226 research outputs found
Vertical pairing of identical particles suspended in the plasma sheath
It is shown experimentally that vertical pairing of two identical
microspheres suspended in the sheath of a radio-frequency (rf) discharge at low
gas pressures (a few Pa), appears at a well defined instability threshold of
the rf power. The transition is reversible, but with significant hysteresis on
the second stage. A simple model, which uses measured microsphere resonance
frequencies and takes into account besides Coulomb interaction between
negatively charged microspheres also their interaction with positive ion wake
charges, seems to explain the instability threshold quite well.Comment: 4 pages, 6 figures. to appear in Phys. Rev. Lett. 86, May 14th (2001
Nonlinear dynamics of large amplitude dust acoustic shocks and solitary pulses in dusty plasmas
We present a fully nonlinear theory for dust acoustic (DA) shocks and DA
solitary pulses in a strongly coupled dusty plasma, which have been recently
observed experimentally by Heinrich et al. [Phys. Rev. Lett. 103, 115002
(2009)], Teng et al. [Phys. Rev. Lett. 103, 245005 (2009)], and Bandyopadhyay
et al. [Phys. Rev. Lett. 101, 065006 (2008)]. For this purpose, we use a
generalized hydrodynamic model for the strongly coupled dust grains, accounting
for arbitrary large amplitude dust number density compressions and potential
distributions associated with fully nonlinear nonstationary DA waves.
Time-dependent numerical solutions of our nonlinear model compare favorably
well with the recent experimental works (mentioned above) that have reported
the formation of large amplitude non-stationary DA shocks and DA solitary
pulses in low-temperature dusty plasma discharges.Comment: 9 pages, 4 figures. To be published in Physical Review
Instability of ion kinetic waves in a weakly ionized plasma
The fundamental higher-order Landau plasma modes are known to be generally
heavily damped. We show that these modes for the ion component in a weakly
ionized plasma can be substantially modified by ion-neutral collisions and a dc
electric field driving ion flow so that some of them can become unstable. This
instability is expected to naturally occur in presheaths of gas discharges at
sufficiently small pressures and thus affect sheaths and discharge structures.Comment: Published in Phys. Rev. E, see
http://link.aps.org/doi/10.1103/PhysRevE.85.02641
Dynamic transitions between metastable states in a superconducting ring
Applying the time-dependent Ginzburg-Landau equations, transitions between
metastable states of a superconducting ring are investigated in the presence of
an external magnetic field. It is shown that if the ring exhibits several
metastable states at a particular magnetic field, the transition from one
metastable state to another one is governed by both the relaxation time of the
absolute value of the order parameter tau_{|psi|} and the relaxation time of
the phase of the order parameter tau_{phi}. We found that the larger the ratio
tau_{|psi|}tau_{phi} the closer the final state will be to the absolute minimum
of the free energy, i.e. the thermodynamic equilibrium. The transition to the
final state occurs through a subsequent set of single phase slips at a
particular point along the ring.Comment: 7 pages, 6 figures, Revtex 4.0 styl
Optical properties and dust temperatures in clumpy diffuse medium
In this study, we explore the impact of inhomogeneities in the spatial distribution of interstellar dust on spatial scales of ≤1 au caused by ion shadowing forces on the optical properties of diffuse interstellar medium (ISM) as well as on the dust temperature. We show that recently proposed possibility that interstellar dust grains in the diffuse ISM are grouped in spherical cloudlets (clumps) may significantly affect the observed optical properties of the diffuse ISM in comparison to that calculated under the commonly accepted assumption on the uniform dust/gas mixture if the size of clumps ≳0.1 au. We found that opacity of an arbitrary region of diffuse ISM quickly decreases with growth of dusty clumps. We also studied the dependence of opacity and dust temperature inside the dusty clumps on their size. We show that the clumps larger than 0.1 au are opaque for far-ultraviolet radiation. Dust temperature exhibits a gradient inside a clump, decreasing from the edge to the centre by several degrees for a clump of a size of 0.1 au and larger. We argue that dust temperatures and high opacity within clumps larger than 0.1 au may facilitate somewhat more efficient synthesis of molecules on surfaces of interstellar grains in the diffuse ISM than it was anticipated