382 research outputs found
LHC Beam Diffusion Dependence on RF Noise: Models and Measurements
Radio Frequency (RF) accelerating system noise and non-idealities can have detrimental impact on the LHC performance through longitudinal motion and longitudinal emittance growth. A theoretical formalism has been developed to relate the beam and RF loop dynamics with the bunch length growth [1]. Measurements were conducted at LHC to validate the formalism, determine the performance limiting RF components, and provide the foundation for beam diffusion estimates for higher energies and intensities. A brief summary of these results is presented in this work
Strain fields in twisted bilayer graphene
Van der Waals heteroepitaxy allows deterministic control over lattice
mismatch or azimuthal orientation between atomic layers to produce long
wavelength superlattices. The resulting electronic phases depend critically on
the superlattice periodicity as well as localized structural deformations that
introduce disorder and strain. Here, we introduce Bragg interferometry, based
on four-dimensional scanning transmission electron microscopy, to capture
atomic displacement fields in twisted bilayer graphene with twist angles <
2{\deg}. Nanoscale spatial fluctuations in twist angle and uniaxial
heterostrain are statistically evaluated, revealing the prevalence of
short-range disorder in this class of materials. By quantitatively mapping
strain tensor fields we uncover two distinct regimes of structural relaxation
-- in contrast to previous models depicting a single continuous process -- and
we disentangle the electronic contributions of the rotation modes that comprise
this relaxation. Further, we find that applied heterostrain accumulates
anisotropically in saddle point regions to generate distinctive striped shear
strain phases. Our results thus establish the reconstruction mechanics
underpinning the twist angle dependent electronic behaviour of twisted bilayer
graphene, and provide a new framework for directly visualizing structural
relaxation, disorder, and strain in any moir\'e material.Comment: 29 pages, 6 figures plus supporting information (42 pages, 28
figures
A Blueprint for the Problem Formulation Phase of EPA-Type Ecological Risk Assessments for 316(b) Determinations
The difference between management objectives focused on sustainability of fish populations and the indigenous aquatic community, and a management objective focused on minimizing entrainment and impingement losses accounts for much of the ongoing controversy surrounding §316(b). We describe the EPA’s ecological risk assessment framework and recommend that this framework be used to more effectively address differences in management objectives and structure §316(b) determinations. We provide a blueprint for the problem formulation phase of EPA-type ecological risk assessments for cooling-water intake structures (CWIS) at existing power plant facilities. Our management objectives, assessment endpoints, conceptual model, and generic analysis plan apply to all existing facilities. However, adapting the problem formulation process for a specific facility requires consideration of the permitting agency’s guidelines and level of regulatory concern, as well as site-specific ecological and technical differences. The facility-specific problem formulation phase is designed around the hierarchy of biolo gical levels of organization in the generic conceptual model and the sequence of cause-effect events and risk hypotheses represented by this model. Problem formulation is designed to be flexible in that it can be tailored for facilities where §316(b) regulatory concern is low or high. For some facilities, we anticipate that the assessment can be completed based on consideration of susceptibility alone. At the other extreme, a high level of regulatory concern combined with the availability of extensive information and consideration of costly CWIS mitigation options may result in the ecological risk assessment relying on analyses at all levels. Decisions on whether to extend the ecological risk assessment to additional levels should be based on whether regulatory or generator concerns merit additional analyses and whether available information is adequate to support such analyses. In making these decisions, the functional dependence between levels of analysis must be considered in making the transition to the analysis phase and risk estimation component of the ecological risk assessment. Regardless of how the generic analysis plan is modified to develop a facility-specific analysis plan, the resulting plan should be viewed as a tool for comparing representative species and alternative CWIS options by focusing on relative changes (i.e., proportional or percent changes) in various measures. The analysis plan is specifically designed to encourage consideration of multiple lines of evidence and to characterize uncertainties in each line of evidence. Multiple lines of evidence from different levels of analysis, obtained using both prospective and retrospective techniques, provide a broader perspective on the magnitude of potential effects and associated uncertainties and risks. The implications of the EPA’s recent (April 2002) proposed regulations for existing facilities on the applicability of this blueprint are briefly considered
A twist in chiral interaction between biological helices
Using an exact solution for the pair interaction potential, we show that
long, rigid, chiral molecules with helical surface charge patterns have a
preferential interaxial angle ~((RH)^1/2)/L, where L is the length of the
molecules, R is the closest distance between their axes, and H is the helical
pitch. Estimates based on this formula suggest a solution for the puzzle of
small interaxial angles in a-helix bundles and in cholesteric phases of DNA.Comment: 7 pages, 2 figures, PDF file onl
Buckling Instabilities of a Confined Colloid Crystal Layer
A model predicting the structure of repulsive, spherically symmetric,
monodisperse particles confined between two walls is presented. We study the
buckling transition of a single flat layer as the double layer state develops.
