Collisional avalanches are identified to be responsible for an 8-fold increase of the initial loss rate of a large 87 Rb condensate. We show that the collisional opacity of an ultra-cold gas exhibits a critical value. When exceeded, losses due to inelastic collisions are substantially enhanced. Under these circumstances, reaching the hydrodynamic regime in conventional BEC experiments is highly questionable.

We present an experimental study of the loading of a magneto-optical trap ͑MOT͒ from a brightened and slowed beam of metastable neon atoms. The unprecedented high numbers of 9ϫ10 9 20 Ne and 3ϫ10 9 22 Ne metastable atoms are trapped under unconventional trap conditions as compared to metastable helium traps, such as low intensity and small detuning. These cause the MOT to have an extraordinarily large volume on the order of 1 cm 3 and a typical peak density of 10 10 atoms/cm 3 . A simple Doppler model is discussed which explains why the optimum is found under these conditions. The model includes the seventh beam necessary for the last slowing step before loading.

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