Field Confinement of Ions

Abstract

As already emphasized, the spectral line width of magnetic dipole transitions in atomic systems is determined in practice not by the natural lifetime of the upper state, but the available perturbation-free time for the observation. This has led to the development of devices that depend on diffusion through an inert gas in the case of the Rb standard and confinement in a bulb with inert wall coating in the case of the hydrogen maser. The short-term stability achieved in the hydrogen maser has been extraordinary and still surpasses any other device in the short term, but it has two important drawbacks: first and most serious is the shift in frequency due to collisions with the walls of the containment bulb, a shift which disqualifies it as an absolute standard, and second, its lack of portability due to the size of the cavity and the need for elaborate magnetic shielding. The experimental approach to overcome these limitations has evolved along two distinct directions: first the extension of the atomic beam observation time using a vertical beam, the atomic fountain standard, and second, the use of electrically charged atoms, that is ions, trapped by electromagnetic fields. We will treat ion confinement in fields in this chapter and deal with the atomic fountain technique after we have reviewed the subject of lasers in the next chapter.