roughening modifies the interaction processes upon electron impact, leading to a modification
of the secondary electron emission. In this study, the nanostructuring as well as the secondary electron
yield (SEY) variation of polycrystalline copper surfaces was investigated by irradiation with 1030 nm
infrared ultrashort laser pulses at a constant repetition rate of 100 kHz. The influence of varying the
pulse duration between 238 fs and 10 ps, the laser power and the number of laser pulses per unit area
(induced by varying the scanning speed) on the surface topography and the SEY was investigated.
Irrespective of the pulse duration, irradiation with low scan speed (v ≤ 20 mm/s) and high laser power
(P ≥ 2.6 W) results in the formation of a surface with compact nanostructures and a very low maximum
SEY δmax < 0.7. The δmax increased slightly with increasing pulse duration at similar laser parameters.
Increasing the pulse duration also resulted in a slight decrease in the ablation threshold and volume.
The observed SEY dependence is probably explained by the pulse duration dependence of the ablation.
The results suggest that nanostructured copper surfaces with very low SEY can be produced with
ultrashort laser pulses over a wide range of pulse duration
Generic accelerated sequence alignment in SeqAn using vectorization and multi-threading
Motivation
Pairwise sequence alignment is undoubtedly a central tool in many bioinformatics analyses. In this paper, we present a generically accelerated module for pairwise sequence lignments applicable for a broad range of applications. In our module, we unified the standard dynamic programming kernel used for pairwise sequence alignments and extended it with a generalized inter-sequence vectorization layout, such that many alignments can be computed simultaneously by exploiting SIMD (Single Instruction Multiple Data) instructions of modern processors. We then extended the module by adding two layers of thread-level parallelization, where we a) distribute many independent alignments on multiple threads and b) inherently parallelize a single alignment computation using a work stealing approach producing a dynamic wavefront progressing along the minor diagonal.
Results
We evaluated our alignment vectorization and parallelization on different processors, including the newest Intel® Xeon® (Skylake) and Intel® Xeon Phi™ (KNL) processors, and use cases. The instruction set AVX512-BW (Byte and Word), available on Skylake processors, can genuinely improve the performance of vectorized alignments. We could run single alignments 1600 times faster on the Xeon Phi™ and 1400 times faster on the Xeon® than executing them with our previous sequential alignment module.
Availability
The module is programmed in C++ using the SeqAn (Reinert et al., 2017) library and distributed with version 2.4. under the BSD license. We support SSE4, AVX2, AVX512 instructions and included UME::SIMD, a SIMD-instruction wrapper library, to extend our module for further instruction sets. We thoroughly test all alignment components with all major C++ compilers on various platforms