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UBC Theses and Dissertations

An efficient ionizer for an atomic beam of helium and a source of doubly charged helium ions. Vermette, Clifford William Harvey

Abstract

Part I An efficient ionizer designed for the ionization of a low intensity molecular beam has been developed. The ionizer will have an efficiency for an argon beam at room temperature and 0.8 amperes plate electron current of greater than 5.3% and for an helium 4 beam at room temperature and 0.8 amperes plate electron current of greater than 0.12%. The electron bombardment ionization occurs between two flat plates both of which are at 200 volts potential with respect to the cathode. The two plates are at a 2.5° angle to each other so that the ions once formed experience a field gradient produced by this angle which accelerates them out the open side of the ionizer 90° to the direction of the incident neutral beam. The electrons were emitted from 6, 0.030 x 0.004 inches, thoriated tungsten ribbon filaments heated by d.c. power. Successful activation of the filaments, however, was not achieved and so they were used as pure tungsten filaments. The ionizer ran at between 1900° Kelvin and 2600°Kelvin for about 8 hours without significant distortion, filament sag, or appreciable outgassing. Part II The design of a source of doubly charged helium ions was carried out to provide an helium beam of twice the terminal energy of the Van de Graafe Accelerator. The unit, to be installed in the top terminal of the Van de Graaffe, employs a radio frequency ion source followed by a double focusing magnet. The magnet has plane parallel pole pieces with a fringing field that produces focusing in a vertical plane. The unit is designed so that at 2.5 kilovolts extraction voltage a magnetic field of 3060 gauss is required to bend the He⁺⁺ through an angle of 90° and bring them to a focus at 5.8 cm. from the exit face of the magnet. The beam acceptance angle of the magnet is 10° at a source distance from the entrance face of 5 cm.

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