Crystalline Ion Beams in the RF Quadrupole Storage Ring PALLAS

We report on the crystallization of laser cooled Mg+ ion beams circulating in the table-top rf quadrupole storage ring PALLAS at a velocity of 2800 m/s (beam energy 1 eV). A sudden collapse of the transverse beam size and the low velocity spread clearly indicate the phase transition to a onedimensional linear string of ions. This crystalline beam shows exceptional stability, surviving for more than 3000 revolutions without cooling. Close to the phase transition, the spatial beam profile of non-crystalline beams was found to exhibit an unexpected two component Gaussian distribution. Although its origin is not yet fully understood, we can exploit this effect for the identification of twoand three-dimensional crystalline beams. Proceeding “Non-Neutral Plasma Physics IV” edited by F. Anderegg, L. Schweikhard, C.F. Driscoll, July 30. – Aug. 2. 2001, UC San Diego, CA/USA, AIP Press 606 235 (2002) INTRODUCTION THE QUEST FOR CRYSTALLINE BEAMS Threading ions like pearls on a string in high-energy storage rings by freezing out the inter-particle motion [1, 2, 3, 4, 5] opens opportunities far beyond the means of standard accelerator physics [6, 7]. The usual heating due to collisions of particles within the beam almost completely vanishes, giving rise to a state of unprecedented brilliance and exceptional stability. To reach this ultimate goal, beam cooling techniques were improved continuously. With electron cooling, very dilute beams of highly charged ions were observed to exhibit liquid-like order [8, 9] with unique applications in mass spectrometry [10]. With refined laser cooling methods [11, 12], an ambiguous reduction of intra-beam heating [13] was reported for 9Be+ ion beams. Space-charge limited densities were reached for laser cooled 24Mg+ ion beams [14], but no clear evidence for beam crystallization has been found in high-energy storage rings so far. This fact stands in contrast to the routine generation of elongated ion crystals at rest in ring [15] and linear traps [16, 17]. As an illustration and as a reference, in fig. 1 we present images of ion crystals at rest [6, 18], gained with our storage ring PALLAS (PAul Laser cooLing Acceleration System), described below. Ions appear ordered since their mutual Coulomb-repulsion overcomes their mean kinetic energy. The overall Coulombrepulsion is compensated by an external parabolic trapping potential Ψ(r;z). A strong asymmetry in the radial and longitudinal confinement [18] leads to the prolate shape of the crystals. The formation of the crystalline structure is well understood [3, 15, 17, 19, 20]. The Coulomb-crystal evolves from a linear string of ions over a zig-zag band to three-dimensional helices when the dimensionless linear ion density λ = (N=z) a [19], N denoting the number of particles and a the Wigner-Seitz radius, increases. This can be achieved either by adding more ions, or by reducing the radial confinement.