728 research outputs found
Tokamak plasma boundary reconstruction using toroidal harmonics and an optimal control method
This paper proposes a new fast and stable algorithm for the reconstruction of
the plasma boundary from discrete magnetic measurements taken at several
locations surrounding the vacuum vessel. The resolution of this inverse problem
takes two steps. In the first one we transform the set of measurements into
Cauchy conditions on a fixed contour close to the measurement
points. This is done by least square fitting a truncated series of toroidal
harmonic functions to the measurements. The second step consists in solving a
Cauchy problem for the elliptic equation satisfied by the flux in the vacuum
and for the overdetermined boundary conditions on previously
obtained with the help of toroidal harmonics. It is reformulated as an optimal
control problem on a fixed annular domain of external boundary and
fictitious inner boundary . A regularized Kohn-Vogelius cost
function depending on the value of the flux on and measuring the
discrepency between the solution to the equation satisfied by the flux obtained
using Dirichlet conditions on and the one obtained using Neumann
conditions is minimized. The method presented here has led to the development
of a software, called VacTH-KV, which enables plasma boundary reconstruction in
any Tokamak.Comment: Fusion Science and Technology, 201
Thermal conductivity of graphene in Corbino membrane geometry
Local laser excitation and temperature readout from the intensity ratio of
Stokes to anti-Stokes Raman scattering signals are employed to study the
thermal properties of a large graphene membrane. The concluded value of the
heat conductivity coefficient \kappa ~ 600 W/m \cdot K is smaller than
previously reported but still validates the conclusion that graphene is a very
good thermal conductor.Comment: 4 pages, 3 figure
Graphite from the viewpoint of Landau level spectroscopy: An effective graphene bilayer and monolayer
We describe an infrared transmission study of a thin layer of bulk graphite
in magnetic fields up to B = 34 T. Two series of absorption lines whose energy
scales as sqrtB and B are present in the spectra and identified as
contributions of massless holes at the H point and massive electrons in the
vicinity of the K point, respectively. We find that the optical response of the
K point electrons corresponds, over a wide range of energy and magnetic field,
to a graphene bilayer with an effective inter-layer coupling 2\gamma_1, twice
the value for a real graphene bilayer, which reflects the crystal ordering of
bulk graphite along the c-axis. The K point electrons thus behave as massive
Dirac fermions with a mass enhanced twice in comparison to a true graphene
bilayer.Comment: 4 pages, 2 figure
A mechanistic modelling and data assimilation approach to estimate the carbon/chlorophyll and carbon/nitrogen ratios in a coupled hydrodynamical-biological model
The principal objective of hydrodynamical-biological models is to provide estimates of the main carbon fluxes such as total and export oceanic production. These models are nitrogen based, that is to say that the variables are expressed in terms of their nitrogen content. Moreover models are calibrated using chlorophyll data sets. Therefore carbon to chlorophyll (C:Chl) and carbon to nitrogen (C:N) ratios have to be assumed. This paper addresses the problem of the representation of these ratios. In a 1D framework at the DYFAMED station (NW Mediterranean Sea) we propose a model which enables the estimation of the basic biogeochemical fluxes and in which the spatio-temporal variability of the C:Chl and C:N ratios is fully represented in a mechanical way. This is achieved through the introduction of new state variables coming from the embedding of a phytoplankton growth model in a more classical Redfieldian NNPZD-DOM model (in which the C:N ratio is assumed to be a constant). Following this modelling step, the parameters of the model are estimated using the adjoint data assimilation method which enables the assimilation of chlorophyll and nitrate data sets collected at DYFAMED in 1997.Comparing the predictions of the new Mechanistic model with those of the classical Redfieldian NNPZD-DOM model which was calibrated with the same data sets, we find that both models reproduce the reference data in a comparable manner. Both fluxes and stocks can be equally well predicted by either model. However if the models are coinciding on an average basis, they are diverging from a variability prediction point of view. In the Mechanistic model biology adapts much faster to its environment giving rise to higher short term variations. Moreover the seasonal variability in total production differs from the Redfieldian NNPZD-DOM model to the Mechanistic model. In summer the Mechanistic model predicts higher production values in carbon unit than the Redfieldian NNPZD-DOM model. In winter the contrary holds
