Miniaturized magnet-less RF electron trap. I. Modeling and analysis

Ionization of trace gases by electron impact followed by ion extraction is an important pumping approach in ultrahigh vacuum (UHV) systems, which operate at nTorr pressure levels. However, pumping efficiency can only be achieved if the lifetime of electrons is sufficiently long to allow ionizing collisions with neutral species. In miniaturized systems, the electron lifetime is limited due to wall collisions. A traditional approach for an extended electron lifetime via trapping uses crossed electric and magnetic fields. These magnetic fields are undesirable in certain miniaturized systems such as atomic clocks. In this paper, the authors report a method and miniaturized structure for electron trapping in UHV conditions, which does not rely on magnetic fields. Electrons from an electron-beam source are transferred through a grid electrode into a central region of the device where they are trapped in lengthened trajectories using applied radio frequency (RF) electric fields. This paper describes analytical and numerical modeling to identify critical operating constraints between the trap geometry and driving RF voltage and frequency. An analytical relation is derived between RF voltage and frequency that should result in electron trapping for a given trap geometry. A plasma transport model is used to numerically investigate the trapping efficiency of the method with a two-dimensional geometry representative of experimental prototypes. A parametric study of RF voltage and frequency, electron beam current and initial energy, and background gas pressure demonstrates the efficacy of this approach in a miniaturized trap (approximate to 1 cm(3) trap volume). The authors find an increase of 3-4 orders of magnitude in electron density in the trap (2 x 10(7) cm(-3)) compared to the density of the electron beam (1 x 10(3) cm(-3)) with a proper choice of the applied voltage amplitude and RF frequency (typically 150 V and 150 MHz). These results indicate that miniature magnet-less electron traps can be effective. (C) 2017 American Vacuum Society.