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High brightness ion beams are used in applications such as focused ion beam systems, the preferred tool to mill and deposit material on the nanometer scale. The smallest achievable spot size of focused ion beam technology is currently limited by chromatic aberrations in the lens column due to the energy spread of the industry standard liquid metal ions source (LMIS).
Here we present a new ion source concept, the laser cooled ion source, which is based on near-threshold ionization of laser cooled atoms [1][2]. This can achieve much lower energy spread, and is suitable for many different ionic species, so both light and heavy ion beams are possible. Compared to conventional sources, the ions are extremely cold: <1 mK. In our experiment we cool and trap rubidium atoms inside an accelerator structure. When the cold atom cloud is ionized by a pulsed laser and accelerated in an electric field, the time-of-flight to the detector can be used to measure the energy spread of the bunches. Bunches with an energy of only 5 eV are produced with a root-mean-square energy spread as low as 0.02 eV [3]. These pulses are also ideal to study space charge effects. The small velocities of the ions inside the accelerator make it possible to manipulate the ions with time dependent fields. Measurements and simulations show that the relative energy spread can be reduced by switching off (or reversing) the accelerator field before the ions have left the accelerator. At the same time, this reduces (or even reverses) the negative lens effect of the accelerator structure itself. References: 1.S. B. van der Geer et al, J. Appl. Phys. 102, 094312 (2007) 2.J.L. Hanssen et al, Phys. Rev. A 74, 063416 (2006). 3. M. P. Reijnders et al, Phys. Rev. Let. 102, 034802 (2009) |
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