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Ian H. Hutchinson. Becerra, Gabriel E. (Becerra Toledo) Massachusetts Institute of Technology. Dept. of Nuclear Science and Engineering. Massachusetts Institute of Technology. Department of Nuclear Science and Engineering 2008-04-23T14:35:28Z 2008-04-23T14:35:28Z 2007 2007 http://hdl.handle.net/1721.1/41248 213331663 Thesis (S.M. and S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2007. Includes bibliographical references (p. 101-107). Multipactor discharges consists of electron multiplication between two surfaces by secondary electron emission in resonance with an alternating electric field. They are detrimental to the performance of radio frequency (RF) systems, such as the ICRF (ion cyclotron range of frequencies) antennas for heating of plasmas in the Alcator C-Mod tokamak and other nuclear fusion devices. This work investigates multipactor discharges in the coaxial geometry in the presence of a constant and uniform magnetic field transverse to the direction of electromagnetic wave propagation. Studies on the Coaxial Multipactor Experiment (CMX) show that the magnetic field decreases the degree to which the discharge detunes the RF circuit. However, it enhances the susceptibility of the system to multipactor-induced gas breakdown at low pressures, which appears to cause the observed neutral pressure limits on antenna performance in Alcator C-Mod. Different surface treatment methods involving roughening and in-situ cleaning failed to suppress the multipactor discharges in a consistent and reliable way in experiments on CMX, despite the success of similar techniques in the parallel-plate geometry. Electron trajectories are significantly more complicated in the presence of magnetic fields of different strengths, as shown by a three-dimensional particle-tracking simulation using Monte Carlo sampling techniques. The trends in electron path length, time of flight, impact energy, secondary emission yield and population growth do not account for the experimental observations between the low and high field limits. These appear to be better explained by collective effects not included in the simulations, such as the effect of the magnetic field on charged particle diffusion. by Gabriel E. Becerra. S.M. S.B. 107 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 Nuclear Science and Engineering. Studies of coaxial multipactor in the presence of a magnetic field Thesis application/pdf Publication unknown unknown <Publication xmlns="https://www.openaire.eu/cerif-profile/1.1/" id="47806647-856f-4434-8456-bcc993bb6b7c"> <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>Studies of coaxial multipactor in the presence of a magnetic field</Title> <PublishedIn> <Publication> </Publication> </PublishedIn> <PublicationDate>2007</PublicationDate> <Authors> <Author> <DisplayName>Becerra, Gabriel E. (Becerra Toledo)</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>Nuclear Science and Engineering.</Keyword> <Abstract>Multipactor discharges consists of electron multiplication between two surfaces by secondary electron emission in resonance with an alternating electric field. They are detrimental to the performance of radio frequency (RF) systems, such as the ICRF (ion cyclotron range of frequencies) antennas for heating of plasmas in the Alcator C-Mod tokamak and other nuclear fusion devices. This work investigates multipactor discharges in the coaxial geometry in the presence of a constant and uniform magnetic field transverse to the direction of electromagnetic wave propagation. Studies on the Coaxial Multipactor Experiment (CMX) show that the magnetic field decreases the degree to which the discharge detunes the RF circuit. However, it enhances the susceptibility of the system to multipactor-induced gas breakdown at low pressures, which appears to cause the observed neutral pressure limits on antenna performance in Alcator C-Mod. Different surface treatment methods involving roughening and in-situ cleaning failed to suppress the multipactor discharges in a consistent and reliable way in experiments on CMX, despite the success of similar techniques in the parallel-plate geometry. Electron trajectories are significantly more complicated in the presence of magnetic fields of different strengths, as shown by a three-dimensional particle-tracking simulation using Monte Carlo sampling techniques. The trends in electron path length, time of flight, impact energy, secondary emission yield and population growth do not account for the experimental observations between the low and high field limits. These appear to be better explained by collective effects not included in the simulations, such as the effect of the magnetic field on charged particle diffusion.</Abstract> <Access xmlns="http://purl.org/coar/access_right" > </Access> </Publication>