94 research outputs found
Modeling the thermal conduction in the solar atmosphere with the code MANCHA3D
Thermal conductivity is one of the important mechanisms of heat transfer in
the solar corona. In the limit of strongly magnetized plasma, it is typically
modeled by Spitzer's expression where the heat flux is aligned with the
magnetic field. This paper describes the implementation of the heat conduction
into the code MANCHA3D with an aim of extending single-fluid MHD simulations
from the upper convection zone into the solar corona. Two different schemes to
model heat conduction are implemented: (1) a standard scheme where a parabolic
term is added to the energy equation, and (2) a scheme where the hyperbolic
heat flux equation is solved. The first scheme limits the time step due to the
explicit integration of a parabolic term, which makes the simulations
computationally expensive. The second scheme solves the limitations on the time
step by artificially limiting the heat conduction speed to computationally
manageable values. The validation of both schemes is carried out with standard
tests in one, two, and three spatial dimensions. Furthermore, we implement the
model for heat flux derived by Braginskii (1965) in its most general form, when
the expression for the heat flux depends on the ratio of the collisional to
cyclotron frequencies of the plasma, and, therefore on the magnetic field
strength. Additionally, our implementation takes into account the heat
conduction in parallel, perpendicular, and transverse directions, and provides
the contributions from ions and electrons separately. The model also
transitions smoothly between field-aligned conductivity and isotropic
conductivity for regions with a low or null magnetic field. Finally, we present
a two-dimensional test for heat conduction using realistic values of the solar
atmosphere where we prove the robustness of the two schemes implemented.Comment: 11 pages, 8 figure
Fast electrochemical doping due to front instability in organic semiconductors
The electrochemical doping transformation in organic semiconductor devices is
studied in application to light-emitting cells. It is shown that the device
performance can be significantly improved by utilizing new fundamental
properties of the doping process. We obtain an instability, which distorts the
doping fronts and increases the doping rate considerably. We explain the
physical mechanism of the instability, develop theory, provide experimental
evidence, and perform numerical simulations. We further show how improved
device design can amplify the instability thus leading to a much faster doping
process and device kinetics.Comment: 4 pages, 4 figure
Ultra-fast spin avalanches in crystals of molecular magnets in terms of magnetic detonation
Recent experiments (Decelle et al., Phys. Rev. Lett. 102, 027203 (2009))
discovered an ultra-fast regime of spin avalanches in crystals of magnetic
magnets, which was three orders of magnitude faster than the traditionally
studied magnetic deflagration. The new regime has been hypothetically
identified as magnetic detonation. Here we demonstrate the possibility of
magnetic detonation in the crystals, as a front consisting of a leading shock
and a zone of Zeeman energy release. We study the dependence of the magnetic
detonation parameters on the applied magnetic field. We find that the magnetic
detonation speed only slightly exceeds the sound speed in agreement with the
experimental observations.Comment: 4 pages, 4 figure
Evolution of the magnetic field generated by the Kelvin-Helmholtz instability
The Kelvin-Helmholtz instability in an ionized plasma is studied with a focus on the magnetic field generation via the Biermann battery (baroclinic) mechanism. The problem is solved by using direct numerical simulations of two counter-directed flows in 2D geometry. The simulations demonstrate the formation of eddies and their further interaction and merging resulting in a large single vortex. In contrast to general belief, it is found that the instability generated magnetic field may exhibit significantly different structures from the vorticity field, despite the mathematically identical equations controlling the magnetic field and vorticity evolution. At later stages of the nonlinear instability development, the magnetic field may keep growing even after the hydrodynamic vortex strength has reached its maximum and started decaying due to dissipation
A model for the dynamics and internal structure of planar doping fronts in organic semiconductors
The dynamics and internal structure of doping fronts in organic
semiconductors are investigated theoretically using an extended drift-diffusion
model for ions, electrons and holes. The model also involves the injection
barriers for electrons and holes in the partially doped regions in the form of
the Nernst equation, together with a strong dependence of the electron and hole
mobility on concentrations. Closed expressions for the front velocities and the
ion concentrations in the doped regions are obtained. The analytical theory is
employed to describe the acceleration of the p- and n-fronts towards each
other. The analytical results show very good agreement with the experimental
data. Furthermore, it is shown that the internal structure of the doping fronts
is determined by the diffusion and mobility processes. The asymptotic behavior
of the concentrations and the electric field is studied analytically inside the
doping fronts. The numerical solution for the front structure confirms the most
important predictions of the analytical theory: a sharp head of the front in
the undoped region, a smooth relaxation tail in the doped region, and a plateau
at the critical point of transition from doped to undoped regions.Comment: 13 pages, 11 figure
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