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Crystalline ion beams


By freezing out the motion between particles in a high-energy storage ring, it should be possible1,2,3,4 to create threads of ions, offering research opportunities beyond the realm of standard accelerator physics. The usual heating due to intra-beam collisions should completely vanish, giving rise to a state of unprecedented brilliance. Despite a continuous improvement of beam cooling techniques, such as electron cooling and laser cooling, the ultimate goal5 of beam crystallization has not yet been reached in high-energy storage rings. Electron-cooled dilute beams of highly charged ions show liquid-like order6,7 with unique applications8. An experiment5 using laser cooling9,10 suggested a reduction of intra-beam heating, although the results were ambiguous. Here we demonstrate the crystallization of laser-cooled Mg+ beams circulating in the radiofrequency quadrupole storage ring PALLAS11,12 at a velocity of 2,800 m s-1, which corresponds to a beam energy of 1 eV. A sudden collapse of the transverse beam size and the low longitudinal velocity spread clearly indicate the phase transition. The continuous ring-shaped crystalline beam shows exceptional stability, surviving for more than 3,000 revolutions without cooling.

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Figure 1: Images of ion crystals at rest in PALLAS.
Figure 2: Axial and radial cut through the r.f. quadrupole storage ring PALLAS.
Figure 3: Fluorescence rate of the ion beam as a function of the frequency detuning of the co-propagating laser.
Figure 4: Transverse beam profiles and velocity profiles before (a, c) and after (b, d) the phase transition.
Figure 5: Fluorescence signal of the ion crystal at rest and the crystalline beam after blocking and unblocking the cooling lasers.


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We thank R. Neugart for technical support, and P. Kienle and H. Walther for discussions. The work was partially funded by the Deutsche Forschungsgemeinschaft and the Maier Leibmitz Labor.

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Schätz, T., Schramm, U. & Habs, D. Crystalline ion beams. Nature 412, 717–720 (2001).

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