659 research outputs found
Development of an X-band Photoinjector at SLAC
As part of a National Cancer Institute contract to develop a compact source
of monoenergetic X-rays via Compton backscattering, we have completed the
design and construction of a 5.5 cell Photoinjector operating at 11.424 GHz.
Successful completion of this project will result in the capability of
generating a monoenergetic X-ray beam, continuously tunable from 20 - 85 KeV.
The immediate goal is the development of a Photoinjector producing 7 MeV, 0.5
nC, sub-picosecond electron bunches with normalized RMS emittances of
approximately 1 pi-mm-mR at repetition rates up to 60 Hz. This beam will then
be further accelerated to 60 MeV using a 1.05 m accelerating structure. This
Photoinjector is somewhat different than the traditional 1.5 cell design both
because of the number of cells and the symmetrically fed input coupler cell.
Its operating frequency is also unique. Since the cathode is non-removable,
cold-test tuning was somewhat more difficult than in other designs. We will
present results of "bead-drop" measurements used in tuning this structure.
Initial beam measurements are currently in progress and results will be
presented as well as results of RF conditioning to high gradients at X-band.
Details of the RF system, emittance-compensating solenoid, and cathode laser
system as well as PARMELA simulations will also be presented.Comment: 3 pages, 6 figures, 1 Table, LINAC 200
Genes, psychological traits and civic engagement
Civic engagement is a classic example of a collective action problem: while civic participation improves life in the community as a whole, it is individually costly and thus there is an incentive to free ride on the actions of others. Yet, we observe significant inter-individual variation in the degree to which people are in fact civically engaged. Early accounts reconciling the theoretical prediction with empirical reality focused either on variation in individuals\u27 material resources or their attitudes, but recent work has turned to genetic differences between individuals. We show an underlying genetic contribution to an index of civic engagement (0.41), as well as for the individual acts of engagement of volunteering for community or public service activities (0.33), regularly contributing to charitable causes (0.28) and voting in elections (0.27). There are closer genetic relationships between donating and the other two activities; volunteering and voting are not genetically correlated. Further, we show that most of the correlation between civic engagement and both positive emotionality and verbal IQ can be attributed to genes that affect both traits. These results enrich our understanding of the way in which genetic variation may influence the wide range of collective action problems that individuals face in modern community life
Brewster effects for deep metallic gratings
Total absorption of light by highly modulated metallic gratings is demonstrated both theoretically and experimentally. This phenomenon occurs when only the zeroth-order propagates and is linked with the excitation of surface plasmons
Optical Lattices for Atom Based Quantum Microscopy
We describe new techniques in the construction of optical lattices to realize
a coherent atom-based microscope, comprised of two atomic species used as
target and probe atoms, each in an independently controlled optical lattice.
Precise and dynamic translation of the lattices allows atoms to be brought into
spatial overlap to induce atomic interactions. For this purpose, we have
fabricated two highly stable, hexagonal optical lattices, with widely separted
wavelengths but identical lattice constants using diffractive optics. The
relative translational stability of 12nm permits controlled interactions and
even entanglement operations with high fidelity. Translation of the lattices is
realized through a monolithic electro-optic modulator array, capable of moving
the lattice smoothly over one lattice site in 11 microseconds, or rapidly on
the order of 100 nanoseconds.Comment: 7 pages, 9 figure
SLAC/CERN high gradient tests on an X-band accelerating section
High frequency linear collider schemes envisage the use of rather high accelerating gradients: 50 to 100 MV/m for X-band and 80 MV/m for CLIC. Because these gradients are well above those commonly used in accelerators, high gradient studies of high frequency structures have been initiated and test facilities have been constructed at KEK [1], SLAC [2] and CERN [3]. The studies seek to demonstrate that the above mentioned gradients are both achievable and practical. There is no well-defined criterion for the maximum acceptable level of dark current but it must be low enough not to generate unacceptable transverse wakefields, disturb beam position monitor readings or cause RF power losses. Because there are of the order of 10,000 accelerating sections in a high frequency linear collider, the conditioning process should not be too long or difficult. The test facilities have been instrumented to allow investigation of field emission and RF breakdown mechanisms. With an understanding of these effects, the high gradient performance of accelerating sections may be improved through modifications in geometry, fabrication methods and surface finish. These high gradient test facilities also allow the ultimate performance of high frequency/short pulse length accelerating structures to be probed. This report describes the high gradient test at SLAC of an X-band accelerating section built at CERN using technology developed for CLIC
Gap maps and intrinsic diffraction losses in one-dimensional photonic crystal slabs
A theoretical study of photonic bands for one-dimensional (1D) lattices
embedded in planar waveguides with strong refractive index contrast is
presented. The approach relies on expanding the electromagnetic field on the
basis of guided modes of an effective waveguide, and on treating the coupling
to radiative modes by perturbation theory. Photonic mode dispersion, gap maps,
and intrinsic diffraction losses of quasi-guided modes are calculated for the
case of self-standing membranes as well as for Silicon-on-Insulator structures.
