344 research outputs found
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Bent solenoids for spectrometers and emittance exchange sections
Bent solenoids can be used to transport low energy beams as they provide both confinement and dispersion of particle orbits. Solenoids are being considered both as emittance exchange sections and spectrometers in the muon cooling system as part of the study of the muon collider. They present the results of a study of bent solenoids which considers the design of coupling sections between bent solenoids to straight solenoids, drift compensation fields, aberrations, and factors relating to the construction, such as field ripple, stored energy, coil forces and field errors
Testing a model of antecedents and consequences of defensive pessimism and self-handicapping in school physical education
There has been very limited research on the use of self-worth protection strategies in the achievement context of school physical education (PE). Thus, this study aimed to examine some antecedents and consequences of defensive pessimism and self-handicapping. The sample comprised 534 (females n = 275; males n = 259) British pupils recruited from two schools who responded to established questionnaires. Results of structural equation modelling analysis indicated that self-handicapping and defensive pessimism were positively predicted by fear of failure and negatively predicted by competence valuation. In addition, defensive pessimism was negatively predicted by physical self-concept. In turn, defensive pessimism negatively predicted enjoyment in PE and intentions to participate in future optional PE programs. Self-handicapping did not predict enjoyment or intentions. Results from multi-sample structural equation modelling showed the specified model to be largely invariant across males and females. The findings indicate that although both strategies aim to protect one’s self-worth, some of their antecedents and consequences in PE may differ
Computational problems in modeling arcs
We explore the reasons why there seems to be no common model for vacuum arcs, in spite of the importance of the field and the level of effort expended over more than one hundred years
Modeling Vacuum Arcs
We are developing a model of vacuum arcs. This model assumes that arcs
develop as a result of mechanical failure of the surface due to Coulomb
explosions, followed by ionization of fragments by field emission and the
development of a small, dense plasma that interacts with the surface primarily
through self sputtering and terminates as a unipolar arc capable of producing
breakdown sites with high enhancement factors. We have attempted to produce a
self consistent picture of triggering, arc evolution and surface damage. We are
modeling these mechanisms using Molecular Dynamics (mechanical failure, Coulomb
explosions, self sputtering), Particle-In-Cell (PIC) codes (plasma evolution),
mesoscale surface thermodynamics (surface evolution), and finite element
electrostatic modeling (field enhancements). We can present a variety of
numerical results. We identify where our model differs from other descriptions
of this phenomenon.Comment: 4 pages, 5 figure
New mechanism of cluster-field evaporation in rf breakdown
Using a simple field evaporation model and molecular dynamics simulations of nanoscale copper tip evolution in a high electric field gradient typical for linacs, we have studied a new mechanism for rf-field evaporation. The mechanism consists of simultaneous (collective) field evaporation of a large group of tip atoms in high-gradient fields. Thus, evaporation of large clusters is energetically more favorable when compared with the conventional, ‘‘one-by-one’’ mechanism. The studied mechanism could also be considered a new mechanism for the triggering of rf-vacuum breakdown. This paper discusses the mechanism and the experimental data available for electric field evaporation of field emission microscopy tips
Ion Solid Interaction And Surface Modification At RF Breakdown In High‐Gradient Linacs
Ion solid interactions have been shown to be an important new mechanism of unipolar arc
formation in high-gradient rf linear accelerators through surface self-sputtering by plasma ions, in addition to an intense surface field evaporation. We believe a non-Debye plasma is formed in close vicinity to the surface and strongly affects surface atomic migration via intense bombardment by ions, strong electric field, and high surface temperature. Scanning electron microscope studies of copper surface of an rf cavity were conducted that show craters, arc pits, and both irregular and regular ripple structures with a characteristic length of 2 microns on the surface. Strong field enhancements are characteristic of the edges, corners, and crack systems at surfaces subjected to rf breakdown
Vacuum arcs and gradient limits
We have been extending and refining our model of vacuum breakdown and gradient limits and describe recent developments. The model considers a large number of mechanisms, but finds that vacuum arcs can be described fairly simply and self-consistently, however simulations of individual mechanisms can be involved, in some cases. Although based on accelerator rf data, we believe our model of vacuum arcs should have general applicability. The paper explores breakdown in plasmas, and self-sputtering and damage by parasitic arcs
Tunneling study of cavity grade Nb: possible magnetic scattering at the surface
Tunneling spectroscopy was performed on Nb pieces prepared by the same
processes used to etch and clean superconducting radio frequency (SRF)
cavities. Air exposed, electropolished Nb exhibited a surface superconducting
gap delta=1.55 meV, characteristic of clean, bulk Nb. However the tunneling
density of states (DOS) was broadened significantly. The Nb pieces treated with
the same mild baking used to improve the Q-slope in SRF cavities, reveal a
sharper DOS. Good fits to the DOS were obtained using Shiba theory, suggesting
that magnetic scattering of quasiparticles is the origin of the gapless surface
superconductivity and a heretofore unrecognized contributor to the Q-slope
problem of Nb SRF cavities.Comment: 3 pages, 3 figure
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