The focal properties of electron bombardment ion sources for mass spectrometers

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Abstract
The focal properties of electron bombardment ion sources of the type commonly used in mass spectrometers are calculated by numerical ray tracing. Low electron current density conditions were assumed so that the effect of space charge within the electron beam could be neglected. Calculation of paraxial rays show that one, two or three real images can be formed within the ion source, depending on the voltage applied to the source electrodes. A curve relating the source sensitivity to the repeller voltage is computed by using ray tracing to a higher order of approximation; the theoretical curve displays the same characteristics as those of the curve obtained experimentally on the AEI MS902 mass spectrometer. This treatment demonstrates that the structure of the repeller curve is related to the multiplicity of real images that can be formed within the source. Precise mechanisms determining the source efficiency are deduced from the computed focal properties.