The magnetic trapping mode of an electron beam ion trap: New opportunities for highly charged ion research

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Abstract
Using x‐ray spectroscopic techniques, we have investigated the properties of an electron beamion trap (EBIT) after the electron beam is switched off. In the absence of the electron beam, bare, and hydrogenlike Kr35+ and Kr36+ ions remain trapped due to externally applied magnetic and electric fields for at least 5 s; xenon ions with an open L shell, i.e., Xe45+–Xe52+, remain trapped at least as long as 20 s. The ion storage time in this ‘‘magnetic trapping mode’’ depends on the pressure of background atoms as well as on the value of the externally applied trapping potential, and even longer ion storage times appear possible. The magnetic trappingmode enables a variety of new opportunities for atomic physics research involving highly charged ions, which include the study of charge transferreactions, Doppler‐shift‐free measurements of the Lamb shift, measurements of radiative lifetimes of long‐lived metastable levels, or ion‐ion collision studies, by x‐ray or laser spectroscopy, and mass spectrometry. Because the trap is filled in situ during the electron trapping phase, transfer losses associated with filling the trap from an external source are avoided. We present spectra of the K‐shell emission from heliumlike and hydrogenlike Kr34+ and Kr35+ as well as Xe52+ and Xe53+ that are produced by charge transferreactions in collisions between ions and neutral atoms. Marked differences with K‐shell spectra produced by electron‐impact excitation are evident. We use the measurements to infer the Lamb shift contribution to the energy of the 1s 1/2–2p 3/2 transition in hydrogenlike Xe53+ and determine it to be 31 276(12) eV. The measurement technique can be applied to any ion produced in an EBIT so that Doppler‐shift‐free Lamb shift measurements of hydrogenlike U91+ are within reach. We also illustrate the utility of the magnetic mode for lifetime determinations by measuring the 3.92(13) ms radiative decay of the 1s2s 3 S 1 level in heliumlike N5+.

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