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The dependence of the 2D spatial structure of ion-scale turbulence on equilibrium ExB flow shear in MAST
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Gyrokinetic turbulence and transport in the Mega Ampere Spherical Tokamak
Full text linkIn this thesis we consider energy transport due to turbulence in the Mega Ampere Spherical Tokamak (MAST), using a multi-scale framework based on local δf gyrokinetics. Transport is modelled globally by means of local flux-tube simulations at each magnetic flux surface. In view of the evidence indicating that turbulent ion transport can be substantially suppressed by rotation shear, our flux tubes span only electron-gyroradius scales. This both simplifies our physics model and reduces the computational resource requirements to a manageable level. Our electrostatic simulations of a MAST H-mode discharge exhibit turbulent electron heat transport comparable with experiment in the outer core region, and are broadly consistent with the paradigm of an electron temperature profile governed by pedestal height and critical electron temperature gradient. Neoclassical transport dominates the ions, and is also comparable with experiment. The two species are decoupled in the sense that the collisional equilibration between ions and electrons is small compared to the sources across most of the plasma. Focusing on a single flux surface in this region, still using both kinetic electrons and kinetic ions, we find that at early simulation times the heat flux "quasi-saturates" with the turbulence dominated by streamer-like radially elongated structures. However, the zonal fluctuation component continues to grow slowly until much later times, eventually leading to a new saturated state dominated by the zonal component. Simplifying further to an adiabatic ion model (which shows the same slow evolution behaviour), we find that in the final saturated state (which determines the macroscopic energy transport) the electron heat flux is approximately proportional to the collision rate. We outline an explanation of this effect based on zonalnonzonal interactions and a scaling of the zonal damping rate with electron-ion collisionality. Improved energy confinement with decreasing collisionality has previously been observed experimentally in STs, and is favourable towards the performance of future devices, which are expected to be hotter and thus less collisional
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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Instabilities and transport in magnetized plasmas
Full text linkIn a magnetized plasma, naturally occurring pressure anisotropies facilitate in- stabilities that are expected to modify the transport properties of the system. In this thesis we examine two such instabilities and, where appropriate, their effects on transport.
First we consider the collisional (fluid) magnetized magnetorotational instability (MRI) in the presence of the Braginskii viscosity. We conduct a global linear analysis of the instability in a galactic rotation profile for three magnetic field configurations: purely azimuthal, purely vertical and slightly pitched. Our analysis, numerical and asymptotic, shows that the first two represent singular configurations where the Braginskii viscosity’s primary role is dissipative and the maximum growth rate is proportional to the Reynolds number when this is small. For a weak pitched field, the Braginskii viscosity is destabilising and when its effects dominate over the Lorentz force, the growth rate of the MRI can be up to 2√2 times faster than the inviscid limit. If the field is strong, an over-stability develops and both the real and imaginary parts of the frequency increase with the coefficient of the viscosity.
Second, in the context of the ICM of galaxy clusters, we consider the pressure-anisotropy-driven firehose instability. The linear instability is fast (∼ ion cyclotron period) and small-scale (ion Larmor radius ρi) and so fluid theory is
inapplicable. We determine its nonlinear evolution in an ab initio kinetic calculation (for parallel gradients only). We use a particular physical asymptotic ordering to derive a closed nonlinear equation for the firehose turbulence, which we solve. We find secular (∝ t) growth of magnetic fluctuations and a k−∥3 spectrum,
starting at scales >~ ρi. When a parallel ion heat flux is present, the parallel firehose instability mutates into the new gyrothermal instability. Its nonlinear evolution also involves secular magnetic energy growth, but its spectrum is eventually dominated by modes with a maximal scale ∼ρilT/λmfp,(lT is the parallel temperature gradient scale). Throughout we discuss implications for modelling, transport and other areas of magnetized plasma physics
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Characteristics of plasma turbulence in the Mega Amp Spherical Tokamak
Full text linkTurbulence is a major factor limiting the achievement of better tokamak performance as it enhances the transport of particles, momentum and heat which hinders the foremost objective of tokamaks. Hence, understanding and possibly being able to control turbulence in tokamaks is of paramount importance, not to mention our intellectual curiosity of it. We take the first step by making measurements of turbulence using the 2D ( radial poloidal channels) beam emission spectroscopy (BES) system on the Mega Amp Spherical Tokamak (MAST). Measured raw data are statistically processed, generating spatio-temporal correlation functions to obtain the physical characteristics of the turbulence such as spatial and temporal correlation lengths as well as its motion. The reliability of statistical techniques employed in this work is examined by generating and utilizing synthetic 2D BES data. The apparent poloidal velocity of fluctuating density patterns is estimated using the cross-correlation time delay method. The experimental results indicate that the poloidal motion of fluctuating density patterns in the lab frame arises because the patterns are advected by the strong toroidal plasma flows while the patterns are aligned with the background magnetic fields which are not parallel to the flows. Furthermore, various time scales associated with the turbulence are calculated using statistically estimated spatial correlation lengths and correlation times of turbulence. We find that turbulence correlation time, the drift time associated with ion temperature or density gradients, the ion streaming time along the magnetic field line and the magnetic drift time are comparable and possibly scale together suggesting that the turbulence, determined by the local equilibrium, is critically balanced. Finally, we argue that we have produced a critical manifold in the experimentally obtained local equilibrium parameter space separating dominant turbulent transport from a non-turbulent or weakly turbulent state. It shows that the inverse ion-temperature-gradient scale length is correlated inversely with (safety factor/inverse aspect ratio) and positively with the plasma rotational shear. Practically, this means that we can attain the stiffer ion-temperature-gradient, thus hotter plasma core, without increasing the rotational shear
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