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UBC Theses and Dissertations

Design study of accelerator electron guns Goud, Paulus Arie

Abstract

In part I of this thesis the theory underlying electron beam design is presented. The paraxial ray equation is derived, and is used to determine the focal properties of an aperture and of an Einzel lens, both of which are assumed to be thin lenses. An aperture can exert a positive or a negative focusing action on an electron beam, depending on the potential gradients existing on either side of the aperture. In an Einzel lens the focusing action is always positive, whether the focusing anode potential is greater or smaller than the beam potential. If the focusing anode is made very negative w.r.t. the beam voltage the electrons are reflected. In the above space-charge and thermal velocity effects are ignored. It is well known that in practical guns electron interaction is important and the Pierce method of electron gun design is examined. The effect of thermal velocity on obtainable current density is also considered. In part II of this thesis the theory is applied to the design of an electron source for a resonant linear accelerator. The perveance and cross-over point are calculated for an existing, partly assembled, bombarded cathode, Pierce type, electron gun. Methods are considered for adapting this gun as a 2 kV. electron source for the resonant linear accelerator. Electrostatic focusing methods for this application are inefficient, as less than 5% of the beam current is utilised. If Brillouin type magnetic focusing is used, a current efficiency of up to 75% is possible. A tungsten filament electron gun, capable of producing a 10 ma beam at a gun voltage of 2 kV, is designed and constructed. This gun has a filamentary cathode consisting of seven turns of 0o013" tungsten wire wound on a threaded mandrel (#4-40). Four anodes are used; they are made of Copel, and perform the functions of current control and of focusing the electron beam.

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