Laser-cooled Ion Source

Pulsed and continuous ion beams are used in applications, such as focused ion beams. The smallest achievable spot size in focused ion beam technology is limited by the monochromaticity of the ion source. Here we present energy spread measurements on a new concept, the ultra-cold ion source, which is based on near-threshold ionization of laser cooled atoms [1]. The low energy spread is important for focused ion beam technology because it enables milling and ion-beam-induced deposition at sub-nm length scales with many ionic species, both light and heavy. In the experiment, Rubidium atoms are captured in a magneto optical trap (MOT) inside an accelerator structure where they are ionized by a pulsed laser in a DC electric field. Time-of-flight measurements show two orders of magnitude lower energy spread than in the current industry standard reached with Gallium liquid-metal ion sources. Bunches with energy of only 5 eV are routinely produced with a root-mean-square energy spread as low as 0.02 eV. This proves the feasibility of this new ion source concept [2]. In addition, we show that the slowly moving, low-energy-spread ion bunches are ideal for studying intricate space charge effects in pulsed beams, i.e. the transition from space charge dominated dynamics to ballistic motion.