7,248 research outputs found
Two-dimensional two-component plasma with adsorbing impurities
We study the behavior of the two-dimensional two-component plasma in the
presence of some adsorbing impurities. Using a solvable model, we find analytic
expressions for the thermodynamic properties of the plasma such as the -body
densities, the grand potential, and the pressure. We specialize in the case
where there are one or two adsorbing point impurities in the plasma, and in the
case where there are one or two parallel adsorbing lines. In the former case we
study the effective interaction between the impurities, due to the charge
redistribution around them. The latter case is a model for electrodes with
adsorbing sticky sites on their surface
Numerical calculation of ion runaway distributions
Ions accelerated by electric fields (so-called runaway ions) in plasmas may
explain observations in solar flares and fusion experiments, however
limitations of previous analytic work have prevented definite conclusions. In
this work we describe a numerical solver of the 2D non-relativistic linearized
Fokker-Planck equation for ions. It solves the initial value problem in
velocity space with a spectral-Eulerian discretization scheme, allowing
arbitrary plasma composition and time-varying electric fields and background
plasma parameters. The numerical ion distribution function is then used to
consider the conditions for runaway ion acceleration in solar flares and
tokamak plasmas. Typical time scales and electric fields required for ion
acceleration are determined for various plasma compositions, ion species and
temperatures, and the potential for excitation of toroidal Alfv\'en eigenmodes
during tokamak disruptions is considered.Comment: 25 pages, 8 figure
Plasma Resonance in Layered Normal Metals and Superconductors
A microscopic theory of the plasma resonance in layered metals is presented.
It is shown that electron-impurity scattering can suppress the plasma resonance
in the normal state and sharpen it in the superconducting state. Analytic
properties of the conductivity for the electronic transport perpendicular to
the layers are investigated. The dissipative part of the electromagnetic
response in c-direction has been found to depend on frequency in a highly
non-trivial manner. This sort of behavior cannot be incorporated in the widely
used phenomenological Gorter-Kazimir model.Comment: 34 pages including 12 figures in uuencoded.file. A revised version.
Several formulas and a number of misprints are corrected. A problem with
printing of figures is fixe
First principles of modelling the stabilization of microturbulence by fast ions
The observation that fast ions stabilize ion-temperature-gradient-driven
microturbulence has profound implications for future fusion reactors. It is
also important in optimizing the performance of present-day devices. In this
work, we examine in detail the phenomenology of fast ion stabilization and
present a reduced model which describes this effect. This model is derived from
the high-energy limit of the gyrokinetic equation and extends the existing
"dilution" model to account for nontrivial fast ion kinetics. Our model
provides a physically-transparent explanation for the observed stabilization
and makes several key qualitative predictions. Firstly, that different classes
of fast ions, depending on their radial density or temperature variation, have
different stabilizing properties. Secondly, that zonal flows are an important
ingredient in this effect precisely because the fast ion zonal response is
negligible. Finally, that in the limit of highly-energetic fast ions, their
response approaches that of the "dilution" model; in particular, alpha
particles are expected to have little, if any, stabilizing effect on plasma
turbulence. We support these conclusions through detailed linear and nonlinear
gyrokinetic simulations.Comment: 29 pages, 10 figures, 3 table
Bulk and edge correlations in the compressible half-filled quantum Hall state
We study bulk and edge correlations in the compressible half-filled state,
using a modified version of the plasma analogy. The corresponding plasma has
anomalously weak screening properties, and as a consequence we find that the
correlations along the edge do not decay algebraically as in the Laughlin
(incompressible) case, while the bulk correlations decay in the same way. The
results suggest that due to the strong coupling between charged modes on the
edge and the neutral Fermions in the bulk, reflected by the weak screening in
the plasma analogue, the (attractive) correlation hole is not well defined on
the edge. Hence, the system there can be modeled as a free Fermi gas of {\em
electrons} (with an appropriate boundary condition). We finally comment on a
possible scenario, in which the Laughlin-like dynamical edge correlations may
nevertheless be realized.Comment: package now includes the file epsfig.sty, needed to incorporate
properly the 8 magnificent figure
Multiexciton molecules in the hexaborides
We investigate multiexciton bound states in a semiconducting phase of
divalent hexaborides. Due to three degenerate valleys in both the conduction
and valence bands the binding energy of a 6-exciton molecule is greatly
enhanced by the shell effect. The ground state energies of multiexciton
molecules are calculated using the density functional formalism. We also show
that charged impurities stabilize multiexciton complexes leading to
condensation of localized excitons. These complexes can act as nucleation
centers of local moments.Comment: RevTEX, 7 pages with 3 figure
Impurity transport and bulk ion flow in a mixed collisionality stellarator plasma
The accumulation of impurities in the core of magnetically confined plasmas,
resulting from standard collisional transport mechanisms, is a known threat to
their performance as fusion energy sources. Whilst the axisymmetric tokamak
systems have been shown to benefit from the effect of temperature screening,
that is an outward flux of impurities driven by the temperature gradient,
impurity accumulation in stellarators was thought to be inevitable, driven
robustly by the inward pointing electric field characteristic of hot fusion
plasmas. We have shown in Helander et. al. (2017b) that such screening can in
principle also appear in stellarators, in the experimentally relevant mixed
collisionality regime, where a highly collisional impurity species is present
in a low collisionality bulk plasma. Details of the analytic calculation are
presented here, along with the effect of the impurity on the bulk ion flow,
which will ultimately affect the bulk contribution to the bootstrap current
Anisotropic thermal emission from magnetized neutron stars
The thermal emission from isolated neutron stars is not well understood. The
X-ray spectrum is very close to a blackbody but there is a systematic optical
excess flux with respect to the extrapolation to low energy of the best
blackbody fit. This fact, in combination with the observed pulsations in the
X-ray flux, can be explained by anisotropies in the surface temperature
distribution.We study the thermal emission from neutron stars with strong
magnetic fields in order to explain the origin of the anisotropy. We find
(numerically) stationary solutions in axial symmetry of the heat
transportequations in the neutron star crust and the condensed envelope. The
anisotropy in the conductivity tensor is included consistently. The presence of
magnetic fields of the expected strength leads to anisotropy in the surface
temperature. Models with toroidal components similar to or larger than the
poloidal field reproduce qualitatively the observed spectral properties and
variability of isolated neutron stars. Our models also predict spectral
features at energies between 0.2 and 0.6 keV.Comment: 18 pages, 19 figures, version accepted for publication in A&
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