38 research outputs found
Nonlinear evolution of the m=1 internal kink mode in the presence of magnetohydrodynamic turbulence
The nonlinear evolution of the m=1 internal kink mode is studied numerically
in a setting where the tokamak core plasma is surrounded by a turbulent region
with low magnetic shear. As a starting point we choose configurations with
three nearby q=1 surfaces where triple tearing modes (TTMs) with high poloidal
mode numbers m are unstable. While the amplitudes are still small, the fast
growing high-m TTMs enhance the growth of the m=1 instability. This is
interpreted as a fast sawtooth trigger mechanism. The TTMs lead to a partial
collapse, leaving behind a turbulent belt with q ~= 1 around the unreconnected
core plasma. Although, full reconnection can occur if the core displacement
grows large enough, it is shown that the turbulence may actively prevent
further reconnection. This is qualitatively similar to experimentally observed
partial sawtooth crashes with post-cursor oscillations due to a saturated
internal kink.Comment: 14 pages, 13 figure
The role of coherent vorticity in turbulent transport in resistive drift-wave turbulence
The coherent vortex extraction method, a wavelet technique for extracting
coherent vortices out of turbulent flows, is applied to simulations of
resistive drift-wave turbulence in magnetized plasma (Hasegawa-Wakatani
system). The aim is to retain only the essential degrees of freedom,
responsible for the transport. It is shown that the radial density flux is
carried by these coherent modes. In the quasi-hydrodynamic regime, coherent
vortices exhibit depletion of the polarization-drift nonlinearity and vorticity
strongly dominates strain, in contrast to the quasiadiabatic regime
Fast growing double tearing modes in a tokamak plasma
Configurations with nearby multiple resonant surfaces have broad spectra of
linearly unstable coupled tearing modes with dominant high poloidal mode
numbers m. This was recently shown for the case of multiple q = 1 resonances
[Bierwage et al., Phys. Rev. Lett. 94 (6), 65001 (2005)]. In the present work,
similar behavior is found for double tearing modes (DTM) on resonant surfaces
with q >= 1. A detailed analysis of linear instability characteristics of DTMs
with various mode numbers m is performed using numerical simulations. The mode
structures and dispersion relations for linearly unstable modes are calculated.
Comparisons between low- and higher-m modes are carried out, and the roles of
the inter-resonance distance and of the magnetic Reynolds number S_Hp are
investigated. High-m modes are found to be destabilized when the distance
between the resonant surfaces is small. They dominate over low-m modes in a
wide range of S_Hp, including regimes relevant for tokamak operation. These
results may be readily applied to configurations with more than two resonant
surfaces.Comment: 11 pages, 15 figure
Dynamics of resistive double tearing modes with broad linear spectra
The nonlinear evolution of resistive double tearing modes (DTMs) with safety
factor values q=1 and q=3 is studied in a reduced cylindrical model of a
tokamak plasma. We focus on cases where the resonant surfaces are a small
distance apart. Recent numerical studies have shown that in such configurations
high-m modes are strongly unstable. In this paper, it is first demonstrated
that linear DTM theory predicts the dominance of high-m DTMs. A semi-empirical
formula for estimating the poloidal mode number of the fastest growing mode,
m_peak, is obtained from the existing linear theory. Second, using nonlinear
simulations, it is shown that the presence of fast growing high-m modes leads
to a rapid turbulent collapse in an annular region, whereby small magnetic
island structures form. Furthermore, consideration is given to the evolution of
low-m modes, in particular the global m=1 internal kink, which can undergo
nonlinear driving through coupling to fast growing linear high-m DTMs. Factors
influencing the details of the dynamics are discussed. These results may be
relevant for the understanding of the magnetohydrodynamic (MHD) activity near
the minimum of q and may thus be of interest to studies concerned with
stability and confinement in advanced tokamaks.Comment: 11 pages, 10 figure
Nonlinear Dynamics of Magnetic Islands Imbedded in Small-Scale Turbulence
International audienceThe nonlinear dynamics of magnetic tearing islands imbedded in a pressure gradient driven turbulence is investigated numerically in a reduced magnetohydrodynamic model. The study reveals regimes where the linear and nonlinear phases of the tearing instability are controlled by the properties of the pressure gradient. In these regimes, the interplay between the pressure and the magnetic flux determines the dynamics of the saturated state. A secondary instability can occur and strongly modify the magnetic island dynamics by triggering a poloidal rotation. It is shown that the complex nonlinear interaction between the islands and turbulence is nonlocal and involves small scales
Anomalous transport in Charney-Hasegawa-Mima flows
Transport properties of particles evolving in a system governed by the
Charney-Hasegawa-Mima equation are investigated. Transport is found to be
anomalous with a non linear evolution of the second moments with time. The
origin of this anomaly is traced back to the presence of chaotic jets within
the flow. All characteristic transport exponents have a similar value around
, which is also the one found for simple point vortex flows in the
literature, indicating some kind of universality. Moreover the law
linking the trapping time exponent within jets to the transport
exponent is confirmed and an accumulation towards zero of the spectrum of
finite time Lyapunov exponent is observed. The localization of a jet is
performed, and its structure is analyzed. It is clearly shown that despite a
regular coarse grained picture of the jet, motion within the jet appears as
chaotic but chaos is bounded on successive small scales.Comment: revised versio
Electron Temperature Gradient Mode Transport
Anomalous electron thermal losses plays a central role in the history of the controlled fusion program being the first and most persistent form of anomalous transport across all toroidal magnetic confinement devices. In the past decade the fusion program has made analysis and simulations of electron transport a high priority with the result of a clearer understanding of the phenomenon, yet still incomplete. Electron thermal transport driven by the electron temperature gradient is examined in detail from theory, simulation and power balance studies in tokamaks with strong auxiliary heating.Physic