Browsing by Subject "Plasma turbulence"
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Item Beam emission spectroscopy on the Alcator C-Mod Tokamak(2004) Sampsell, Matthew Brian; Bravenec, Ronald V.; Gentle, Kenneth W.Item Experimental characterization of drift-wave turbulence in the sheared, cylindrical slab(2009-05) Lee, Kevin Michael; Gentle, Kenneth W.Plasma turbulence on a uniform density gradient with unfavorable magnetic curvature is investigated extensively in the Helimak device. The turbulence is strong with density and electrostatic potential fluctuation levels in excess of 40%. Measurements of the dispersion relation, k[subscript z], and k[subscript parallel lines] identify the the fluctuations with drift-waves, which propagate in the poloidal direction at the diamagnetic drift velocity and have a small, but nite parallel wavenumber. A non-zero phase shift between the density and potential fluctuations gives rise to turbulent cross-eld particle transport, which is measured using spectral techniques. In addition, the electrostatic drift-wave fluctuations have a small magnetic component that is driven by the turbulent parallel current [scientific symbols]. An examination of nonlinear processes associated with the plasma turbulence uncovers high levels of intermittency near the plasma edge and long-time persistence of the density fluctuations on the order of the parallel confinement time. An analysis of the bispectrum conrms the existence local and nonlocal three-wave interactions between unstable drift-waves although the turbulent saturation of the density fluctuation spectrum is likely due nonlinear processes acting in the time domain.Item Non-Markovian velocity diffusion in plasma turbulence(Texas Tech University, 1993-05) Xia, HuajuanThe behavior of charged particles placed in random electric fields in a fully ionized plasma can be observed through test particle computer simulations. In a weak field, the observed diffusion rate in velocity space is known to be in good agreement with the conventional quasilinear theory. However, as the field amplitude increases, the velocity diffusion is found to reveal its time-dependent nature, or non-Markovian property. Such a non-Markovian process is studied in detail both analytically and numerically.Item Plasma turbulence in the equatorial electrojet observations, theories, models, and simulations(2015-12) Hassan, Ehab Mohamed Ali Hussein; Morrison, Philip J.; Horton, Wendell; Fitzpatrick, Richard; Bengtson, Roger; Humphreys, ToddThe plasma turbulence in the equatorial electrojet due to the presence of two different plasma instability mechanisms has been observed and studied for more than seven decades. The sharp density-gradient and large conductivity give rise to gradient-drift and Farley-Buneman instabilities, respectively, of different scale-lengths. A new 2-D fluid model is derived by modifying the standard two-stream fluid model with the ion viscosity tensor and electron polarization drift, and is capable of describing both instabilities in a unified system. Numerical solution of the model in the linear regime demonstrates the capacity of the model to capture the salient characteristics of the two instabilities. Nonlinear simulations of the unified model of the equatorial electrojet instabilities reproduce many of the features that are found in radar observations and sounding rocket measurements under multiple solar and ionospheric conditions. The linear and nonlinear numerical results of the 2-D unified fluid model are found to be comparable to the fully kinetic and hybrid models which have high computational cost and small coverage area of the ionosphere. This gives the unified fluid model a superiority over those models. The distribution of the energy content in the system is studied and the rate of change of the energy content in the evolving fields obeys the law of energy conservation. The dynamics of the ions were found to have the largest portion of energy in their kinetic and internal thermal energy components. The redistribution of energy is characterized by a forward cascade generating small-scale structures. The bracket of the system dynamics in the nonlinear partial differential equation was proved to be a non-canonical Hamiltonian system as that bracket satisfies the Jacobi identity. The penetration of the variations in the interplanetary magnetic and electric fields in the solar winds to the dip equator is observed as a perfect match with the variations in the horizontal components of the geomagnetic and electric fields at the magnetic equator. Three years of concurrent measurements of the solar wind parameters at Advanced Composition Explorer (ACE) and Interplanetary Monitoring Platform (IMP) space missions used to establish a Kernel Density Estimation (KDE) functions for these parameters at the IMP-8 location. The KDE functions can be used to generate an ensemble of the solar wind parameters which has many applications in space weather forecasting and data-driven simulations. Also, categorized KDE functions ware established for the solar wind categories that have different origin from the Sun.Item Real space diffusion in drift wave turbulence(Texas Tech University, 1987-12) Robinson, Edward KyleNumerical experiments have been performed to examine the particle transport in real space in drift wave turbulence in the presence of a sheared magnetic field, B = Bo(0, x/Lg, 1), and random electrostatic waves, 0(x, t) = SOf^.n expIK^mV + kpZ - cOmnt)]. where the summation is over the mode numbers m and n, and O^n^n 's the random electric potential of the (m, n) mode. Two cases are examined: one with single n-mode, where n = ng, and the other for an infinite number of n-modes. A diffusion coefficient is determined for both cases. For infinite n-modes, diffusion Is not greatly dependent on the value of m, whereas for single n, diffusion greatly depends on the choice of m and n.