The exploitation of ion confinement for the development of a new kind of portable atomic frequency standard was first proposed by Major in 1969 as an approach that promised extraordinary accuracy in a light, compact device suitable for aerospace applications (Major, 1969). It was predicted that a microwave resonance of unprecedented spectral resolution was possible at around 40.5 GHz on mercury ions of isotopic mass 199, when observed under the perturbation-free environment of field confinement in vacuum, where free observation times of tens of seconds were routinely obtainable. Moreover, the relatively large mass of the mercury ion has the further advantage of a small (second-order) Doppler width for a given distribution of kinetic energy, a subject we will consider in greater detail later. The resonance is at the high end of the microwave region of the spectrum, where for a given line width the resonance Q is high, and yet falls in a range that can conveniently be reached by common frequency-synthesis techniques. Finally, the short wavelength of the resonant microwave field (7.4 mm) permits the physical dimensions of the microwave components to be correspondingly small. For all these reasons it was argued that the choice of mercury is particularly suited to fully exploit the new technique of field confinement in the development of a spacecraft clock.
KeywordsShot Noise Microwave Field Trapping Region Paul Trap Crystal Oscillator
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