1,532 research outputs found
Numerical Studies of Weakly Stochastic Magnetic Reconnection
We study the effects of turbulence on magnetic reconnection using
three-dimensional numerical simulations. This is the first attempt to test a
model of fast magnetic reconnection proposed by Lazarian & Vishniac (1999),
which assumes the presence of weak, small-scale magnetic field structure near
the current sheet. This affects the rate of reconnection by reducing the
transverse scale for reconnection flows and by allowing many independent flux
reconnection events to occur simultaneously. We performed a number of
simulations to test the dependencies of the reconnection speed, defined as the
ratio of the inflow velocity to the Alfven speed, on the turbulence power, the
injection scale and resistivity. Our results show that turbulence significantly
affects the topology of magnetic field near the diffusion region and increases
the thickness of the outflow region. We confirm the predictions of the Lazarian
& Vishniac model. In particular, we report the growth of the reconnection speed
proportional to ~ V^2, where V is the amplitude of velocity at the injection
scale. It depends on the injection scale l as ~ (l/L)^(2/3), where L is the
size of the system, which is somewhat faster but still roughly consistent with
the theoretical expectations. We also show that for 3D reconnection the Ohmic
resistivity is important in the local reconnection events only, and the global
reconnection rate in the presence of turbulence does not depend on it.Comment: 8 pages, 8 figure
The role of pressure anisotropy in the turbulent intracluster medium
In low-density plasma environments, such as the intracluster medium (ICM),
the Larmour frequency is much larger than the ion-ion collision frequency. In
such a case, the thermal pressure becomes anisotropic with respect to the
magnetic field orientation and the evolution of the turbulent gas is more
correctly described by a kinetic approach. A possible description of these
collisionless scenarios is given by the so-called kinetic magnetohydrodynamic
(KMHD) formalism, in which particles freely stream along the field lines, while
moving with the field lines in the perpendicular direction. In this way a
fluid-like behavior in the perpendicular plane is restored. In this work, we
study fast growing magnetic fluctuations in the smallest scales which operate
in the collisionless plasma that fills the ICM. In particular, we focus on the
impact of a particular evolution of the pressure anisotropy and its
implications for the turbulent dynamics of observables under the conditions
prevailing in the ICM. We present results from numerical simulations and
compare the results which those obtained using an MHD formalism.Comment: 7 pages, 14 figures, Journal of Physics: Conference Serie
Accuracy of computerized tomography in determining hepatic tumor size in patients receiving liver transplantation or resection
Computerized tomography (CT) of liver is used in oncologic practice for staging tumors, evaluating response to treatment, and screening patients for hepatic resection. Because of the impact of CT liver scan on major treatment decisions, it is important to assess its accuracy. Patients undergoing liver transplantation or resection provide a unique opportunity to test the accuracy of hepatic-imaging techniques by comparison of finding of preoperative CT scan with those at gross pathologic examination of resected specimens. Forty-one patients who had partial hepatic resection (34 patients) or liver transplantation (eight patients) for malignant (30 patients) or benign (11 patients) tumors were evaluable. Eight (47%) of 17 patients with primary malignant liver tumors, four (31%) of 13 patients with metastatic liver tumors, and two (20%) of 10 patients with benign liver tumors had tumor nodules in resected specimens that were not apparent on preoperative CT studies. These nodules varied in size from 0.1 to 1.6 cm. While 11 of 14 of these nodules were 1.0 cm. These results suggest that conventional CT alone may be insufficient to accurately determine the presence or absence of liver metastases, extent of liver involvement, or response of hepatic metastases to treatment
Research on properties of multi-core superconducting wires made from materials based on magnesium and boron (MgB2)
The article presents the results of laboratory research on the production of multi-core superconducting wires. Multicore wires containing boron and magnesium powders in a copper matrix were obtained in the drawing process combined with intermediate heat treatment. The wires contains powder cores were sintered under high isostatic pressure to produce the MgB2 superconducting phase. The critical temperature for the composite’s superconducting state was determined. The macrostructure and energy dispersion (EDX) analysis of multi-core wires was also presented
Reconnection current sheet structure in a turbulent medium
