3,808 research outputs found
Vibrational Stability of NLC Linac accelerating structure
The vibration of components of the NLC linac, such as accelerating structures
and girders, is being studied both experimentally and analytically. Various
effects are being considered including structural resonances and vibration
caused by cooling water in the accelerating structure. This paper reports the
status of ongoing work.Comment: 3 pages 8 figures Presented at EPAC 2002 Paris Franc
Theory of adhesion: role of surface roughness
We discuss how surface roughness influence the adhesion between elastic
solids. We introduce a Tabor number which depends on the length scale or
magnification, and which gives information about the nature of the adhesion at
different length scales. We consider two limiting cases relevant for (a)
elastically hard solids with weak adhesive interaction (DMT-limit) and (b)
elastically soft solids or strong adhesive interaction (JKR-limit). For the
former cases we study the nature of the adhesion using different adhesive force
laws (, , where is the wall-wall separation). In
general, adhesion may switch from DMT-like at short length scales to JKR-like
at large (macroscopic) length scale. We compare the theory predictions to the
results of exact numerical simulations and find good agreement between theory
and the simulation results
Towards a modeling of the time dependence of contact area between solid bodies
I present a simple model of the time dependence of the contact area between
solid bodies, assuming either a totally uncorrelated surface topography, or a
self affine surface roughness. The existence of relaxation effects (that I
incorporate using a recently proposed model) produces the time increase of the
contact area towards an asymptotic value that can be much smaller than
the nominal contact area. For an uncorrelated surface topography, the time
evolution of is numerically found to be well fitted by expressions of
the form [, where the exponent depends on
the normal load as , with close to 0.5. In
particular, when the contact area is much lower than the nominal area I obtain
, i.e., a logarithmic time increase of the
contact area, in accordance with experimental observations. The logarithmic
increase for low loads is also obtained analytically in this case. For the more
realistic case of self affine surfaces, the results are qualitatively similar.Comment: 18 pages, 9 figure
Magnetic friction in Ising spin systems
A new contribution to friction is predicted to occur in systems with magnetic
correlations: Tangential relative motion of two Ising spin systems pumps energy
into the magnetic degrees of freedom. This leads to a friction force
proportional to the area of contact. The velocity and temperature dependence of
this force are investigated. Magnetic friction is strongest near the critical
temperature, below which the spin systems order spontaneously.
Antiferromagnetic coupling leads to stronger friction than ferromagnetic
coupling with the same exchange constant. The basic dissipation mechanism is
explained. If the coupling of the spin system to the heat bath is weak, a
surprising effect is observed in the ordered phase: The relative motion acts
like a heat pump cooling the spins in the vicinity of the friction surface.Comment: 4 pages, 4 figure
Vibrational Stability of NLC Linac and Final Focus Components
Vertical vibration of linac components (accelerating structures, girders and
quadrupoles) in the NLC has been studied experimentally and analytically.
Effects such as structural resonances and vibration caused by cooling water
both in accelerating structures and quadrupoles have been considered.
Experimental data has been compared with analytical predictions and simulations
using ANSYS. A design, incorporating the proper decoupling of structure
vibrations from the linac quadrupoles, is being pursued.Comment: 3 pages, 8 figures presented at the LINAC 2002 conference, Gyeongju
Kore
Effect of Cooling Water on Stability of NLC Linac Components
Vertical vibration of linac components (accelerating structures, girders and
quadrupoles) in the NLC has been studied experimentally and analytically.
Effects such as structural resonances and vibration caused by cooling water
both in accelerating structures and quadrupoles have been considered.
