298 research outputs found
A Kinetic Study of Microwave Start-up of Tokamak Plasmas
A kinetic model for studying the time evolution of the distribution function for microwave startup is presented. The model for the distribution function is two dimensional in momentum space, but, for simplicity and rapid calculations, has no spatial dependence. Experiments on the Mega Amp Spherical Tokamak have shown that the plasma current is carried mainly by electrons with energies greater than 70 keV, and effects thought to be important in these experiments are included, i.e. particle sources, orbital losses, the loop voltage and microwave heating, with suitable volume averaging where necessary to give terms independent of spatial dimensions. The model predicts current carried by electrons with the same energies as inferred from the experiments, though the current drive effciency is smaller
Perturbing microwave beams by plasma density fluctuations
The propagation of microwaves across a turbulent plasma density layer is investigated with full-wave simulations. To properly represent a fusion edge-plasma, drift-wave turbulence is considered based on the Hasegawa-Wakatani model. Scattering and broadening of a microwave beam whose amplitude distribution is of Gaussian shape is studied in detail as a function of certain turbulence properties. Parameters leading to the strongest deterioration of the microwave beam are identified and implications for existing experiments are given
Quantum Vacuum Experiments Using High Intensity Lasers
The quantum vacuum constitutes a fascinating medium of study, in particular
since near-future laser facilities will be able to probe the nonlinear nature
of this vacuum. There has been a large number of proposed tests of the
low-energy, high intensity regime of quantum electrodynamics (QED) where the
nonlinear aspects of the electromagnetic vacuum comes into play, and we will
here give a short description of some of these. Such studies can shed light,
not only on the validity of QED, but also on certain aspects of nonperturbative
effects, and thus also give insights for quantum field theories in general.Comment: 9 pages, 8 figur
Reversible magnetization of MgB2 single crystals with a two-gap nature
We present reversible magnetization measurements on MgB2 single crystals in
magnetic fields up to 2.5 T applied parallel to the crystal's c-axis. This
magnetization is analyzed in terms of the Hao-Clem model, and various
superconducting parameters, such as the critical fields [Hc(0) and Hc2(0)], the
characteristic lengths [xi(0) and lambda(0)], and the Ginzburg-Landau
parameter, kappa, are derived. The temperature dependence of the magnetic
penetration depth, lambda(T), obtained from the Hao-Clem analysis could not be
explained by theories assuming a single gap. Our data are well described by
using a two-gap model.Comment: 20 pages, 1 table, 4 figures, will be published in Phys. Rev.
Growing electrostatic modes in the isothermal pair plasma of the pulsar magnetosphere
It is shown that a strongly magnetized isothermal pair plasma near the
surface of a pulsar supports low-frequency (in comparison to electron cyclotron
frequency) toroidal electrostatic plasma modes in the equatorial region.
Physically, the thermal pressure coupled with the magnetic pressure creates the
low frequency oscillations which may grow for particular case of
inhomogeneities of the equilibrium magnetic field and the pair plasma density.Comment: 8 pages, 8 figures, Accepted for publication in Astrophysics & Space
Science 201
Suppression of Jahn-Teller distortion by chromium and magnesium doping in spinel LiMn2O4: A first-principles study using GGA and GGA+U
The effect of doping spinel LiMn2O4 with chromium and magnesium has been
studied using the first-principles spin density functional theory within GGA
(generalized gradient approximation) and GGA+U. We find that GGA and GGA+U give
different ground states for pristine LiMn2O4 and same ground state for doped
systems. For LiMn2O4 the body centered tetragonal phase was found to be the
ground state structure using GGA and face centered orthorhombic using GGA+U,
while for LiM0.5Mn1.5O4 (M= Cr or Mg) it was base centered monoclinic and for
LiMMnO4 (M= Cr or Mg) it was body centered orthorhombic in both GGA and GGA+U.
We find that GGA predicts the pristine LiMn2O4 to be metallic while GGA+U
predicts it to be the insulating which is in accordance with the experimental
observations. For doped spinels, GGA predicts the ground state to be half
metallic while GGA+U predicts it to be insulating or metallic depending on the
doping concentration. GGA+U predicts insulator-metal-insulator transition as a
function of doping in case of Cr and in case of Mg the ground state is found to
go from insulating to a half metallic state as a function of doping. Analysis
of the charge density and the density of states suggest a charge transfer from
the dopants to the neighboring oxygen atoms and manganese atoms. We have
calculated the Jahn-Teller active mode displacement Q3 for doped compounds
using GGA and GGA+U. The bond lengths calculated from GGA+U are found to be in
better agreement with the experimental bond lengths. Based on the bond lengths
of metal and oxygen, we have also estimated the average oxidation states of the
dopants.Comment: 26 pages, 8 figure
The Effects of Disorder on the Quantum Hall State
A disorder-averaged Hartree-Fock treatment is used to compute the density of
single particle states for quantum Hall systems at filling factor . It
is found that transport and spin polarization experiments can be simultaneously
explained by a model of mostly short-range effective disorder. The slope of the
transport gap (due to quasiparticles) in parallel field emerges as a result of
the interplay between disorder-induced broadening and exchange, and has
implications for skyrmion localization.Comment: 4 pages, 3 eps figure
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