1,227,349 research outputs found
A cryocooler for applications requiring low magnetic and mechanical interference
A very low-power, low-interference Stirling cryocooler is being developed based on principles and techniques described in several previous publications over the last four years. It differs in several important details from those built previously. It uses a tapered displacer based upon an analytical optimization procedure. The displacer is driven by an auxiliary piston and cylinder (rather than by mechanical linkage) using some of the working fluid itself to provide the driving force. This provides smooth, vibration-free motion, and, more importantly, allows complete mechanical and spatial separation of the cryostat from the pressure-wave generator. Either of two different pressure-wave generators can be used. One is a non-contaminating, unlubricated ceramic piston and cylinder. The other is a compressed-air-operated rubber diaphragm with motor-driven valves to cycle the pressure between appropriate limits
Complex temperatures zeroes of partition function in spin-glass models
An approximate method is proposed for investigating complex-temperature
properties of real-dimensional spin-glass models. The method uses the
complex-temperature data of the ferromagnetic model on the same lattice. The
universality line in the complex-temperature space is obtained.Comment: latex, corrected some misprint
Axially Symmetric Solutions for SU(2) Yang-Mills Theory
By casting the Yang-Mills-Higgs equations of an SU(2) theory in the form of
the Ernst equations of general relativity, it is shown how the known exact
solutions of general relativity can be used to give similiar solutions for
Yang-Mills theory. Thus all the known exact solutions of general relativity
with axial symmetry (e.g. the Kerr metric, the Tomimatsu-Sato metric) have
Yang-Mills equivalents. In this paper we only examine in detail the Kerr-like
solution. It will be seen that this solution has surfaces where the gauge and
scalar fields become infinite, which correspond to the infinite redshift
surfaces of the normal Kerr solution. It is speculated that this feature may be
connected with the confinement mechanism since any particle which carries an
SU(2) color charge would tend to become trapped once it passes these surfaces.
Unlike the Kerr solution, our solution apparently does not have any intrinsic
angular momentum, but rather appears to give the non-Abelian field
configuration associated with concentric shells of color charge.Comment: 15 pages LaTe
Strangeness Balance in HADES Experiments and the Xi- Enhancement
HADES data on a strangeness production in Ar+KCl collisions at 1.76A GeV are
analyzed within a minimal statistical model. The total negative strangeness
content is fixed by the observed K^+ multiplicities on event-by-event basis.
Particles with negative strangeness are assumed to remain in chemical
equilibrium with themselves and in thermal equilibrium with the environment
until a common freeze-out. Exact strangeness conservation in each collision
event is explicitly preserved. This implies that Xi baryons can be released
only in events where two or more kaons are produced. An increase of the
fireball volume due to application of a centrality trigger in HADES experiments
is taken into account. We find that experimental ratios of K-/K+, Lambda/K+ and
Sigma/K+ can be satisfactorily described provided in-medium potentials are
taken into account. However, the calculated Xi-/Lambda/K+ ratio proves to be
significantly smaller compared to the measured value (8 times lower than the
experimental median value and 3 times lower than the lower error bar). Various
scenarios to explain observed Xi enhancement are discussed. Arguments are given
in favor of the Xi production in direct reactions. The rates of the possible
production processes are estimated and compared.Comment: 13 pages, 3 figure
An approach to optimization of low-power Stirling cryocoolers
A method for optimizing the design (shape of the displacer) of low power Stirling cryocoolers relative to the power required to operate the systems is described. A variational calculation which includes static conduction, shuttle and radiation losses, as well as regenerator inefficiency, was completed for coolers operating in the 300 K to 10 K range. While the calculations apply to tapered displacer machines, comparison of the results with stepped displacer cryocoolers indicates reasonable agreement
Characterization of open cluster remnants
Despite progress in the theoretical knowledge of open cluster remnants and
the growing search for observational identifications in recent years, open
questions still remain. The methods used to analyze open cluster remnants and
criteria to define them as physical systems are not homogeneous. In this work
we present a systematic method for studying these objects that provides a view
of their properties and allows their characterization. Eighteen remnant
candidates are analyzed by means of photometric and proper motion data. These
data provide information on objects and their fields. We establish criteria for
characterizing open cluster remnants, taking observational uncertainties into
account. 2MASS J and H photometry is employed (i) to study structural
properties of the objects by means of radial stellar density profiles, (ii) to
test for any similarity between objects and fields with a statistical
comparison method applied to the distributions of stars in the CMDs, and (iii)
to obtain ages, reddening values, and distances from the CMD, taking an index
of isochrone fit into account. The UCAC2 proper motions allowed an objective
comparison between objects and large solid angle offset fields. The objective
analysis based on the present methods indicates 13 open-cluster remnants in the
sample. Evidence of the presence of binary stars is found, as expected for
dynamically evolved systems. Finally, we infer possible evolutionary stages
among remnants from the structure, proper motion, and CMD distributions. The
low stellar statistics for individual objects is overcome by means of the
construction of composite proper motion and CMD diagrams. The distributions of
remnants in the composite diagrams resemble the single-star and unresolved
binary star distributions of open clusters.Comment: 16 pages, 15 figures, A&A accepte
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