Experimental data has been compared with analytical predictions and simulations
using ANSYS. A design, incorporating the proper decoupling of structure
vibrations from the linac quadrupoles, is being pursued.Comment: 6 Pages 13 Figures Presented at The Nanobeam 2002 Workshop (Lausanne
Switzerland
Finite-element analysis of contact between elastic self-affine surfaces
Finite element methods are used to study non-adhesive, frictionless contact
between elastic solids with self-affine surfaces. We find that the total
contact area rises linearly with load at small loads. The mean pressure in the
contact regions is independent of load and proportional to the rms slope of the
surface. The constant of proportionality is nearly independent of Poisson ratio
and roughness exponent and lies between previous analytic predictions. The
contact morphology is also analyzed. Connected contact regions have a fractal
area and perimeter. The probability of finding a cluster of area drops as
where increases with decreasing roughness exponent. The
distribution of pressures shows an exponential tail that is also found in many
jammed systems. These results are contrasted to simpler models and experiment.Comment: 13 pages, 15 figures. Replaced after changed in response to referee
comments. Final two figures change
African Swine Fever
At the time of writing this chapter, a global pandemic of African swine fever (ASF) is ongoing with the virus having moved from Eastern Europe, Asia, and into the Caribbean—leaving swine production in devastation along the way. Due to the global spread of African swine fever virus (ASFV), the persistence of the virus, and the increasing number of endemic countries, this disease poses an imminent threat of introduction into North America and other countries that are currently ASF free. Throughout the chapter, we reference Eurasian wild boar (Sus scrofa) which are charismatic megafauna that are native to Europe and Asia. Wild boar were introduced into numerous areas in the southeastern United States and California by early settlers and they subsequently augmented and hybridized with established feral domestic swine (Sus scrofa) to give rise to contemporary populations of feral swine, a highly invasive species that are present across much of the United States. Feral swine are referred to by various terms, including wild hogs, feral pigs, wild boar, wild swine, razorbacks, and other regional names in North America. African swine fever has never been introduced into the United States; as such, we do not discuss feral swine in specific within the chapter. However, experimental inoculations demonstrate that feral swine are acutely susceptible to ASFV and given its current rapid global movement we anticipate similar patterns of exposure, infection, and risk amongst these populations
Global Alfven Eigenmodes in the H-1 heliac
Recent upgrades in H-1 power supplies have enabled the operation of the H-1
experiment at higher heating powers than previously attainable. A heating power
scan in mixed hydrogen/helium plasmas reveals a change in mode activity with
increasing heating power. At low power (<50 kW) modes with beta-induced Alfven
eigenmode (BAE) frequency scaling are observed. At higher power modes
consistent with an analysis of nonconventional Global Alfven Eigenmodes (GAEs)
are observed, the subject of this work. We have computed the mode continuum,
and identified GAE structures using the ideal MHD solver CKA and the
gyrokinetic code EUTERPE. An analytic model for ICRH-heated minority ions is
used to estimate the fast ion temperature from the hydrogen species. Linear
growth rate scans using a local flux surface stability calculation, LGRO, are
performed. These studies demonstrate growth from circulating particles whose
speed is significantly less than the Alfven speed, and are resonant with the
mode through harmonics of the Fourier decomposition of the strongly-shaped
heliac magnetic field. They reveal drive is possible with a small, hot
energetic tail of the hydrogen species. Local linear growth rate scans are also
complemented with global calculations from CKA and EUTERPE. These qualitatively
confirm the findings from the LGRO study, and show that the inclusion of finite
Larmor radius effects can reduce the growth rate by a factor of three, but do
not affect marginal stability. Finally, a study of damping of the global mode
with the thermal plasma is conducted, computing continuum, and the damping
arising from parallel electric fields. We find that continuum damping is of
order 0.1% for the configuration studied. The inclusion of resistivity lifts
the damping to 19%. Such large damping is consistent with experimental
observations that in absence of drive the mode decays rapidly (~0.1 ms).Comment: 18 pages, 15 figures, submitted 07/04/2017 to Plasma Physics and
Controlled Fusio
High-threshold mechanosensitive ion channels blocked by a novel conopeptide mediate pressure-evoked pain
Little is known about the molecular basis of somatosensory mechanotransduction in mammals. We screened a library of peptide toxins for effects on mechanically activated currents in cultured dorsal root ganglion neurons. One conopeptide analogue, termed NMB-1 for noxious mechanosensation blocker 1, selectively inhibits (IC50 1 µM) sustained mechanically activated currents in a subset of sensory neurons. Biotinylated NMB-1 retains activity and binds selectively to peripherin-positive nociceptive sensory neurons. The selectivity of NMB-1 was confirmed by the fact that it has no inhibitory effects on voltage-gated sodium and calcium channels, or ligand-gated channels such as acid-sensing ion channels or TRPA1 channels. Conversely, the tarantula toxin, GsMTx-4, which inhibits stretch-activated ion channels, had no effects on mechanically activated currents in sensory neurons. In behavioral assays, NMB-1 inhibits responses only to high intensity, painful mechanical stimulation and has no effects on low intensity mechanical stimulation or thermosensation. Unexpectedly, NMB-1 was found to also be an inhibitor of rapid FM1-43 loading (a measure of mechanotransduction) in cochlear hair cells. These data demonstrate that pharmacologically distinct channels respond to distinct types of mechanical stimuli and suggest that mechanically activated sustained currents underlie noxious mechanosensation. NMB-1 thus provides a novel diagnostic tool for the molecular definition of channels involved in hearing and pressure-evoked pain
- …