The Electron-Beam Ion Trap

The mention of few‐electron atoms usually brings to mind hydrogen, helium or other light elements in neutral form. However, these simple atoms are part of a sequence of ions having the same number of electrons but different nuclear charges. For example, the hydrogen‐like sequence spans neutral hydrogen through hydrogen‐like uranium, ${\mathrm{U}}^{\text{91+}}.$ Both the atomic physics and the applications of the most highly charged ions in such isoelectronic sequences are receiving increasing attention. Recently the electronbeam ion trap has made it possible to produce and study any such ion in a modest‐sized apparatus (figure 1).