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Miklos Porkolab. Edlund, Eric Matthias Massachusetts Institute of Technology. Dept. of Physics. Massachusetts Institute of Technology. Department of Physics 2011-05-23T18:00:32Z 2011-05-23T18:00:32Z 2009 2009 http://hdl.handle.net/1721.1/63004 720730164 Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2009. Cataloged from PDF version of thesis. Includes bibliographical references (p. 199-209). Shear Alfvén waves are a fundamental mode of magnetized plasma oscillation and may exist in tokamak plasmas as eigenmodes with a global structure and discrete frequencies. The inhomogeneity of the plasma profiles in conjunction with tokamak geometry tends to focus the Alfvén waves in regions of near uniformity defined by local extrema in the Alfvén continuum, a quantity which describes the local Alfvén resonance frequency for particular mode numbers. Modes excited in the vicinity of these near uniform regions may be weakly damped and excited by energetic ions. The reversed shear Alfvén eigenmode (RSAE), localized deep in the plasma core, is typically associated with a minimum in q in reversed magnetic shear configurations. RSAEs have proven especially useful for MHD spectroscopy, that is the inference of plasma equilibrium properties through their frequency spectra, due to their high sensitivity to the value of qmin. Reversed shear equilibria during the current ramp-up phase and sawtoothing phase are studied through the spatial and temporal characteristics of the RSAEs. Analysis of the spatial structures of RSAEs measured by phase contrast imaging, and interpreted with a synthetic diagnostic using numerical results from the ideal MHD code NOVA, provides constraints on the evolution of qmi,. Additional observations including mode coupling, mode tunneling, and minimum frequency scaling are discussed in light of theoretical and numerical models, with commentary on possible future experiments. by Eric Matthias Edlund. Ph.D. 209 p. eng Massachusetts Institute of Technology M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission. http://dspace.mit.edu/handle/1721.1/7582 Physics. A study of reversed shear Alfvén eigenmodes in Alcator C-Mod with phase contrast imaging Thesis application/pdf Publication unknown unknown <Publication xmlns="https://www.openaire.eu/cerif-profile/1.1/" id="48ca4947-0875-4162-988d-4802291581a0"> <Type xmlns="https://www.openaire.eu/cerif-profile/vocab/COAR_Publication_Types">http://purl.org/coar/resource_type/c_1843</Type> <Language>eng</Language> <Title>A study of reversed shear Alfvén eigenmodes in Alcator C-Mod with phase contrast imaging</Title> <PublishedIn> <Publication> </Publication> </PublishedIn> <PublicationDate>2009</PublicationDate> <Authors> <Author> <DisplayName>Edlund, Eric Matthias</DisplayName> <Affiliation> <OrgUnit> </OrgUnit> </Affiliation> </Author> </Authors> <Editors> </Editors> <Publishers> <Publisher> <DisplayName>Massachusetts Institute of Technology</DisplayName> <OrgUnit /> </Publisher> </Publishers> <License>http://dspace.mit.edu/handle/1721.1/7582</License> <Keyword>Physics.</Keyword> <Abstract>Shear Alfvén waves are a fundamental mode of magnetized plasma oscillation and may exist in tokamak plasmas as eigenmodes with a global structure and discrete frequencies. The inhomogeneity of the plasma profiles in conjunction with tokamak geometry tends to focus the Alfvén waves in regions of near uniformity defined by local extrema in the Alfvén continuum, a quantity which describes the local Alfvén resonance frequency for particular mode numbers. Modes excited in the vicinity of these near uniform regions may be weakly damped and excited by energetic ions. The reversed shear Alfvén eigenmode (RSAE), localized deep in the plasma core, is typically associated with a minimum in q in reversed magnetic shear configurations. RSAEs have proven especially useful for MHD spectroscopy, that is the inference of plasma equilibrium properties through their frequency spectra, due to their high sensitivity to the value of qmin. Reversed shear equilibria during the current ramp-up phase and sawtoothing phase are studied through the spatial and temporal characteristics of the RSAEs. Analysis of the spatial structures of RSAEs measured by phase contrast imaging, and interpreted with a synthetic diagnostic using numerical results from the ideal MHD code NOVA, provides constraints on the evolution of qmi,. Additional observations including mode coupling, mode tunneling, and minimum frequency scaling are discussed in light of theoretical and numerical models, with commentary on possible future experiments.</Abstract> <Access xmlns="http://purl.org/coar/access_right" > </Access> </Publication>