3,145 research outputs found
High energy collision cascades in tungsten: dislocation loops structure and clustering scaling laws
Recent experiments on in-situ high-energy self-ion irradiation of tungsten
(W) show the occurrence of unusual cascade damage effects resulting from single
ion impacts, shedding light on the nature of radiation damage expected in the
tungsten components of a fusion reactor. In this paper, we investigate the
dynamics of defect production in 150 keV collision cascades in W at atomic
resolution, using molecular dynamics simulations and comparing predictions with
experimental observations. We show that cascades in W exhibit no subcascade
break-up even at high energies, producing a massive, unbroken molten area,
which facilitates the formation of large defect clusters. Simulations show
evidence of the formation of both 1/2 and interstitial-type
dislocation loops, as well as the occurrence of cascade collapse resulting in
vacancy-type dislocation loops, in excellent agreement with experimental
observations. The fractal nature of the cascades gives rise to a scale-less
power law type size distribution of defect clusters.Comment: 6 pages, 3 figure
Could the Hercules satellite be a stellar stream in the Milky Way halo?
We investigate the possibility that Hercules, a recently discovered Milky Way
(MW) satellite, is a stellar stream in the process of formation. This
hypothesis is motivated by Hercules' highly elongated shape as well as the
measurement of a tentative radial velocity gradient along its body. The
application of simple analytical techniques on radial velocity data of its
member stars provides tight constraints on the tangential velocity of the
system (v_t = -16^{+6}_{-22} km/s, relative to the Galactic Standard of Rest).
Combining this with its large receding velocity (145 km/s) and distance (138
kpc) yields an orbit that would have taken Hercules to within 6^{+9}_{-2} kpc
of the Galactic centre approximately 0.6 Gyr ago. This very small
perigalacticon can naturally explain the violent tidal destruction of the dwarf
galaxy in the MW's gravitational potential, inducing its transformation into a
stellar stream.Comment: Conference proceedings of "A Universe of dwarf galaxies:
Observations, Theories, Simulations" held in Lyon, France (June 14-18, 2010
Analysis of Multipath Mitigation Techniques with Land Mobile Satellite Channel Model
Multipath is undesirable for Global Navigation Satellite System (GNSS) receivers, since the reception of multipath can create a significant distortion to the shape of the correlation function leading to an error in the receivers’ position estimate. Many multipath mitigation techniques exist in the literature to deal with the multipath propagation problem in the context of GNSS. The multipath studies in the literature are often based on optimistic assumptions, for example, assuming a static two-path channel or a fading channel with a Rayleigh or a Nakagami distribution. But, in reality, there are a lot of channel modeling issues, for example, satellite-to-user geometry, variable number of paths, variable path delays and gains, Non Line-Of-Sight (NLOS) path condition, receiver movements, etc. that are kept out of consideration when analyzing the performance of these techniques. Therefore, this is of utmost importance to analyze the performance of different multipath mitigation techniques in some realistic measurement-based channel models, for example, the Land Multipath is undesirable for Global Navigation Satellite System (GNSS) receivers, since the reception of multipath can create a significant distortion to the shape of the correlation function leading to an error in the receivers’ position estimate. Many multipath mitigation techniques exist in the literature to deal with the multipath propagation problem in the context of GNSS. The multipath studies in the literature are often based on optimistic assumptions, for example, assuming a static two-path channel or a fading channel with a Rayleigh or a Nakagami distribution. But, in reality, there are a lot of channel modeling issues, for example, satellite-to-user geometry, variable number of paths, variable path delays and gains, Non Line-Of-Sight (NLOS) path condition, receiver movements, etc. that are kept out of consideration when analyzing the performance of these techniques. Therefore, this is of utmost importance to analyze the performance of different multipath mitigation techniques in some realistic measurement-based channel models, for example, the Land Mobile Satellite (LMS) channel model [1]-[4], developed at the German Aerospace Center (DLR). The DLR LMS channel model is widely used for simulating the positioning accuracy of mobile satellite navigation receivers in urban outdoor scenarios. The main objective of this paper is to present a comprehensive analysis of some of the most promising techniques with the DLR LMS channel model in varying multipath scenarios. Four multipath mitigation techniques are chosen herein for performance comparison, namely, the narrow Early-Minus-Late (nEML), the High Resolution Correlator, the C/N0-based two stage delay tracking technique, and the Reduced Search Space Maximum Likelihood (RSSML) delay estimator. The first two techniques are the most popular and traditional ones used in nowadays GNSS receivers, whereas the later two techniques are comparatively new and are advanced techniques, recently proposed by the authors. In addition, the implementation of the RSSML is optimized here for a narrow-bandwidth receiver configuration in the sense that it now requires a significantly less number of correlators and memory than its original implementation. The simulation results show that the reduced-complexity RSSML achieves the best multipath mitigation performance in moderate-to-good carrier-to-noise density ratio with the DLR LMS channel model in varying multipath scenarios
Direct observation of size scaling and elastic interaction between nano-scale defects in collision cascades
Using in-situ transmission electron microscopy, we have directly observed
nano-scale defects formed in ultra-high purity tungsten by low-dose high energy
self-ion irradiation at 30K. At cryogenic temperature lattice defects have
reduced mobility, so these microscope observations offer a window on the
initial, primary damage caused by individual collision cascade events. Electron
microscope images provide direct evidence for a power-law size distribution of
nano-scale defects formed in high-energy cascades, with an upper size limit
independent of the incident ion energy, as predicted by Sand et al. [Eur. Phys.
Lett., 103:46003, (2013)]. Furthermore, the analysis of pair distribution
functions of defects observed in the micrographs shows significant
intra-cascade spatial correlations consistent with strong elastic interaction
between the defects
The Density Profiles of Massive, Relaxed Galaxy Clusters. I. The Total Density Over Three Decades in Radius
Clusters of galaxies are excellent locations to probe the distribution of
baryons and dark matter (DM) over a wide range of scales. We study a sample of
seven massive, relaxed galaxy clusters with centrally-located brightest cluster
galaxies (BCGs) at z=0.2-0.3. Using the observational tools of strong and weak
gravitational lensing, combined with resolved stellar kinematics within the
BCG, we measure the total radial density profile, comprising both dark and
baryonic matter, over scales of ~3-3000 kpc. Lensing-derived mass profiles
typically agree with independent X-ray estimates within ~15%, suggesting that
departures from hydrostatic equilibrium are small and that the clusters in our
sample (except A383) are not strongly elongated along the line of sight. The
inner logarithmic slope gamma_tot of the total density profile measured over
r/r200=0.003-0.03, where rho_tot ~ r^(-gamma_tot), is found to be nearly
universal, with a mean = 1.16 +- 0.05 (random) +0.05-0.07
(systematic) and an intrinsic scatter of < 0.13 (68% confidence). This is
further supported by the very homogeneous shape of the observed velocity
dispersion profiles, obtained via Keck spectroscopy, which are mutually
consistent after a simple scaling. Remarkably, this slope agrees closely with
numerical simulations that contain only dark matter, despite the significant
contribution of stellar mass on the scales we probe. The Navarro-Frenk-White
profile characteristic of collisionless cold dark matter is a better
description of the total mass density at radii >~ 5-10 kpc than that of dark
matter alone. Hydrodynamical simulations that include baryons, cooling, and
feedback currently provide a poorer match. We discuss the significance of our
findings for understanding the assembly of BCGs and cluster cores, particularly
the influence of baryons on the inner DM halo. [abridged]Comment: Updated to matched the published version in Ap
- …