Photonic band gaps in a waveguide are found to depend strongly on the core
thickness and on polarization, so that the gaps for transverse electric and
transverse magnetic modes most often do not overlap. Radiative losses of
quasi-guided modes above the light line depend in a nontrivial way on structure
parameters, mode index and wavevector. The results of this study may be useful
for the design of integrated 1D photonic structures with low radiative losses.Comment: 9 pages, 8 figures, submitted to Physical Review
Hindered Coulomb explosion of embedded Na clusters -- stopping, shape dynamics and energy transport
We investigate the dynamical evolution of a Na cluster embedded in Ar
matrices of various sizes from N=30 to 1048. The system is excited by an
intense short laser pulse leading to high ionization stages. We analyze the
subsequent highly non-linear motion of cluster and Ar environment in terms of
trajectories, shapes, and energy flow. The most prominent effects are:
temporary stabilization of high charge states for several ps, sudden stopping
of the Coulomb explosion of the embedded Na clusters associated with an
extremely fast energy transfer to the Ar matrix, fast distribution of energy
throughout the Ar layers by a sound wave. Other ionic-atomic transfer and
relaxation processes proceed at slower scale of few ps. The electron cloud is
almost thermally decoupled from ions and thermalizes far beyond the ps scale.Comment: 12 pages, 10 figures, accepted in Euro. Phys. J.
Employing culturally responsive pedagogy to foster literacy learning in schools
 In recent years it has become increasingly obvious that, to enable students in schools from an increasingly diverse range of cultural backgrounds to acquire literacy to a standard that will support them to achieve academically, it is important to adopt pedagogy that is responsive to, and respectful of, them as culturally situated. What largely has been omitted from the literature, however, is discussion of a relevant model of learning to underpin this approach. For this reason this paper adopts a socio-cultural lens (Vygotsky, 1978) through which to view such pedagogy and refers to a number of seminal texts to justify of its relevance. Use of this lens is seen as having a particular rationale. It forces a focus on the agency of the teacher as a mediator of learning who needs to acknowledge the learner’s cultural situatedness (Kozulin, 2003) if school literacy learning for all students is to be as successful as it might be. It also focuses attention on the predominant value systems and social practices that characterize the school settings in which students’ literacy learning is acquired. The paper discusses implications for policy and practice at whole-school, classroom and individual student levels of culturally-responsive pedagogy that is based on a socio-cultural model of learning. In doing so it draws on illustrations from the work of a number of researchers, including that of the author
Grammatical meaning and the second language classroom : introduction
This special issue assembles empirical work on second language teaching and learning from a generative linguistic perspective. The focus is on properties that constitute grammar–meaning interaction, that differ in the native and target language grammars, and that have not been highlighted in the pedagogical literature so far. Common topics address whether and how learners acquire grammatical meanings in the second language, including difficult misalignments between native and target-language constructions and functional morphemes. We propose that teaching and learning a second language can be enhanced by focusing on the relationship between grammatical forms and their meanings, as elucidated by contemporary linguistic theory
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