34,937 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
A discontinuous Galerkin method for the Vlasov-Poisson system
A discontinuous Galerkin method for approximating the Vlasov-Poisson system
of equations describing the time evolution of a collisionless plasma is
proposed. The method is mass conservative and, in the case that piecewise
constant functions are used as a basis, the method preserves the positivity of
the electron distribution function and weakly enforces continuity of the
electric field through mesh interfaces and boundary conditions. The performance
of the method is investigated by computing several examples and error estimates
associated system's approximation are stated. In particular, computed results
are benchmarked against established theoretical results for linear advection
and the phenomenon of linear Landau damping for both the Maxwell and Lorentz
distributions. Moreover, two nonlinear problems are considered: nonlinear
Landau damping and a version of the two-stream instability are computed. For
the latter, fine scale details of the resulting long-time BGK-like state are
presented. Conservation laws are examined and various comparisons to theory are
made. The results obtained demonstrate that the discontinuous Galerkin method
is a viable option for integrating the Vlasov-Poisson system.Comment: To appear in Journal for Computational Physics, 2011. 63 pages, 86
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A unified gas kinetic scheme for transport and collision effects in plasma
In this study, the Vlasov-Poisson equation with or without collision term for
plasma is solved by the unified gas kinetic scheme (UGKS). The Vlasov equation
is a differential equation describing time evolution of the distribution
function of plasma consisting of charged particles with long-range interaction.
The distribution function is discretized in discrete particle velocity space.
After the Vlasov equation is integrated in finite volumes of physical space,
the numerical flux across a cell interface and source term for particle
acceleration are computed to update the distribution function at next time
step. The flux is decided by Riemann problem and variation of distribution
function in discrete particle velocity space is evaluated with central
difference method. A electron-ion collision model is introduced in the Vlasov
equation. This finite volume method for the UGKS couples the free transport and
long-range interaction between particles. The electric field induced by charged
particles is controlled by the Poisson's equation. In this paper, the Poisson's
equation is solved using the Green's function for two dimensional plasma system
subjected to the symmetry or periodic boundary conditions. Two numerical tests
of the linear Landau damping and the Gaussian beam are carried out to validate
the proposed method. The linear electron plasma wave damping is simulated based
on electron-ion collision operator. Compared with previous methods, it is shown
that the current method is able to obtain accurate results of the
Vlasov-Poisson equation with a time step much larger than the particle
collision time. Highly non-equilibrium and rarefied plasma flows, such as
electron flows driven by electromagnetic field, can be simulated easily.Comment: 33 pages, 13 figure
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