Autocalibration with the Minimum Number of Cameras with Known Pixel Shape
In 3D reconstruction, the recovery of the calibration parameters of the
cameras is paramount since it provides metric information about the observed
scene, e.g., measures of angles and ratios of distances. Autocalibration
enables the estimation of the camera parameters without using a calibration
device, but by enforcing simple constraints on the camera parameters. In the
absence of information about the internal camera parameters such as the focal
length and the principal point, the knowledge of the camera pixel shape is
usually the only available constraint. Given a projective reconstruction of a
rigid scene, we address the problem of the autocalibration of a minimal set of
cameras with known pixel shape and otherwise arbitrarily varying intrinsic and
extrinsic parameters. We propose an algorithm that only requires 5 cameras (the
theoretical minimum), thus halving the number of cameras required by previous
algorithms based on the same constraint. To this purpose, we introduce as our
basic geometric tool the six-line conic variety (SLCV), consisting in the set
of planes intersecting six given lines of 3D space in points of a conic. We
show that the set of solutions of the Euclidean upgrading problem for three
cameras with known pixel shape can be parameterized in a computationally
efficient way. This parameterization is then used to solve autocalibration from
five or more cameras, reducing the three-dimensional search space to a
two-dimensional one. We provide experiments with real images showing the good
performance of the technique.Comment: 19 pages, 14 figures, 7 tables, J. Math. Imaging Vi
Integer Quantum Hall Effect in Trilayer Graphene
The Integer Quantum Hall Effect (IQHE) is a distinctive phase of
two-dimensional electronic systems subjected to a perpendicular magnetic field.
Thus far, the IQHE has been observed in semiconductor heterostructures and in
mono- and bi-layer graphene. Here we report on the IQHE in a new system:
trilayer graphene. Experimental data are compared with self-consistent Hartree
calculations of the Landau levels for the gated trilayer. The plateau structure
in the Hall resistivity determines the stacking order (ABA versus ABC). We find
that the IQHE in ABC trilayer graphene is similar to that in the monolayer,
except for the absence of a plateau at filling factor v=2. At very low filling
factor, the Hall resistance vanishes due to the presence of mixed electron and
hole carriers induced by disorder.Comment: 5 pages, 4 figure
Quasi-classical cyclotron resonance of Dirac fermions in highly doped graphene
Cyclotron resonance in highly doped graphene has been explored using infrared
magnetotransmission. Contrary to previous work, which only focused on the
magneto-optical properties of graphene in the quantum regime, here we study the
quasi-classical response of this system. We show that it has a character of
classical cyclotron resonance, with an energy which is linear in the applied
magnetic field and with an effective cyclotron mass defined by the position of
the Fermi level m = E_F/v_F^2.Comment: 6 pages, 4 figure
Real time plasma equilibrium reconstruction in a Tokamak
The problem of equilibrium of a plasma in a Tokamak is a free boundary
problemdescribed by the Grad-Shafranov equation in axisymmetric configurations.
The right hand side of this equation is a non linear source, which represents
the toroidal component of the plasma current density. This paper deals with the
real time identification of this non linear source from experimental
measurements. The proposed method is based on a fixed point algorithm, a finite
element resolution, a reduced basis method and a least-square optimization
formulation
How perfect can graphene be?
Fabrication of graphene structures has triggered vast research efforts
focused on the properties of two-dimensional systems with massless Dirac
fermions. Nevertheless, further progress in exploring this quantum
electrodynamics system in solid-state laboratories seems to be limited by
insufficient electronic quality of manmade structures and the crucial question
arises whether existing technologies have reached their limits or major
advances are in principle possible. Here we show that graphene in a
significantly purer state can be found in nature on the surface of bulk
graphite, in form of flakes decoupled from the substrate material. Probing such
flakes with Landau level spectroscopy in the THz range at very low magnetic
fields, we demonstrate a superior electronic quality of these ultra low density
layers (n~3x10^9 cm^-2) expressed by the carrier mobility in excess of 10^7
cm^2/(V.s). This finding represents an important challenge for further
improvements of current graphene technologies.Comment: 5 pages, 4 figures, to appear in PR
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