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This paper reviews the history of multipactor discharge theory, focusing on recent models of multipactor accessibility and saturation. Two cases are treated in detail: That of a first-order, two-surface multipactor, and that of a single-surface multipactor on a dielectric. In both cases, susceptibility curves are constructed to indicate the regions of external parameter space where multipactor is likely to occur, taking into account the dependence on surface materials, and the effects of space charge and cavity loading. In the case of a dielectric, multipactor is found to deliver about 1% of the rf power to the surface. The two cases are contrasted in light of experimental observations.

Recent experiments at the University of Maryland using photoemission from a dispenser cathode have yielded some interesting results regarding the effects of the area of emission and of the ratio between the pulse length and the gap transit time on the amount of current that may be drawn from an electron gun before a virtual cathode forms. The experiments show that a much higher current density may be drawn from a short pulse or limited emitter area than is anticipated by the Child-Langmuir limiting current. There is also evidence that the current may be increased even after virtual cathode formation, which leads a distinction between a limiting current density and a current density critical for virtual cathode formation. The experiments have also yielded some interesting results on the longitudinal structure of the current pulse passed through the anode. Some empirical and theoretical scaling laws regarding the formation of virtual cathodes in an electron gun will be presented. This work was motivated by the needs of the University of Maryland Electron Ring ͑UMER͒ ͓P. G. O'Shea, M. Reiser, R. A. Kishek et al., Nucl. Instrum. Methods Phys. Res. A 464, 646 ͑2001͔͒ where the goal is to generate pulses that are well-localized in time and space.

This paper proposes a novel theory of single-surface multipactor discharge on a dielectric, such as an rf window. Using a Monte Carlo simulation, we obtain the susceptibility diagram, applicable to a wide range of materials, in terms of the rf electric field and of the dc electric field that may result from dielectric charging. The electron multiplication mechanism assumes realistic yield curves of secondary electrons, including distributions of emission velocities and angles for these electrons. The susceptibility diagram thus constructed allows an immediate assessment of the range of rf power over which multipactor may be expected to occur. A simple analytic theory is constructed to explain the simulation results. [S0031-9007(97)04847-3] PACS numbers: 84.40.Zc

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