39 research outputs found
Electrically Enhanced Free Dendrite Growth in Polar and Non-polar Systems
We describe the electrically enhanced growth of needle crystals from the
vapor phase, for which there exists a morphological instability above a
threshold applied potential. Our improved theoretical treatment of this
phenomenon shows that the instability is present in both polar and non-polar
systems, and we provide an extension of solvability theory to include
electrical effects. We present extensive experimental data for ice needle
growth above the electrical threshold, where at C high-velocity
shape-preserving growth is observed. These data indicate that the needle tip
assumes an effective radius} which is nearly independent of both
supersaturation and the applied potential. The small scale of and
its response to chemical additives suggest that the needle growth rate is being
limited primarily by structural instabilities, possibly related to surface
melting. We also demonstrate experimentally that non-polar systems exhibit this
same electrically induced morphological instability
Current-density functional for disordered systems
The effective action for the current and density is shown to satisfy an
evolution equation, the functional generalization of Callan-Symanzik equation.
The solution describes the dependence of the one-particle irreducible vertex
functions on the strength of the quenched disorder and the annealed Coulomb
interaction. The result is non-perturbative, no small parameter is assumed. The
a.c. conductivity is obtained by the numerical solution of the evolution
equation on finite lattices in the absence of the Coulomb interaction. The
static limit is performed and the conductivity is found to be vanishing beyond
a certain threshold of the impurity strength.Comment: final version, 28 pages, 17 figures, to appear in Phys. Rev.
One-loop corrections to the metastable vacuum decay
We evaluate the one-loop prefactor in the false vacuum decay rate in a theory
of a self interacting scalar field in 3+1 dimensions. We use a numerical
method, established some time ago, which is based on a well-known theorem on
functional determinants. The proper handling of zero modes and of
renormalization is discussed. The numerical results in particular show that
quantum corrections become smaller away from the thin-wall case. In the
thin-wall limit the numerical results are found to join into those obtained by
a gradient expansion.Comment: 31 pages, 7 figure
Crypto-unitary forms of quantum evolution operators
For the description of quantum evolution, the use of a manifestly
time-dependent quantum Hamiltonian
is shown equivalent to the work with its simplified, time-independent
alternative . A tradeoff analysis is performed recommending
the latter option. The physical unitarity requirement is shown fulfilled in a
suitable ad hoc representation of Hilbert space.Comment: 15 p
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
Helium identification with LHCb
The identification of helium nuclei at LHCb is achieved using a method based on measurements of ionisation losses in the silicon sensors and timing measurements in the Outer Tracker drift tubes. The background from photon conversions is reduced using the RICH detectors and an isolation requirement. The method is developed using pp collision data at √(s) = 13 TeV recorded by the LHCb experiment in the years 2016 to 2018, corresponding to an integrated luminosity of 5.5 fb-1. A total of around 105 helium and antihelium candidates are identified with negligible background contamination. The helium identification efficiency is estimated to be approximately 50% with a corresponding background rejection rate of up to O(10^12). These results demonstrate the feasibility of a rich programme of measurements of QCD and astrophysics interest involving light nuclei
Curvature-bias corrections using a pseudomass method
Momentum measurements for very high momentum charged particles, such as muons from electroweak vector boson decays, are particularly susceptible to charge-dependent curvature biases that arise from misalignments of tracking detectors. Low momentum charged particles used in alignment procedures have limited sensitivity to coherent displacements of such detectors, and therefore are unable to fully constrain these misalignments to the precision necessary for studies of electroweak physics. Additional approaches are therefore required to understand and correct for these effects. In this paper the curvature biases present at the LHCb detector are studied using the pseudomass method in proton-proton collision data recorded at centre of mass energy √(s)=13 TeV during 2016, 2017 and 2018. The biases are determined using Z→μ + μ - decays in intervals defined by the data-taking period, magnet polarity and muon direction. Correcting for these biases, which are typically at the 10-4 GeV-1 level, improves the Z→μ + μ - mass resolution by roughly 18% and eliminates several pathological trends in the kinematic-dependence of the mean dimuon invariant mass
Momentum scale calibration of the LHCb spectrometer
For accurate determination of particle masses accurate knowledge of the momentum scale of the detectors is crucial. The procedure used to calibrate the momentum scale of the LHCb spectrometer is described and illustrated using the performance obtained with an integrated luminosity of 1.6 fb-1 collected during 2016 in pp running. The procedure uses large samples of J/ψ → μ + μ - and B+ → J/ψ K + decays and leads to a relative accuracy of 3 × 10-4 on the momentum scale
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The effects of laser beam non-uniformities on x-ray conversion efficiency
High gain Inertial Confinement Fusion (ICF) targets require a highly uniform drive. In the case of direct drive, the inherent non-uniformities in a high-power glass laser beam are large enough to prevent high compression of targets. In recent years two methods for smoothing the laser drive, Induced Spatial Incoherence (ISI) and Smoothing by Spectral Dispersion (SSD), have been proposed. Both methods break the original laser beam up into many beamlets that then interfere at the target to produce an illumination pattern with large instantaneous intensity variations over a wide range of spatial scales. This interferences pattern dances around at the coherence time of the laser and averages out to produce a smooth beam on longer time scales. Indirect drive schemes shine the laser on high-Z material, usually gold, which converts the laser energy into x-rays. The x-rays are then used to drive the target. Non-uniformities in the laser beam can imprint themselves on the emitted x-rays and potentially cause problems, although the spatial transport of the x-rays to the target tends to smooth out these non-uniformities. As a result, ISI and SSD schemes are also being considered for indirect drive laser systems. We address this problem by modeling the effects on the x-ray conversion efficiency of shining a laser beam with a sinusoidal intensity modulation on a gold slab. Our principal results are that electron heat transport is quite efficient in smoothing out non-uniformities in the laser deposition before they reach the ablation surface if the spatial scale of the laser modulation is less than roughly 500 {mu}m. We also show that the gold plasma is below the Raman and Brillouin thresholds throughout the pulse