53 research outputs found
Electrostatic stability of electron-positron plasmas in dipole geometry
The electrostatic stability of electron-positron plasmas is investigated in
the point-dipole and Z-pinch limits of dipole geometry. The kinetic dispersion
relation for sub-bounce-frequency instabilities is derived and solved. For the
zero-Debye-length case, the stability diagram is found to exhibit singular
behavior. However, when the Debye length is non-zero, a fluid mode appears,
which resolves the observed singularity, and also demonstrates that both the
temperature and density gradients can drive instability. It is concluded that a
finite Debye length is necessary to determine the stability boundaries in
parameter space. Landau damping is investigated at scales sufficiently smaller
than the Debye length, where instability is absent
Semianalytical calculation of the zonal-flow oscillation frequency in stellarators
Due to their capability to reduce turbulent transport in magnetized plasmas,
understanding the dynamics of zonal flows is an important problem in the fusion
programme. Since the pioneering work by Rosenbluth and Hinton in axisymmetric
tokamaks, it is known that studying the linear and collisionless relaxation of
zonal flow perturbations gives valuable information and physical insight.
Recently, the problem has been investigated in stellarators and it has been
found that in these devices the relaxation process exhibits a characteristic
feature: a damped oscillation. The frequency of this oscillation might be a
relevant parameter in the regulation of turbulent transport, and therefore its
efficient and accurate calculation is important. Although an analytical
expression can be derived for the frequency, its numerical evaluation is not
simple and has not been exploited systematically so far. Here, a numerical
method for its evaluation is considered, and the results are compared with
those obtained by calculating the frequency from gyrokinetic simulations. This
"semianalytical" approach for the determination of the zonal-flow frequency
reveals accurate and faster than the one based on gyrokinetic simulations.Comment: 30 pages, 14 figure
Persistent form of bovine viral diarrhea
The review provides an analysis of literature data on the persistent form of Bovine Viral diarrhea/Mucosal disease (BVD) and is focused on virus and host factors, including those related to immune response, that contribute the persistence of the virus. BVD is a cattle disease widespread throughout the world that causes significant economic damage to dairy and beef cattle. The disease is characterized by a variety of clinical signs, including damage to the digestive and respiratory organs, abortions, stillbirths and other failures of reproductive functions
Linear and nonlinear excitation of TAE modes by external electromagnetic perturbations using ORB5
The excitation of toroidicity induced Alfv{\'e}n eigenmodes (TAEs) using
prescribed external electromagnetic perturbations (hereafter ``antenna") acting
on a confined toroidal plasma as well as its nonlinear couplings to other modes
in the system is studied. The antenna is described by an electrostatic
potential resembling the target TAE mode structure along with its corresponding
parallel electromagnetic potential computed from Ohm's law. Numerically stable
long-time linear simulations are achieved by integrating the antenna within the
framework of a mixed representation and pullback scheme [A. Mishchenko, et al.,
Comput. Phys. Commun. \textbf{238} (2019) 194]. By decomposing the plasma
electromagnetic potential into symplectic and Hamiltonian parts and using Ohm's
law, the destabilizing contribution of the potential gradient parallel to the
magnetic field is canceled in the equations of motion. Besides evaluating the
frequencies as well as growth/damping rates of excited modes compared to
referenced TAEs, we study the interaction of antenna-driven modes with fast
particles and indicate their margins of instability. Furthermore, we show first
nonlinear simulations in the presence of a TAE-like antenna exciting other TAE
modes, as well as Global Alfv\'en Eigenmodes (GAE) having different toroidal
wave numbers from that of the antenna
Singularities of bi-Hamiltonian systems
We study the relationship between singularities of bi-Hamiltonian systems and
algebraic properties of compatible Poisson brackets. As the main tool, we
introduce the notion of linearization of a Poisson pencil. From the algebraic
viewpoint, a linearized Poisson pencil can be understood as a Lie algebra with
a fixed 2-cocycle. In terms of such linearizations, we give a criterion for
non-degeneracy of singular points of bi-Hamiltonian systems and describe their
types
Giant magnetoresistance of Dirac plasma in high-mobility graphene
The most recognizable feature of graphene's electronic spectrum is its Dirac
point around which interesting phenomena tend to cluster. At low temperatures,
the intrinsic behavior in this regime is often obscured by charge inhomogeneity
but thermal excitations can overcome the disorder at elevated temperatures and
create electron-hole plasma of Dirac fermions. The Dirac plasma has been found
to exhibit unusual properties including quantum critical scattering and
hydrodynamic flow. However, little is known about the plasma's behavior in
magnetic fields. Here we report magnetotransport in this quantum-critical
regime. In low fields, the plasma exhibits giant parabolic magnetoresistivity
reaching >100% in 0.1 T even at room temperature. This is orders of magnitude
higher than magnetoresistivity found in any other system at such temperatures.
We show that this behavior is unique to monolayer graphene, being underpinned
by its massless spectrum and ultrahigh mobility, despite frequent
(Planckian-limit) scattering. With the onset of Landau quantization in a few T,
where the electron-hole plasma resides entirely on the zeroth Landau level,
giant linear magnetoresistivity emerges. It is nearly independent of
temperature and can be suppressed by proximity screening, indicating a
many-body origin. Clear parallels with magnetotransport in strange metals and
so-called quantum linear magnetoresistance predicted for Weyl metals offer an
interesting playground to further explore relevant physics using this
well-defined quantum-critical 2D system.Comment: 8 pages, 3 figure
- …