30 research outputs found
A self-organized criticality model for ion temperature gradient (ITG) mode driven turbulence in confined plasma
A new Self-Organized Criticality (SOC) model is introduced in the form of a
Cellular Automaton (CA) for ion temperature gradient (ITG) mode driven
turbulence in fusion plasmas. Main characteristics of the model are that it is
constructed in terms of the actual physical variable, the ion temperature, and
that the temporal evolution of the CA, which necessarily is in the form of
rules, mimics actual physical processes as they are considered to be active in
the system, i.e. a heating process and a local diffusive process that sets on
if a threshold in the normalized ion temperature gradient R/L_T is exceeded.
The model reaches the SOC state and yields ion temperature profiles of
exponential shape, which exhibit very high stiffness, in that they basically
are independent of the loading pattern applied. This implies that there is
anomalous heat transport present in the system, despite the fact that diffusion
at the local level is imposed to be of a normal kind. The distributions of the
heat fluxes in the system and of the heat out-fluxes are of power-law shape.
The basic properties of the model are in good qualitative agreement with
experimental results.Comment: In press at Physics of Plasmas, July 2010; 11 pages, 5 figure
Formation and Evolution of Coherent Structures in 3D Strongly Turbulent Magnetized Plasmas
We review the current literature on the formation of Coherent Structures
(CoSs) in strongly turbulent 3D magnetized plasmas. CoSs (Current Sheets (CS),
magnetic filaments, large amplitude magnetic disturbances, vortices, and
shocklets) appear intermittently inside a turbulent plasma and are collectively
the locus of magnetic energy transfer (dissipation) into particle kinetic
energy, leading to heating and/or acceleration of the latter. CoSs and
especially CSs are also evolving and fragmenting, becoming locally the source
of new clusters of CoSs. Strong turbulence can be generated by the nonlinear
coupling of large amplitude unstable plasma modes, by the explosive
reorganization of large scale magnetic fields, or by the fragmentation of CoSs.
A small fraction of CSs inside a strongly turbulent plasma will end up
reconnecting. Magnetic Reconnection (MR) is one of the potential forms of
energy dissipation of a turbulent plasma. Analysing the evolution of CSs and MR
in isolation from the surrounding CoSs and plasma flows may be convenient for
2D numerical studies, but it is far from a realistic modeling of 3D
astrophysical, space and laboratory environments, where strong turbulence can
be exited, as e.g. in the solar wind, the solar atmosphere, solar flares and
Coronal Mass Ejections (CMEs), large scale space and astrophysical shocks, the
magnetosheath, the magnetotail, astrophysical jets, Edge Localized Modes (ELMs)
in confined laboratory plasmas (TOKAMAKS), etc.Comment: 27 pages, 31 figures; review; accepted for publication in Physics of
Plasmas 202
Particle Acceleration in an Evolving Network of Unstable Current Sheets
We study the acceleration of electrons and protons interacting with
localized, multiple, small-scale dissipation regions inside an evolving,
turbulent active region. The dissipation regions are Unstable Current Sheets
(UCS), and in their ensemble they form a complex, fractal, evolving network of
acceleration centers. Acceleration and energy dissipation are thus assumed to
be fragmented. A large-scale magnetic topology provides the connectivity
between the UCS and determines in this way the degree of possible multiple
acceleration. The particles travel along the magnetic field freely without
loosing or gaining energy, till they reach a UCS. In a UCS, a variety of
acceleration mechanisms are active, with the end-result that the particles
depart with a new momentum. The stochastic acceleration process is represented
in the form of Continuous Time Random Walk (CTRW), which allows to estimate the
evolution of the energy distribution of the particles. It is found that under
certain conditions electrons are heated and accelerated to energies above 1 MeV
in much less than a second. Hard X-ray (HXR) and microwave spectra are
calculated from the electrons' energy distributions, and they are found to be
compatible with the observations. Ions (protons) are also heated and
accelerated, reaching energies up to 10 MeV almost simultaneously with the
electrons. The diffusion of the particles inside the active region is extremely
fast (anomalous super-diffusion). Although our approach does not provide
insight into the details of the specific acceleration mechanisms involved, its
benefits are that it relates acceleration to the energy release, and it well
describes the stochastic nature of the acceleration process.Comment: 37 pages, 10 figures, one of them in color; in press at ApJ (2004
Particle heating and acceleration by reconnecting and nonreconnecting current sheets
In this article, we study the physics of charged particle energization inside a strongly turbulent plasma, where current sheets naturally appear in evolving large-scale magnetic topologies, but they are split into two populations of fractally distributed reconnecting and nonreconnecting current sheets (CS). In particular, we implemented a Monte Carlo simulation to analyze the effects of the fractality and we study how the synergy of energization at reconnecting CSs and at nonreconnecting CSs affects the heating, the power-law high energy tail, the escape time, and the acceleration time of electrons and ions. The reconnecting current sheets systematically accelerate particles and play a key role in the formation of the power-law tail in energy distributions. On the other hand, the stochastic energization of particles through their interaction with nonreconnecting CSs can account for the heating of the solar corona and the impulsive heating during solar flares. The combination of the two acceleration mechanisms (stochastic and systematic), commonly present in many explosive events of various sizes, influences the steady-state energy distribution, as well as the transport properties of the particles in position- and energy-space. Our results also suggest that the heating and acceleration characteristics of ions and electrons are similar, the only difference being the time scales required to reach a steady state