1,347 research outputs found
Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations
A self-regulating magnetic flux pumping mechanism in tokamaks that maintains
the core safety factor at , thus preventing sawteeth, is analyzed
in nonlinear 3D magnetohydrodynamic simulations using the M3D-C code. In
these simulations, the most important mechanism responsible for the flux
pumping is that a saturated quasi-interchange instability generates
an effective negative loop voltage in the plasma center via a dynamo effect. It
is shown that sawtoothing is prevented in the simulations if is
sufficiently high to provide the necessary drive for the
instability that generates the dynamo loop voltage. The necessary amount of
dynamo loop voltage is determined by the tendency of the current density
profile to centrally peak which, in our simulations, is controlled by the
peakedness of the applied heat source profile.Comment: submitted to Physics of Plasmas (23 pages, 15 Figures
Non-existence of normal tokamak equilibria with negative central current
Recent tokamak experiments employing off-axis, non-inductive current drive
have found that a large central current hole can be produced. The current
density is measured to be approximately zero in this region, though in
principle there was sufficient current drive power for the central current
density to have gone significantly negative. Recent papers have used a large
aspect-ratio expansion to show that normal MHD equilibria (with axisymmetric
nested flux surfaces, non-singular fields, and monotonic peaked pressure
profiles) can not exist with negative central current. We extend that proof
here to arbitrary aspect ratio, using a variant of the virial theorem to derive
a relatively simple integral constraint on the equilibrium. However, this
constraint does not, by itself, exclude equilibria with non-nested flux
surfaces, or equilibria with singular fields and/or hollow pressure profiles
that may be spontaneously generated.Comment: 5 pages, 3 figures. Submitted to Physics of Plasmas, Feb. 14, 2003.
Revised Feb. 24, 2003. Vers. 2: revised May 29 to clarify points raised by
referee, add references to recent work. July 18, accepted for publicatio
A new explanation of the sawtooth phenomena in tokamaks
The ubiquitous sawtooth phenomena in tokamaks are so named because the central temperature rises slowly and falls rapidly, similar to the blades of a saw. First discovered in 1974, it has so far eluded a theoretical explanation that is widely accepted and consistent with experimental observations. We propose here a new theory for the sawtooth phenomena in auxiliary heated tokamaks, which is motivated by our recent understanding of "magnetic flux pumping." In this theory, the role of the (m, n) = (1, 1) mode is to generate a dynamo voltage, which keeps the central safety factor, q(0), just above 1.0 with low central magnetic shear. When central heating is present, the temperature on axis will increase until at some point, and the configuration abruptly becomes unstable to ideal MHD interchange modes with equal poloidal and toroidal mode numbers, m = n > 1. It is these higher order modes and the localized magnetic stochasticity they produce that cause the sudden crash of the temperature profile, not magnetic reconnection. Long time 3D MHD simulations demonstrate these phenomena, which appear to be consistent with many experimental observations.</p
Supersonic regime of the Hall-magnetohydrodynamics resistive tearing instability
An earlier analysis of the Hall-magnetohydrodynamics (MHD) tearing instability [E. Ahedo and J. J. Ramos, Plasma Phys. Controlled Fusion 51, 055018 (2009)] is extended to cover the regime where the growth rate becomes comparable or exceeds the sound frequency. Like in the previous subsonic work, a resistive, two-fluid Hall-MHD model with massless electrons and zero-Larmor-radius ions is adopted and a linear stability analysis about a force-free equilibrium in slab geometry is carried out. A salient feature of this supersonic regime is that the mode eigenfunctions become intrinsically complex, but the growth rate remains purely real. Even more interestingly, the dispersion relation remains of the same form as in the subsonic regime for any value of the instability Mach number, provided only that the ion skin depth is sufficiently small for the mode ion inertial layer width to be smaller than the macroscopic lengths, a generous bound that scales like a positive power of the Lundquist numbe
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Hall MHD Modeling of Two-dimensional Reconnection: Application to MRX Experiment
Two-dimensional resistive Hall magnetohydrodynamics (MHD) code is used to investigate the dynamical evolution of driven reconnection in the Magnetic Reconnection Experiment (MRX). The initial conditions and dimensionless parameters of the simulation are set to be similar to the experimental values. We successfully reproduce many features of the time evolution of magnetic configurations for both co- and counter-helicity reconnection in MRX. The Hall effect is shown to be important during the early dynamic X-phase of MRX reconnection, while effectively negligible during the late ''steady-state'' Y-phase, when plasma heating takes place. Based on simple symmetry considerations, an experiment to directly measure the Hall effect in MRX configuration is proposed and numerical evidence for the expected outcome is given
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
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TSC simulation of feedback stabilization of axisymmetric modes in tokamaks using driven halo currents
The Tokamak Simulation Code (TSC) has been used to model a new method of feedback stabilization of the axisymmetric instability in tokamaks using driven halo (or scrapeoff layer) currents. The method appears to be feasible for a wide range of plasma edge parameters. It may offer significant advantages over the more conventional method of controlling this instability when applied in a reactor environment
The acceleration and storage of radioactive ions for a neutrino factory
The term beta-beam has been coined for the production of a pure beam of
electron neutrinos or their antiparticles through the decay of radioactive ions
circulating in a storage ring. This concept requires radioactive ions to be
accelerated to a Lorentz gamma of 150 for 6He and 60 for 18Ne. The neutrino
source itself consists of a storage ring for this energy range, with long
straight sections in line with the experiment(s). Such a decay ring does not
exist at CERN today, nor does a high-intensity proton source for the production
of the radioactive ions. Nevertheless, the existing CERN accelerator
infrastructure could be used as this would still represent an important saving
for a beta-beam facility. This paper outlines the first study, while some of
the more speculative ideas will need further investigations.Comment: Accepted for publication in proceedings of Nufact02, London, 200
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