55 research outputs found
Magnetohydrodynamic simulations of noninductive helicity injection in the reversed-field pinch and tokamak
Numerical computation is used to investigate resistive magnetohydrodynamic (MHD) fluctuations in the reversed-field pinch (RFP) and in tokamak-like configurations driven solely by direct current (DC) helicity injection. A Lundquist number (S) scan of RFP turbulence without plasma pressure produces the weak scaling of S{sup -0.18} for the root-mean-square magnetic fluctuation level for 2.5x10{sup 3}{le}S{le}4x10{sup 4}. The temporal behavior of fluctuations and the reversal parameter becomes more regular as S is increased, acquiring a {open_quotes}sawtooth{close_quotes} shape at the largest value of S. Simulations with plasma pressure and anisotropic thermal conduction demonstrate energy transport resulting from parallel heat fluctuations. To investigate means of improving RFP energy confinement, three forms of current profile modification are tested. Radio frequency (RF) current drive is modeled with an auxiliary electron force, and linear stability calculations are used
3D simulations of vertical displacement events in tokamaks: A benchmark of M3D-C<sup>1</sup>, NIMROD, and JOREK editors-pick
3D simulations of vertical displacement events in tokamaks: A benchmark of M3D-C, NIMROD and JOREK
In recent years, the nonlinear 3D magnetohydrodynamic codes JOREK, M3D-C
and NIMROD developed the capability of modelling realistic 3D vertical
displacement events (VDEs) including resistive walls. In this paper, a
comprehensive 3D VDE benchmark is presented between these state of the art
codes. The simulated case is based on an experimental NSTX plasma but with a
simplified rectangular wall. In spite of pronounced differences between physics
models and numerical methods, the comparison shows very good agreement in the
relevant quantities used to characterize disruptions such as the 3D wall forces
and energy decay. This benchmark does not only bring confidence regarding the
use of the mentioned codes for disruption studies, but also shows differences
with respect to the used models (e.g. reduced versus full MHD models). The
simulations show important 3D features for a NSTX plasma such as the
self-consistent evolution of the halo current and the origin of the wall
forces. In contrast to other reduced MHD models based on an ordering in the
aspect ratio, the ansatz based JOREK reduced MHD model allows capturing the 3D
dynamics even in the spherical tokamak limit considered here
Magnetohydrodynamic simulations of direct current helicity injection for current drive in tokamaks
Axisymmetric simulations of vertical displacement events in tokamaks: A benchmark of M3D-C1, NIMROD and JOREK
A benchmark exercise for the modeling of vertical displacement events(VDEs) is presented and applied to the 3D nonlinear magneto-hydrodynamic codesM3D-C1, JOREK and NIMROD. The simulations are based on a vertically unstableNSTX equilibrium enclosed by an axisymmetric resistive wall with rectangular crosssection. A linear dependence of the linear VDE growth rates on the resistivity ofthe wall is recovered for sufficiently large wall conductivity and small temperatures inthe open field line region. The benchmark results show good agreement between theVDE growth rates obtained from linear NIMROD and M3D-C1simulations as wellas from the linear phase of axisymmetric nonlinear JOREK, NIMROD and M3D-C1simulations. Axisymmetric nonlinear simulations of a full VDE performed with thethree codes are compared and excellent agreement is found regarding plasma locationand plasma currents as well as eddy and halo currents in the wall.</p
Rotational Stabilization of Magnetically Collimated Jets
We investigate the launching and stability of extragalactic jets through
nonlinear magnetohydrodynamic (MHD) simulation and linear eigenmode analysis.
In the simulations of jet evolution, a small-scale equilibrium magnetic arcade
is twisted by a differentially rotating accretion disk. These simulations
produce a collimated outflow which is unstable to the current driven m=1 kink
mode for low rotational velocities of the accretion disk relative to the Alfven
speed of the coronal plasma. The growth rate of the kink mode in the jet is
shown to be inversely related to the rotation rate of the disk, and the jet is
stable for high rotation rates. Linear MHD calculations investigate the effect
of rigid rotation on the kink mode in a cylindrical plasma. These calculations
show that the Coriolis force distorts the m=1 kink eigenmode and stabilizes it
at rotation frequencies such that the rotation period is longer than a few
Alfven times.Comment: 17 pages, 21 figures, final published version, updated to include a
discussion of the effect of rotation on increasing magnetic pitch equilibri
Computational modeling of fully ionized magnetized plasmas using the fluid approximation
Magnetic Reconnection with Asymmetry in the Outflow Direction
Magnetic reconnection with asymmetry in the outflow direction occurs in the
Earth's magnetotail, coronal mass ejections, flux cancellation events,
astrophysical disks, spheromak merging experiments, and elsewhere in nature and
the laboratory. A control volume analysis is performed for the case of steady
antiparallel magnetic reconnection with asymmetric downstream pressure, which
is used to derive scaling relations for the outflow velocity from each side of
the current sheet and the reconnection rate. Simple relationships for outflow
velocity are presented for the incompressible case and the case of symmetric
downstream pressure but asymmetric downstream density. Asymmetry alone is not
found to greatly affect the reconnection rate. The flow stagnation point and
magnetic field null do not coincide in a steady state unless the pressure
gradient is negligible at the flow stagnation point.Comment: 12 pages, 8 figures. Submitted to JGR. Any comments will be
appreciate
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