Experimental realizations of this model are suspensions of stabilized colloidal
particles squeezed between glass plates. By expanding the thermodynamic
potential about a flat state of confined colloidal particles, we derive
a free energy as a functional of in-plane and out-of-plane displacements. The
wavevectors of these first buckling instabilities correspond to three different
ordered structures. Landau theory predicts that the symmetry of these phases
allows for second order phase transitions. This possibility exists even in the
presence of gravity or plate asymmetry. These transitions lead to critical
behavior and phases with the symmetry of the three-state and four-state Potts
models, the X-Y model with 6-fold anisotropy, and the Heisenberg model with
cubic interactions. Experimental detection of these structures is discussed.Comment: 24 pages, 8 figures on request. EF508
Homeostatic interferon-lambda response to bacterial microbiota stimulates preemptive antiviral defense within discrete pockets of intestinal epithelium
Interferon-lambda (IFN-λ) protects intestinal epithelial cells (IECs) from enteric viruses by inducing expression of antiviral IFN-stimulated genes (ISGs). Here, we find that bacterial microbiota stimulate a homeostatic ISG signature in the intestine of specific pathogen-free mice. This homeostatic ISG expression is restricted to IECs, depends on IEC-intrinsic expression of IFN-λ receptor
A comparison of alternative assays to measure DNA damage in stallion spermatozoa: TUNEL test versus ‘Nicoletti assay’
The aberrations of sperm DNA may cause various problems and have negative consequences on fertility. These influence embryonic development or might lead to early embryo loss. Sperm Chromatin Structure Assay (SCSA) is the flow cytometric method most often used for the detection of DNA lesions; however, some studies using that method reached confusing conclusions. The aim of this pilot study was to adjust and compare two alternative tests, namely the TUNEL test and the Nicoletti assay. The above-mentioned two flow cytometric methods capable of detecting the fragmented DNA of sperm were tested on 12 frozen-thawed stallion semen samples. The TUNEL test demonstrated much higher DNA fragmentation ratio than the Nicoletti assay (mean ± SD: 30.77 ± 13.03% vs. 1.93 ± 0.89%, respectively). A fluorescent microscopic check of the samples showed that TUNEL labelled the plasma membrane and the mitochondria in a nonspecific way, rather than detecting only the fragmented DNA, thus eventually resulting in a false positive sign. The Nicoletti assay is simpler, quicker and does not detect nonspecific binding; however, further analyses are required to determine its diagnostic value
Defects in Chiral Columnar Phases: Tilt Grain Boundaries and Iterated Moire Maps
Biomolecules are often very long with a definite chirality. DNA, xanthan and
poly-gamma-benzyl-glutamate (PBLG) can all form columnar crystalline phases.
The chirality, however, competes with the tendency for crystalline order. For
chiral polymers, there are two sorts of chirality: the first describes the
usual cholesteric-like twist of the local director around a pitch axis, while
the second favors the rotation of the local bond-orientational order and leads
to a braiding of the polymers along an average direction. In the former case
chirality can be manifested in a tilt grain boundary phase (TGB) analogous to
the Renn-Lubensky phase of smectic-A liquid crystals. In the latter case we are
led to a new "moire" state with twisted bond order. In the moire state polymers
are simultaneously entangled, crystalline, and aligned, on average, in a common
direction. In the moire state polymers are simultaneously entangled,
crystalline, and aligned, on average, in a common direction. In this case the
polymer trajectories in the plane perpendicular to their average direction are
described by iterated moire maps of remarkable complexity, reminiscent of
dynamical systems.Comment: plain TeX, (33 pages), 17 figures, some uufiled and included, the
remaining available at ftp://ftp.sns.ias.edu/pub/kamien/ or by request to
[email protected]
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