Buffer gas cooled clocks rely on large numbers of ions, typically ～10~(7), optically pumped by a discharge lamp at scattering rates of a few photons per second per ion. To reduce the 2~(nD) Doppler shift from space charge repulsion of ions from the trap node line, novel multi-pole ion traps are now being developed where ions are weakly bound with confining fields that are effectively zero through the trap interior and grow rapidly near the trap electrode 'walls.'
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