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Diagnostics for magnetically confined high-temperature plasmas
During the last 20 years, magnetically confined laboratory plasmas of steadily increasing temperatures and densities have been obtained, most notably in tokamak configurations, and now approach the conditions necessary to sustain a fusion reaction. Even more important to the goal of understanding the physics of such systems, remarkable advances in plasma diagnostics, the techniques for determining the properties of such plasmas, have accompanied these developments. More parameters can be determined with greater accuracy and finer spatial and temporal resolution. The magnetic configuration, the primary local thermodynamic quantities (density, temperature, and drift velocity), and other necessary quantities can now be measured with sufficient accuracy to determine particle and energy fluxes within the plasma and to characterize the basic transport processes. These plasmas are far from thermodynamic equilibrium. This deviation manifests itself in a variety of instabilities on several spatial and temporal scales, many of which are aptly described as turbulence. Many aspects of the turbulence can also be characterized. This article reviews the current state of diagnostics from an epistemoiogical perspective: the capabilities and limitations for measuring each important physical quantity are presented.Physic
Topics in Magnetohydrodynamics
To understand plasma physics intuitively one need to master the MHD behaviors. As sciences advance, gap between published textbooks and cutting-edge researches gradually develops. Connection from textbook knowledge to up-to-dated research results can often be tough. Review articles can help. This book contains eight topical review papers on MHD. For magnetically confined fusion one can find toroidal MHD theory for tokamaks, magnetic relaxation process in spheromaks, and the formation and stability of field-reversed configuration. In space plasma physics one can get solar spicules and X-ray jets physics, as well as general sub-fluid theory. For numerical methods one can find the implicit numerical methods for resistive MHD and the boundary control formalism. For low temperature plasma physics one can read theory for Newtonian and non-Newtonian fluids etc
Relaxed plasma equilibria and entropy-related plasma self-organization principles
The concept of plasma relaxation as a constrained energy minimization is
reviewed. Recent work by the authors on generalizing this approach to partially
relaxed three-dimensional plasma systems in a way consistent with chaos theory
is discussed, with a view to clarifying the thermodynamic aspects of the
variational approach used. Other entropy-related approaches to finding
long-time steady states of turbulent or chaotic plasma systems are also briefly
reviewed.Comment: Contribution to the Proceedings of the AMSI/MASCOS Concepts of
Entropy and their Applications Workshop, Melbourne, Australia,
November-December 2007, to be published in Entropy e-journal
http://www.mdpi.org/entropy
Reconnection Studies Under Different Types of Turbulence Driving
We study a model of fast magnetic reconnection in the presence of weak
turbulence proposed by Lazarian and Vishniac (1999) using three-dimensional
direct numerical simulations. The model has been already successfully tested in
Kowal et al. (2009) confirming the dependencies of the reconnection speed
on the turbulence injection power and the injection scale
expressed by a constraint
and no observed dependency on Ohmic resistivity. In Kowal et al. (2009), in
order to drive turbulence, we injected velocity fluctuations in Fourier space
with frequencies concentrated around , as described in
Alvelius (1999). In this paper we extend our previous studies by comparing fast
magnetic reconnection under different mechanisms of turbulence injection by
introducing a new way of turbulence driving. The new method injects velocity or
magnetic eddies with a specified amplitude and scale in random locations
directly in real space. We provide exact relations between the eddy parameters
and turbulent power and injection scale. We performed simulations with new
forcing in order to study turbulent power and injection scale dependencies. The
results show no discrepancy between models with two different methods of
turbulence driving exposing the same scalings in both cases. This is in
agreement with the Lazarian and Vishniac (1999) predictions. In addition, we
performed a series of models with varying viscosity . Although Lazarian
and Vishniac (1999) do not provide any prediction for this dependence, we
report a weak relation between the reconnection speed with viscosity,
.Comment: 19 pages, 9 figures. arXiv admin note: text overlap with
arXiv:0903.205
Magnetohydrodynamic Waves and Instabilities in Rotating Tokamak Plasmas
One of the most promising ways to achieve controlled nuclear fusion for the commercial production of energy is the tokamak design. In such a device, a hot plasma is confined in a toroidal geometry using magnetic fields. The present generation of tokamaks shows significant plasma rotation, primarily in the toroidal direction. This plasma flow has an important impact on stability and confinement, aspects of which can be described quite well by the theory of magnetohydrodynamics (MHD).
This work contains a comprehensive theoretical analysis, supported by numerical simulations, of the MHD equilibrium, waves, and instabilities of rotating tokamak plasmas. A new general description of the thermodynamic state of the equilibrium is presented. Next, a stability criterion is derived that generalizes various previous results by including toroidal rotation. This criterion shows that a radially decreasing rotation profile can be stabilizing. The previously unknown origin of this stabilization is shown to be the Coriolis effect, with a mediating role for the pressure. Various factors that affect stability also influence stable waves and eigenmodes of the plasma. New modes that are created by rotation are found, and the effect of rotation on a type of experimentally well-known modes is described.
Finally, the step to nonlinear magnetohydrodynamics is made by extending an existing reduced MHD code to the full viscoresistive MHD equations. This allows a study of the nonlinear evolution of the equilibria, waves, and instabilities described in this thesis
Towards a solution of the closure problem for convective atmospheric boundary-layer turbulence
We consider the closure problem for turbulence in the dry convective atmospheric boundary
layer (CBL). Transport in the CBL is carried by small scale eddies near the surface and large
plumes in the well mixed middle part up to the inversion that separates the CBL from the
stably stratified air above. An analytically tractable model based on a multivariate Delta-PDF
approach is developed. It is an extension of the model of Gryanik and Hartmann [1] (GH02)
that additionally includes a term for background turbulence. Thus an exact solution is derived
and all higher order moments (HOMs) are explained by second order moments, correlation
coefficients and the skewness. The solution provides a proof of the extended universality
hypothesis of GH02 which is the refinement of the Millionshchikov hypothesis (quasi-
normality of FOM). This refined hypothesis states that CBL turbulence can be considered as
result of a linear interpolation between the Gaussian and the very skewed turbulence regimes.
Although the extended universality hypothesis was confirmed by results of field
measurements, LES and DNS simulations (see e.g. [2-4]), several questions remained
unexplained. These are now answered by the new model including the reasons of the
universality of the functional form of the HOMs, the significant scatter of the values of the
coefficients and the source of the magic of the linear interpolation. Finally, the closures
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predicted by the model are tested against measurements and LES data. Some of the other
issues of CBL turbulence, e.g. familiar kurtosis-skewness relationships and relation of area
coverage parameters of plumes (so called filling factors) with HOM will be discussed also
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