The phase space density of dense, cylindrical clouds of atoms in a 2D magneto-optic trap is investigated. For a large number of trapped atoms (>10 8 ), the density of a spherical cloud is limited by photon reabsorption. However, as the atom cloud is deformed to reduce the radial optical density, the temperature of the atoms decreases due to the suppression of multiple scattering leading to an increase in the phase space density. A density of 2 10 ÿ4 has been achieved in a magneto-optic trap

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... aining 2 10 8 atoms. Laser cooled samples of atoms confined at high densities in traps are a promising system for applications in atom optics, interferometry, and the manipulation of quantum systems. In particular, the magneto-optic trap [1] (MOT) has developed into a robust and convenient method to trap and cool atoms. In nearly all experiments on degenerate gases, the MOT is used to collect and precool atoms as a precursor to magnetic trapping and evaporative cooling. However, like all optical cooling mechanisms, the efficacy of the magneto-optic cooling process degrades drastically with increasing phase space density due to reabsorption of spontaneous photons. Methods to circumvent this limitation would not only result in large samples of atoms at high phase space densities, but also provide insight into possible schemes for the creation of all optical Bose condensates [2] . In the particular case of a MOT, reabsorption of spontaneous photons manifests itself in the form of a density limiting mechanism [3] and a source of heating [4] . Thus, the phase space density of a spherical MOT with a large number of atoms (>10 8 ) is typically limited to the range of 10 ÿ7 . Higher phase space densities have been achieved [5] , but only at the expense of a fewer number of trapped atoms. To obtain degenerate samples of atoms, the increase of the remaining 7 orders of magnitude in phase space density is obtained by evaporative cooling. In this process, a very large fraction of the initial number of atoms is lost, limiting the number of atoms in the condensate. One method to minimize the effects of reabsorption is to use traps with a large aspect ratio. In such traps, the surface area to volume ratio of the atomic sample is large, thereby making it more likely for spontaneous photons to escape the cloud before they interact with other atoms. In this Letter, we study the phase space density of a MOT as a function of its aspect ratio. As the aspect ratio of the atom cloud is increased, thereby reducing the optical density in the radial dimension, we find that the deleterious effects of photon reabsorption can be suppressed, leading to a drastic improvement in the final phase space density of the atoms. It should be noted that a cylindrical atom cloud can be made optically thin while retaining a large number of atoms at high densities. This is in contrast to a spherical cloud where a reduction of the optical density necessarily entails a decrease in the number of trapped atoms. We have trapped 2 10 8 atoms at a phase space density of 2 10 ÿ4 , limited mainly by light induced collisional losses. This phase space density reflects an improvement of over 3 orders of magnitude compared to a conventional spherical MOT with the same number of atoms. The magneto-optic trap has been studied extensively both theoretically and experimentally [5, 6] . The characteristic parameters which determine the behavior of the MOT are the number of atoms N, the magnetic gradient b, and the laser light shift parameter 2 =jjÿ, where is the Rabi frequency per laser beam, is the detuning, and ÿ is the natural linewidth. Depending on these parameters, the MOT has been observed to be in different regimes with distinct scaling laws for the temperature and density. In particular, if the number of atoms is very low (10 4 -10 5 ), the temperature of the atoms is independent of the atom number and is close to molasses temperatures for the given light shift parameter. This regime has been referred to as a "temperature limited" regime[5] since the size of the atom cloud is determined solely by the temperature. Accordingly, the spatial density and the phase space density are proportional to the atom number and effects of photon reabsorption are insignificant. In contrast to this regime, as the number of trapped atoms increases, the MOT enters the multiple scattering regime. The properties of the atom cloud are then dominated by the effects of photon reabsorption, and the simple model of the MOT as a collection of independent particles ceases to apply. As was observed in [3], the multiple scattering regime is characterized by strong interatomic repulsive forces due to the increased cross section for scattering spontaneous photons. This repulsive P H Y S I C A L