“White-light” Laser Cooling of a Fast Stored Ion Beam

Abstract

We report the experimental demonstration of "white-light" cooling of a high-velocity 7 Li 1 ion beam stored at 6.4% of the speed of light in a storage ring. In a direct comparison with single-mode laser cooling, we show that white-light cooling is much more efficient to counteract strong intrabeam heating and leads to lower longitudinal beam temperatures at higher ion densities, i.e., much higher densities in longitudinal phase space. [S0031-9007(98)05508-2] PACS numbers: 29.20.Dh, 32.80.Pj, 42.50.Vk In ion storage rings, established in the past decade as powerful instruments for precision experiments in atomic and nuclear physics [1], cooling techniques [2] play a key role in the production of beams of high brilliance and quality. In accelerator physics, very cold and highly dense ion beams are of great interest for exploring the beam dynamics at highest phase-space densities and for approaching a regime in which the beam behaves as a strongly coupled one-component plasma. Laser cooling [3-8] provides an extremely fast and efficient cooling and thus offers unique possibilities to enter this regime and to attain beams with liquidlike or solidlike Coulomb ordering [9,10].In order to match the particular requirements of stored high-velocity ion beams, new laser-cooling techniques have to be employed [11,12]. The main problem, which markedly distinguishes the cooling of dense ion beams in storage rings from the cooling of neutral atom beams, is a very strong influence of Coulomb collisions between the stored particles. This intrabeam scattering (IBS) leads to strong heating exceeding thousands of degrees Kelvin per second and, most problematic for laser cooling, to sudden longitudinal velocity changes up to ϳ1000 m͞s in single large-angle Coulomb collisions. As singlemode laser radiation does efficiently excite the ions only within the homogeneous linewidth, corresponding to a narrow velocity interval of typically a few tens m͞s, these scattering events lead to dramatic losses out of the cooling process [13]. A large velocity capture range, as needed to overcome these losses, can be obtained by Doppler tuning the transition frequency with local electrostatic fields and making use of an adiabatic optical excitation [11]. This method has recently served as a prerequisite for the first transverse laser cooling of a stored ion beam [7], but it has the severe drawback of limiting the applicable cooling rate to values far below the maximum dissipation rates offered by laser cooling.We have recently proposed a laser-cooling scheme [12] which is based on a specifically tailored broadband laser [14] for the achievement of efficient "white-light" cooling [15][16][17][18][19]. The basic idea of this broadband cooling method is to resonantly interact with different velocity classes at the same time so that the radiation pressure can counteract the influence of IBS much more efficiently as compared to the single-mode case. The crucial point for obtaining optimum cooling results is to realize a broad spectrum with a sharp cuto...

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