16,805 research outputs found
Experimental and numerical study of error fields in the CNT stellarator
Sources of error fields were indirectly inferred in a stellarator by
reconciling computed and numerical flux surfaces. Sources considered so far
include the displacements and tilts (but not the deformations, yet) of the four
circular coils featured in the simple CNT stellarator. The flux surfaces were
measured by means of an electron beam and phosphor rod, and were computed by
means of a Biot-Savart field-line tracing code. If the ideal coil locations and
orientations are used in the computation, agreement with measurements is poor.
Discrepancies are ascribed to errors in the positioning and orientation of the
in-vessel interlocked coils. To that end, an iterative numerical method was
developed. A Newton-Raphson algorithm searches for the coils' displacements and
tilts that minimize the discrepancy between the measured and computed flux
surfaces. This method was verified by misplacing and tilting the coils in a
numerical model of CNT, calculating the flux surfaces that they generated, and
testing the algorithm's ability to deduce the coils' displacements and tilts.
Subsequently, the numerical method was applied to the experimental data,
arriving at a set of coil displacements whose resulting field errors exhibited
significantly improved quantitative and qualitative agreement with experimental
results.Comment: Special Issue on the 20th International Stellarator-Heliotron
Worksho
Partial purification of tumour-specific transplantation antigens from methylcholanthrene-induced murine sarcomas by immobilized lectins.
Plasma membranes isolated from two immunogenic, non-cross-protecting, MC sarcomas were shown to retain the specific rejection antigens of whole cells as well as serologically detected H-2 antigens. Solubilization of the membranes with sodium deoxycholate gave quantitative release of H-2 and retained the rejection specificity of the tumour from which it was derived. Polyacrylamide-gel electrophoresis (PAGE) showed no extensive degradation of membrane components during solubilization. The solubilized TSTAs were further characterized and purified on columns of 4 different lectins immobilized on sepharose beads. TSTA from both tumours bound to WGA but not to Con A, LCH or RCA columns. Specific activity was retained after elution from the WGA column. Serologically detectable H-2 bound to the Con A and LCH columns only. Clear separation of H-2 from TSTA activity was thus obtained. Furthermore the WGA-binding material represents a source for further purification of TSTA molecules in order to explore the basis for their diversity
Thermal diffusion by Brownian motion induced fluid stress
The Ludwig-Soret effect, the migration of a species due to a temperature
gradient, has been extensively studied without a complete picture of its cause
emerging. Here we investigate the dynamics of DNA and spherical particles sub
jected to a thermal gradient using a combination of Brownian dynamics and the
lattice Boltzmann method. We observe that the DNA molecules will migrate to
colder regions of the channel, an observation also made in the experiments of
Duhr, et al[1]. In fact, the thermal diffusion coefficient found agrees
quantitatively with the experimental value. We also observe that the thermal
diffusion coefficient decreases as the radius of the studied spherical
particles increases. Furthermore, we observe that the thermal
fluctuations-fluid momentum flux coupling induces a gradient in the stress
which leads to thermal migration in both systems.Comment: 6 pages, 5 figue
Dynamics of superconducting nanowires shunted with an external resistor
We present the first study of superconducting nanowires shunted with an
external resistor, geared towards understanding and controlling coherence and
dissipation in nanowires. The dynamics is probed by measuring the evolution of
the V-I characteristics and the distributions of switching and retrapping
currents upon varying the shunt resistor and temperature. Theoretical analysis
of the experiments indicates that as the value of the shunt resistance is
decreased, the dynamics turns more coherent presumably due to stabilization of
phase-slip centers in the wire and furthermore the switching current approaches
the Bardeen's prediction for equilibrium depairing current. By a detailed
comparison between theory and experimental, we make headway into identifying
regimes in which the quasi-one-dimensional wire can effectively be described by
a zero-dimensional circuit model analogous to the RCSJ (resistively and
capacitively shunted Josephson junction) model of Stewart and McCumber. Besides
its fundamental significance, our study has implications for a range of
promising technological applications.Comment: 15 pages, 14 figure
Motility of small nematodes in disordered wet granular media
The motility of the worm nematode \textit{Caenorhabditis elegans} is
investigated in shallow, wet granular media as a function of particle size
dispersity and area density (). Surprisingly, we find that the nematode's
propulsion speed is enhanced by the presence of particles in a fluid and is
nearly independent of area density. The undulation speed, often used to
differentiate locomotion gaits, is significantly affected by the bulk material
properties of wet mono- and polydisperse granular media for .
This difference is characterized by a change in the nematode's waveform from
swimming to crawling in dense polydisperse media \textit{only}. This change
highlights the organism's adaptability to subtle differences in local structure
and response between monodisperse and polydisperse media
Damped finite-time-singularity driven by noise
We consider the combined influence of linear damping and noise on a dynamical
finite-time-singularity model for a single degree of freedom. We find that the
noise effectively resolves the finite-time-singularity and replaces it by a
first-passage-time or absorbing state distribution with a peak at the
singularity and a long time tail. The damping introduces a characteristic
cross-over time. In the early time regime the probability distribution and
first-passage-time distribution show a power law behavior with scaling exponent
depending on the ratio of the non linear coupling strength to the noise
strength. In the late time regime the behavior is controlled by the damping.
The study might be of relevance in the context of hydrodynamics on a nanometer
scale, in material physics, and in biophysics.Comment: 9 pages, 4 eps-figures, revtex4 fil
Metal-semiconductor (semimetal) superlattices on a graphite sheet with vacancies
It has been found that periodically closely spaced vacancies on a graphite
sheet cause a significant rearrange-ment of its electronic spectrum: metallic
waveguides with a high density of states near the Fermi level are formed along
the vacancy lines. In the direction perpendicular to these lines, the spectrum
exhibits a semimetal or semiconductor character with a gap where a vacancy
miniband is degenerated into impurity levels.Comment: 4 pages, 3 figure
Sonoluminescing air bubbles rectify argon
The dynamics of single bubble sonoluminescence (SBSL) strongly depends on the
percentage of inert gas within the bubble. We propose a theory for this
dependence, based on a combination of principles from sonochemistry and
hydrodynamic stability. The nitrogen and oxygen dissociation and subsequent
reaction to water soluble gases implies that strongly forced air bubbles
eventually consist of pure argon. Thus it is the partial argon (or any other
inert gas) pressure which is relevant for stability. The theory provides
quantitative explanations for many aspects of SBSL.Comment: 4 page
- …