= Paul trap
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The Paul trap alternates focusing and defocusing <electric forces>. Their time average along a moving trajectory need not vanish, even when the <electric field> averages to zero at every fixed point. A spatial <gradient> makes the fast displacement correlate with the driving <electric field>; the <RF-averaged trapping potential> calculates the resulting restoring <force>.
<Wolfgang Paul>'s lecture describes hyperbolic ring/endcap electrodes, loading by <Electron>-impact <ionization> of low-pressure gas, and resonant ejection through an endcap hole into an <electron multiplier>. Sweeping RF voltage produces an air <mass spectrum>.<Electromagnetic traps for charged and neutral particles by Wolfgang Paul>[Paul, pp. 608–614, Figs. 6, 11; Columbia scan]{ref}
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