21 research outputs found
Kinetic Solvers with Adaptive Mesh in Phase Space
An Adaptive Mesh in Phase Space (AMPS) methodology has been developed for
solving multi-dimensional kinetic equations by the discrete velocity method. A
Cartesian mesh for both configuration (r) and velocity (v) spaces is produced
using a tree of trees data structure. The mesh in r-space is automatically
generated around embedded boundaries and dynamically adapted to local solution
properties. The mesh in v-space is created on-the-fly for each cell in r-space.
Mappings between neighboring v-space trees implemented for the advection
operator in configuration space. We have developed new algorithms for solving
the full Boltzmann and linear Boltzmann equations with AMPS. Several recent
innovations were used to calculate the discrete Boltzmann collision integral
with dynamically adaptive mesh in velocity space: importance sampling,
multi-point projection method, and the variance reduction method. We have
developed an efficient algorithm for calculating the linear Boltzmann collision
integral for elastic and inelastic collisions in a Lorentz gas. New AMPS
technique has been demonstrated for simulations of hypersonic rarefied gas
flows, ion and electron kinetics in weakly ionized plasma, radiation and light
particle transport through thin films, and electron streaming in
semiconductors. We have shown that AMPS allows minimizing the number of cells
in phase space to reduce computational cost and memory usage for solving
challenging kinetic problems
Electron Kinetics in a Positive Column of AC Discharges in a Dynamic Regime
We have performed hybrid kinetic-fluid simulations of a positive column in AC
Argon discharges over a range of driving frequencies f and gas pressure p for
the conditions when the spatial nonlocality of the Electron Energy Distribution
Function (EEDF) is substantial. Our simulations confirmed that the most
efficient conditions of plasma maintenance are observed in the dynamic regime
when time modulations of mean electron energy (temperature) are substantial.
The minimal values of the root mean square (rms) electric field and the
electron temperature have been observed at f/p values of about 3 kHz/Torr in a
tube of radius R = 1 cm. The ionization rate and plasma density reached maximal
values under these conditions.
The numerical solution of a kinetic equation allowed accounting for the
kinetic effects associated with spatial and temporal nonlocality of the EEDF.
Using the kinetic energy of electrons as an independent variable, we solved an
anisotropic tensor diffusion equation in phase space. We clarified the role of
different flux components during electron diffusion in phase space over
surfaces of constant total energy. We have shown that the kinetic theory
uncovers a more exciting and rich physics than the classical ambipolar
diffusion (Schottky) model. Non-monotonic radial distributions of excitation
rates, metastable densities, and plasma density have been observed in our
simulations at pR > 6 Torr cm. The predicted off-axis plasma density peak in
the dynamic regime has never been observed in experiments so far. We hope our
results stimulate further experimental studies of the AC positive column. The
kinetic analysis could help uncover new physics even for such a well-known
plasma object as a positive column in noble gases
Ionization waves (striations) in low-current DC discharges in noble gases obtained with a hybrid kinetic-fluid model
A hybrid kinetic-fluid model is used to study ionization waves (striations)
in a low-current plasma column of DC discharges in noble gases. Coupled
solutions of a kinetic equation for electrons, a drift-diffusion equation of
ions, and a Poisson equation for the electric field are obtained to clarify the
nature of plasma stratification in the positive column and near-electrode
effects. A simplified two-level excitation-ionization model is used for the
conditions when the nonlinear effects due to stepwise ionization, gas heating,
and Coulomb interactions among electrons are negligible. It is confirmed that
the nonlocal effects are responsible for the formation of moving striations in
DC discharges at low plasma densities. The calculated properties of
self-excited waves of S, P, and R types in Neon and S type in Argon agree with
available experimental data. The reason for Helium plasma stability to
stratification is clarified. It is shown that sustaining stratified plasma is
more efficient than striation-free plasma when the ionization rate is a
nonlinear function of the electric field. However, the nonlinear dependence of
the ionization rate on the electric field is not required for plasma
stratification. Striations of S, P, and R types in Neon exist with minimal or
no ionization enhancement. Effects of the column length on the wave properties
have been demonstrated in our simulations
Distinct functional brain regional integration of Casp3, Ascl1 and S100a6 gene expression in spatial memory
Evaluating the brain structural expression of defined genes involved in basic biological processes of neurogenesis, apoptosis or neural plasticity may facilitate the understanding of genetic mechanisms underlying spatial memory. The aim of the present study was to compare Ascl1, Casp3 and S100a6gene expression in the hippocampus, prefrontal cortex and cerebellum of adult rats in water maze spatial memory performance. After four days training, the mean platform time (<10 s) was evidence of stable long-term spatial memory formation. Real time PCR analysis revealed a positive inter-structural correlation for S100a6/Caspgene expression between the prefrontal cortex and the cerebellum but a negative correlation for S100a6/Ascl1transcribed genes between the prefrontal cortex and hippocampus during swimming in the active controls. However, during spatial memory performance there was only one inter-structural correlation between the prefrontal cortex and cerebellum with respect to Casp3 expression, though there were intra-structural correlations between Casp3/Ascl1transcriptions within the prefrontal cortex and hippocampus as well as between Ascl1/S100a6in the cerebellum. In active learners versus naive controls, the transcrption of all genes was augmented in the prefrontal cortex but Casp3 and Ascl1were also elevated in hippocampus whilst only S100a6increased in the cerebellum. The findings endorsed the role of the hippocampus in memory acquisition in addition to an integrative relationship with the prefrontal cortex and cerebellum. This structural and molecular configuration is important for creation of novel neural circuitry for consolidation and reconsolidation of memory trace with an involvement of coupled processes of neurogenesis, apoptosis or neural plasticity