previously. On the other hand, the presence of clumps with sizes below 0.1 au makes small or negligible influence on the optical properties of the diffuse ISM in comparison to the case with uniform dust/gas mixture. © 2020 The Author(s).The authors are thankful to Prof. Paola Caselli and Dr. Yaroslav Pavlyuchenkov for critical reading of the manuscript and suggestions on its improvement. The authors are also thankful to the anonymous referee for suggestions that helped to improve the manuscript. ABO, AIV, and AVI were supported by the Russian Science Foundation via the project 18-12-00351. AIV and VAS are members of the Max Planck Partner Group at the Ural Federal University. Monte Carlo simulations were made by SYP. SYP acknowledges support by the Ministry of Science and Higher Education of the Russian Federation, FEUZ-2020-0030
Rolling Friction in Loose Media and its Role in Mechanics Problems
Rolling friction between particles is to be set in problems of granular material mechanics alongside with sliding friction. A classical problem of material passive lateral pressure on the retaining wall is submitted as a case in point. 3D method of discrete elements was employed for numerical analysis. Material is a universe of spherical particles with specified size distribution. Viscose-elastic properties of the material and surface friction are included, when choosing contact forces. Particles' resistance to rolling relative to other particles and to the boundary is set into the model. Kinetic patterns of medium deformations are given. It has been proved that rolling friction can significantly affect magnitude and nature of passive lateral pressure on the retaining wall
Our experience of surgical treatment of aortic arch obstruction in children in the presence of antegrade selective cerebral perfusion
This work is about outcomes of surgical treatment aortic arch obstruction in children performed with the use of selective cerebral perfusion and moderate hypothermia. The study included 97 patients with aortic coarctation, recoarctation and hypoplastic aortic arch. The patients were divided into 3 groups. 75 patients with isolated coarctation and recoarctation or with concomitant atrial septal defects, patentforamen ovale and patent ductus arterious were assigned to the 1st group. The 2nd group included 13 patients with aortic arch obstruction and ventricular septal defects. The 3rd group consisted of 9 patients with coarctation, recoarctation and complex intracardiac anomalies. All patients underwent reconstruction of the aortic arch with selective cerebral perfusion and moderate hypothermia. Overall mortality rate was 4%. Postoperative complications took place in 7% of the cases: 1% - postoperative bleeding, 1% - paresis of the diaphragm, 2% - pneumothorax, 2% - chylothorax, and 1% of the cases - vocal cords paresis. Respiratory postoperative complications were registered in 5% of cases. In 6% of the cases patients had neurological postoperative complications. After the operation there was no gradientfound between the pressures on upper and lower extremities. We consider that antegrade selective cerebral perfusion and moderate hypothermia during aortic arch reconstruction are effective methods of brain and visceral organs protection
Metastability in Josephson transmission lines
Thermal activation and macroscopic quantum tunneling in current-biased
discrete Josephson transmission lines are studied theoretically. The degrees of
freedom under consideration are the phases across the junctions which are
coupled to each other via the inductances of the system. The resistively
shunted junctions that we investigate constitute a system of N interacting
degrees of freedom with an overdamped dynamics. We calculate the decay rate
within exponential accuracy as a function of temperature and current. Slightly
below the critical current, the decay from the metastable state occurs via a
unique ("rigid") saddlepoint solution of the Euclidean action describing the
simultaneous decay of the phases in all the junctions. When the current is
reduced, a crossover to a regime takes place, where the decay occurs via an
"elastic" saddlepoint solution and the phases across the junctions leave the
metastable state one after another. This leads to an increased decay rate
compared with the rigid case both in the thermal and the quantum regime. The
rigid-to-elastic crossover can be sharp or smooth analogous to first- or
second- order phase transitions, respectively. The various regimes are
summarized in a current-temperature decay diagram.Comment: 11 pages, RevTeX, 3 PS-figures, revised versio
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