We present an investigation of the cesium magneto-optical trap, with particular regard to the best combination of atomic density and temperature that can be produced. Conditions in the trap depend on four independent parameters: the detuning and intensity of the light, the gradient of the magnetic field, and the number of atoms trapped. We have varied all these parameters and measured the temperature and density distribution of the trapped cloud. Both the nonlinear variation with position of the restoring force and the reabsorption of photons scattered in the cloud limit the maximum density, and we present an empirical model that takes this into account. This in turn limits the density in phase space p (defined as the number of atoms in a box with sides of one thermal de Broglie wavelength). We have observed a maximum p = (1.5 + 0. 5) x 10,with a spatial density of about 2 x 10 atoms/cm . PACS number(s): 32.80.Pj

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A cloud of ultracold atoms confined in a magneto-optical trap (MOT) has a temperature greater than that of a low-density vapour in the same light field. It has been observed (Drewsen M. et al., AppL Phys. B, 59 (1994) 283) that this temperature excess is proportional to the cube root of the total number of trapped atoms (N1/3). We present an explanation for this effect in terms of the scattering which arises when photons spontaneously emitted by atoms have an appreciable probability of being reabsorbed within the cloud. This hypothesis has been tested by time-of-flight measurements of the temperature of clouds of atoms released from a MOT into l-dimensional and 3-dimensional optical molasses in the g + -r -and lin I lin configuration.