In the presence of turbulence, magnetic field lines lose their dynamical identity and particles entrained on field lines diffuse through space at a rate determined by the amplitude of the turbulence. In previous work (Lazarian and Vishniac, 1999; Kowal et al., 2009; Eyink et al., 2011) we showed that this leads to reconnection speeds which are independent of resistivity. In particular, in Kowal et al. (2009) we showed that numerical simulations were consistent with the predictions of this model. Here we examine the structure of the current sheet in simulations of turbulent reconnection. Laminar flows consistent with the Sweet-Parker reconnection model produce very thin and well ordered currents sheets. On the other hand, the simulations of Kowal et al. (2009) show a strongly disordered state even for relatively low levels of turbulence. Comparing data cubes with and without reconnection, we find that large scale field reversals are the cumulative effect of many individual eddies, each of which has magnetic properties which are not very different from turbulent eddies in a homogeneous background. This implies that the properties of stationary and homogeneous MHD turbulence are a reasonable guide to understanding turbulence during large scale magnetic reconnection events. In addition, dissipation and high energy particle acceleration during reconnection events take place over a macroscopic volume, rather than being confined to a narrow zone whose properties depend on microscopic transport coefficients
Numerical Tests of Fast Reconnection in Weakly Stochastic Magnetic Fields
We study the effects of turbulence on magnetic reconnection using 3D
numerical simulations. This is the first attempt to test a model of fast
magnetic reconnection in the presence of weak turbulence proposed by Lazarian &
Vishniac (1999). This model predicts that weak turbulence, generically present
in most of astrophysical systems, enhances the rate of reconnection by reducing
the transverse scale for reconnection events and by allowing many independent
flux reconnection events to occur simultaneously. As a result the reconnection
speed becomes independent of Ohmic resistivity and is determined by the
magnetic field wandering induced by turbulence. To quantify the reconnection
speed we use both an intuitive definition, i.e. the speed of the reconnected
flux inflow, as well as a more sophisticated definition based on a formally
derived analytical expression. Our results confirm the predictions of the
Lazarian & Vishniac model. In particular, we find that Vrec Pinj^(1/2), as
predicted by the model. The dependence on the injection scale for some of our
models is a bit weaker than expected, i.e. l^(3/4), compared to the predicted
linear dependence on the injection scale, which may require some refinement of
the model or may be due to the effects like finite size of the excitation
region. The reconnection speed was found to depend on the expected rate of
magnetic field wandering and not on the magnitude of the guide field. In our
models, we see no dependence on the guide field when its strength is comparable
to the reconnected component. More importantly, while in the absence of
turbulence we successfully reproduce the Sweet-Parker scaling of reconnection,
in the presence of turbulence we do not observe any dependence on Ohmic
resistivity, confirming that our reconnection is fast.Comment: 22 pages, 20 figure
Patching laser-reduced graphene oxide with carbon nanodots
Three-dimensional graphenes are versatile materials for a range of electronic applications and considered among the most promising candidates for electrodes in future electric double layer capacitors (EDLCs) as they are expected to outperform commercially used activated carbon. Parameters such as electrical conductivity and active surface area are critical to the final device performance. By adding carbon nanodots to graphene oxide in the starting material for our standard laser-assisted reduction process, the structural integrity (i.e. lower defect density) of the final 3D-graphene is improved. As a result, the active surface area in the hybrid starting materials was increased by 130% and the electrical conductivity enhanced by nearly an order of magnitude compared to pure laser-reduced graphene oxide. These improved material parameters lead to enhanced device performance of the EDLC electrodes. The frequency response, i.e. the minimum phase angle and the relaxation time, were significantly improved from −82.2° and 128 ms to −84.3° and 7.6 ms, respectively. For the same devices the specific gravimetric device capacitance was increased from 110 to a maximum value of 214 F g−1 at a scan rate of 10 mV s−1
Multi-step particle emission probabilities in superheavy nuclei at moderate excitation energies
The probabilities of -, -, and -evaporation channels in
excited superheavy nuclei were evaluated using the Monte Carlo method. The
calculations utilized microscopically determined nuclear level densities and
were compared with results obtained from the phenomenological Jackson formula.
Effective temperatures derived from the microscopic approach were incorporated
into the Jackson formula for different evaporation channels at low and moderate
excitation energies. Additionally, an analytical formula was introduced to
estimate the average kinetic energy of emitted particles in multi-step
processes.Comment: 10 pages, 3 figure
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